GMEEAL: IHSTSTJCTIOHS FOB THE ELI): WOEE OF THE GO AST AND Q-EOBETie SUEVEY (fforn^U HnlUBreitg ffiibrarg Jlt^ata. S^em Qork THE GIFT OF A\ts. K. B, Tixrner, ENGINEERING LIBRARY ;^e shows when this voltune was taken. lo renew this book copy the call No. and give to the hbrarian, HOME USE RULES fjC Q O 1952 AU Books subject to RecaU All borrowers must regis- ter in the library to borrow books fOT home use. All books must be re- turned at end of college year for inspection and repairs. Limited books must be re- turned within the four week limit and not renewed. Students must return all books before leaving town. Officers should arrange for the return of books wanted during their absence from town. Volumes of periodicals and of pamphlets are held in the library as much as possible. For special pur- poses they are given out f^r a limited time. Borrowers should not use their library privileges for the benefit of other persons. Books of special value and gift books, when the giver wishes it, are not allowed to circulate. Readers are asked to r^ port all cases of booki marked or mutilated. Do not d^ace books by marks and writing. Cornell University Library TA 545.U581 General instructions for the field work 3 1924 004 359 745 Cornell University Library The original of tinis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924004359745 DEPARTMENT OF COMMERCE AND LABOR COAST AND GEODETIC SURVEY O. H. TITTMANN, Superintendent GENEEAL INSTRUCTIONS FOR THE FIELD WORK OF THE COAST AND GEODETIC SURVEY WASHINGTON GOVERNMENT PRINTING OFFICE 1908 A 53 /A,3?35:6 OOlTTEl^TS. Paga Latitude and longitude determinations 5 Triangulation 6 Tertiary triangulation _ 6 Elevations by vertical angles 31 Precise leveling 39 Azimuth 39 Topography - - 42 Hydrography 48 Tide observations 78 Current observations 103 Magnetic observations 106 Descriptive reports 113 Progress sketches 116 Geographic names 117 General remarks - 119 Index 123 GENERAL INSTRUCTIONS FOR THE FIELD WORK OF THE COAST AND GEODETIC SURVEY. These instructions supersede all previous instructions and circulars upon the subjects treated and, together with the Laws and Regula- tions for the Government of the Coast and Geodetic Survey, are to be followed in the execution of field work. O. H. TiTTMANN, Superintendent. August 1, 1908. LATITUDE AND LONGITUDE DETERMINATIONS. 1. In Appendix No. 7, Report for 1897-98, " Determinations of time, longitude, latitude, and azimuth," the standard methods for this work in the Coast and Geodetic Survey are described. Since the date of that publication the longitude methods have been modified as described in " General instructions • for longitude determinations with transit micrometer, 1907" (not printed). 2. In connection with general survey operations, initial positions from which to extend new work will ordinarily be furnished, these positions being derived from previous triangulation or from as- tronomical determinations made by special parties. In the present state of the work in such regions as Alaska and the Philippine Islands occasions may arise where surveys must be made in localities where no determined initial point is available. In such cases, in order not to delay other operations, it may be necessary at first to assume the latitude and longitude of a starting point, taking the best value obtainable from existing charts or other source of in- formation. If means are available for determining by approximate astronomical methods a position likely to be more correct, this should be done as opportunity offers. The relative economy of using such methods, or the standard methods, or depending on the assumed value until connection is made by triangulation, should be considered in each case. If the instruments are available, the meridian telescope can be used for latitude and time with little more expenditure of time than is required for getting less satisfactory results with a theodolite. When, however, a theodolite is used, the latitude should 5 6 COAST AND GEODETIC SURVEY. preferably be obtained by observing meridian altitudes of stars north and south of the zenith, or the altitude of Polaris, and the time by equal altitudes of stars, or a single altitude of a star pref- erably near the prime vertical. For convenient star charts, see United States Hydrographic Office charts Nos. 2100 and 2101. TRIANGULATION. 3. Triangulation in the Coast and Geodetic Survey is divided into three classes: 4. Primary triangulation. — Triangulation of the highest grade of accuracy attainable at a reasonable cost. It is to be used where the triangulation is to extend unsupported over long distances, and is to constitute the main framework for all other surveys, or where it is to supplement for a given locality former triangulation of this class in that locality. (See " General triangulation instructions " of January 10, 1905.) 5. Secondary triangulation. — This is triangulation of somewhat less accuracy than primary triangulation and is to be used for the main scheme of triangulation in the Philippines, and for certain cases in the United States where the total distance covered is not very great, or where it is to be supported at frequent intervals by primary triangulation. (See "General triangulation instructions" of Jan- uary 10, 1905.) 6. Tertiary triangulation. — This is the latest accurate class of tri- angulation to be used in the Survey for the determination of marked points. It is to be used in general for the immediate control of topographic and hydrographic surveys. In Alaska and the Philip- pine Islands tertiary triangulation will be used by all vessels and parties engaged in general surveys, whether continuous Avork along the coast or local harbor surveys, unless otherwise instructed. 7. No lower grade of triangulation should be used unless physical conditions or lack of time or means prevents the maintenance of the standard called for, and in such cases, where practicable, the office should be previously advised. No higher grade of accuracy or greater refinement than here called for should be used for the classes of work specified, unless the special instructions for the work so require. 8. Because of the fact that tertiary triangulation is usually in- cluded in the work of parties engaged in general surveys, instructions for this class of triangulation are given here. TERTIARY TRIANGULATION. 9. Character of figures.— The main scheme of the triangulation shall be made up of figures of from four to seven points each, in which certain stations may be left unoccupied as indicated under paragraph GENERAL INSTKUCTIONS FOR FIELD WORK. 7 11, "Strength of figures." It must not be allowed to degenerate even for a single figure to simple triangles, except where it is ex- tremely difficult to avoid this. On the other hand, there must be no overlapping of figures and no excess of observed lines beyond those necessary to secure a double determination of every length, except that in a four-sided, central-point figure one of the diagonals of the figure may be observed. Observations over lines which will make the main scheme any more complicated than that defined above would practically be wasted. The main scheme should be extended to within sight of all portions of the area to be controlled by the triangulation. If it is necessary to occupy other stations than those in the main scheme in order to reach by intersection cer- tain stations which must be fixed to control hydrographic or topo- graphic operations, connect these additional occupied stations (which will be called supplementary stations) with the main scheme by the simplest figures jjossible in which there is a check. Simple triangles with all the angles measured will, in general, be sufficient for the jDurpose. It frequently happens that tertiary triangulation stops at a place from which it is probable that it will be extended farther at some future date — as, for example, at the head of a bay or part way up a river. In such a case it is desirable to stop on a line rather than a point, and the last figure should be a quadrilateral with one point left unoccupied rather than a simple triangle. 10. In the coast triangulation in the Philippine Islands it is ex- pected that the stations will be located near the coast and on offlying islands or on the first foothills back from the coast. 11. Strength of figures. — In the main scheme of triangulation the value of the quantity R=f — =c^ — j 2 [6''j,-\-Sj^S^-\-S'^^ for any one figure must not in the selected best chain (call it 11^) exceed 50, nor in the second best (call it Ra) exceed 150 in units in the sixth place of logarithms. These are extreme limits never to be exceeded, except when it is extremely difficult under existing conditions to keep within them. Keep the quantities 11^ and R, down to the limits 25 and 80 for the best and second best chains, respectively, whenever the esti- mated total cost does not exceed that for the chain barely within the extreme limits by more than 25 per cent. The values of R may be readily obtained by use of the " Table for determining relative strength of figures." (See paragraphs 59 and 60 for this table and explanation of formula for R.) One station in each figure may be left unoccupied whenever to do so does not increase the values of R beyond the specified limits. In a figure in which all stations are occupied, if any interruption, failure of signal to show, for example, makes it probable that such a procedure would save considerable time, certain lines to a total of not more than three may be observed 8 COAST AND GEODETIC SURVEY. over ill one direction only. In such a case R^ and Rj shall be com- puted as if one outside station of the figure had been left unoccupied, and the values so computed must not exceed the specified limits. The advantage of keeping R small and thus avoiding the necessity of measuring bases at short intervals is indicated in the second follow- ing paragraph. For no triangle used in connecting a supplementary station with the main scheme should the value of R be greater than 50. 12. Lengths of lines. — The lower limit of length of line is fixed by two considerations. On very short lines it is difficult to get observa- tions of the degree of accuracy necessary to close the triangles within the required limit. Very short lines are apt to be accompanied, though not necessarily so, by poor geometrical conditions as expressed by large values of R. The extreme lower limit fixed by these two con- siderations should be avoided. There is no advantage in so far as accuracy is concerned in using very long lines. Long lines are apt to introduce delays due to signals not being visible. With long lines sup- plementary stations to reach required points in all portions of the area covered are much more apt to be needed than with short lines. There- fore endeavor in laying out the main scheme to use the economic length of line — that is, endeavor to use in each region lines of such lengths as to make the total cost of reconnoissance signal building, triangulation, and base measurement a minimum for the area to be covered, subject to the limitations stated in these instructions. 13. Frequency of bases. — If the character of the country is such that a base site can be found near any desired location, ^R^ between base lines, or between a base line and a line of primary or secondary trian- gulation used as a base, should be made about 130. This will be found to correspond to a chain of from 10 to 35 triangles, according to the strength of the figures secured. With strong figures but few base lines will be needed, and a corresponding saving will be made on this part of the work. If topographic conditions make it difficult to secure a base site at the desired location, SRi may be allowed to approach but not exceed 200. There will be danger when this is done that an inter- vening base will be necessary, for the reason stated in the next sen- tence. If in any case the discrepancy between adjacent bases (either measured bases or lines of primary or secondary triangulation used as bases) is found to exceed one part in 5,000, an intervening base must be measured, or the intervening triangulation strengthened. 14. Base sites and base nets. — A base may be measured with steel or invar tapes with the degree of accuracy specified in the following paragraph over rough ground and steep slopes. Smooth, level ground is a convenience for base measurement of this grade of ac- curacy, but not a necessity. There should be no hesitancy in placing the base on rough ground if by so doing the geometrical conditions GENERAL INSTEITCTIONS FOR FIELD WORK. 9 in the base net are improved — that is, values of E made smaller. The length of a base is to be determined primarily by the desirability of securing small values of R in the base net. The longer the base the easier it will be found to secure small values of R, and the smaller the values of R the longer the chain of triangles through which the lengths may be carried before another base becomes necessary. The base net shall consist of a figure or figures of the same character, and subject to the same conditions as to strength as the main scheme previously described. If the net is made up of two or more figures they may overlap in space, but there should be no overlapping of figures in the sense of the existence of observed lines which tie to- gether the separate figures. Broken bases are permissible when found advantageous. 15. Base measurements. — In the base measurements select apparatus and methods which insure that the constant error does not exceed one part in 30,000 and that the accidental errors are not greater than that represented by a probable error of one part in 100,000 in the length of the base. No difficulty will be encountered in keeping both classes of errors within these limits, even if the measurement is over very rough ground and steep slopes, provided that the vertical meas- urements on steep slopes are made with sufficient accuracy, that two measurements are made of each section of the base with 50-meter steel or invar tapes, and that the tapes have been properly standard- ized. 16. Measurements made with steel tapes in daylight, particularly in sunlight, require special precautions to avoid constant, errors in determination of the temperature by mercurial thermometers. These errors may exceed 3° C. and produce a constant error in length as great as one part in 30,000; therefore preference should be given to overcast days when practicable, or to measures at night. Bases will in general be measured by steel or invar tapes suspended from stakes. The tape lengths may be marked on the tops of the stakes by marks on copper strips or zinc plates, or by pins; a 50- meter tape should be supported at 25-meter intervals, the support (conveniently a nail in the side of a stake) being in line vertically between the end supports. "When the topography is such as to require high end supports, the intermediate support may be placed above (never below) the grade of the end supports and each half of the tape corrected for grade. Two thermometers should be used fastened to the tape toward either end. The tape should be carried clear of ground. Two measurements of a base should usually suffice. Set backs and set forwards may be made with quarter-meter scale and dividers, and where the lengths run off the stakes, with a pocket tape having proper subdivisions. A tension of 15 kilograms should be applied with a spring balance attached to the forward end of the 10 COAST AND GEODETIC STJBVEY. tape, using a light bar as a lever and a strip of signal muslin as a brealiing link to insure that no excessive strain is ever applied to the tape. 17. The spring balance used in the base measurement should be verified before and after the measurement by comparison with a spare spring balance. 18. The base measurements may be recorded in " Horizontal angle "' record books by changing the headings. A duplicate of the base measurement record should be made on computing paper, and at- tached to the List of Directions. Each record of base measurement should be preceded by a description of the measurement showing what tapes were used, their lengths and coefficients of expansion and method of support, the tension applied, how the thermometers were used, and all information necessary to a clear understanding of the. measurement. 19. Form for record of base measurement: Thermome- From stake No. To stake No. Set backs. Set forwards. 2183. 2184. NE. base 176 S.O 4.9 17(1 175 4.7 5.0 REMARKS. Time of beginning : 91i. 5m. p. ui. Began at center of N. E. base. Backward measure. Fifteen liilograms tension applied witb spring balance Xo. 170. Tape No. 40.3 on tbree supports. E.i-iunple of computation of length of base. Standardization formula for tape No. 40.3, supported at ends and in middle, witb a tension of 15 kilograms: to 50 m. = 50 m. + S.32 mm. + 0.568 mm. (* — 14°.56 O.) ± .039 mm. ± .003 mm. Twenty tape leugtbs : mean corrected temperature 15°.5S C. 20 (50 m. +8.32 mm.) = 20 (50.00832)= 1000.1064 Temperature correction : (20 (15..").s — 14.56) (0. 568 mm.) =_+ . 0116 Set forwards, sum + .0133 Set backs, sum — . 0060 Inclination corrections (see table, paragraph 76) sum — . 22S3 Length of base !)!)!). i)570 " To be entered on rigbt-band page. GENERAL INSTRUCTIONS FOR FIELD WORK. 11 20. In case the standard length is given for a tape supported throughout its length, the catenary correction G is to be applied to each span of tape, as follows : Where TF= weight in grams per meter of tape (21.6 grams for steel tapes in use). ^=horizontal tension in grams=15,000 grams in general, Z=length of span in meters. 21. Horizontal angle observations — Standard of accuracy. — In select- ing the instrument to be used, the methods of observation, the number of observations, the signals to be used, and the conditions under which to observe, proceed upon the assumption that the maximum speed and minimum cost consistent with the requirement that the closing error of a single triangle in the main scheme shall seldom exceed 15 seconds, and that the average closing error shall be between 4 and .5 seconds is what is required. The observations connecting supple- mentary stations with the main scheme should be of this same degree' of accuracy. This standard of accuracy, used in connection with other portions of these instructions defining the necessary strength of figures, frequency of bases, and accuracy of base measurements, will in general insure that the probable error of any base line (or line of primary or secondary triangulation used as a base) , as computed from an adjacent base (or triangulation line used as a base), is about one part in 20,000, and that the actual discrepancy between such bases is always less than one part in 5,000. 22. Selection of instrument. — Either a direction or a repeating in- strument may be used in triangulation of this class. In selecting the size of instrument to be used two o^Dposing factors must be taken into account. The smaller and lighter the instrument and the less it is protected from sun and wind, the less cost and the inconvenience of triangulation. On the other hand, the larger and better the instru- ment the more fully it is protected from the sun and wind, and the more stable the support provided for it the smaller will be the num- ber of observations necessary to secure the required degree of accuracy and the shorter will be the observing period at the station. 23. Observations in the main scheme with a direction instrument. — An 8-inch direction instrument (No. 140, for example) used on its tripod and protected from sun and wind simply by an umbrella will give the required accuracy with two measurements, a direct and re- verse reading being considered one measurement. Any two positions of the circle may be used with this instrument for which the settings on the initial signal differ by approximately 90° 05', The backward 12 COAST AND GEODETIC SURVEY. (additive) reading of the micrometer only should be taken in each position of each microscope. At least once a month, as a test for run, a few special readings both backward and forward should be taken on various graduations of the circle to determine the run of each microm- eter and placed in the record. If the average value of the run for either micrometer is found to be greater than two divisions (four seconds), the micrometer should be adjusted for run. Under these conditions and with the specified positions of the circle the run will be eliminated from the results with sufficient accuracy by the process of taking means. For any other direction instrument the system of positions to be used may be selected with reference to the number of measurements found to be necessary. With any direction instru- ment when a broken series is observed the missing signals are to be observed later in connection with the chosen initial, or with some other one, and only one, of the signals already observed in that series. With this system of observing no local adjustment is necessary. Little time should be spent in waiting for a doubtful signal to show. If it is not showing within say one minute when wanted, pass to the next. A saving of time results from observing many or all of the signals in each series, provided there are no long waits for signals to show, but not otherwise. Example of record is given below. 24. Horizontal directions. — station : Gunton. Obsorver : Date: April 17, 1902. Instrument : 8-ineh tlieodolite No. 140. Po.si- tlon. Objects observed. Time. Tel. D. or R. o Angle. Back- ward. a Mean D. and R. Direc- Re- tion. marks. h. m. o , „ „ I Benvenue ■2 66 D A B CO 07.0 04.5 oil. 5 Benvenue 3 06 R A B 1,'0 00 05.0 11.0 10. 13.8 00.0 Wliite stone Point D A -1.5 40 12.0 B 07.0 19.0 "White Stone Point R A 2Li5 40 08.5 1 ! B 16.5 25.0 22. 08.2 Stevenson D A B 76 35 26.5 23.0 49.6 Stevenson R A B 256 35 24.0 29.5 53.5 61.5 37.7 Gut D A B 87 05 25.5 24.0 50.0 Gut R A 267 05 23.5 B 26.0 49.5 49.8 86.0 « Eacli division of the micrometer corresponds to 2" of arc and therefore the tor this instrument is the sum of the two readings. GENERAL INSTRUCTIONS FOE FIELD WORK. 13 Posi- tion. Objects observed. Time. Tel. D. or R, Angle. Back- ward. i 3 Mean D. and R. Direc- tion. Re- marks. II Benvenue 3 10 R A B / 270 05 03.0 09.5 12.6 " " Benvenue 3 15 D A B 90 05 06.0 05.6 11.6 12.0 co.o White Stone Point R A B 316 45 05.0 06.0 11.0 White Stone Point D A B 136 45 06.0 07.0 13.0 12.0 00.0 Stevenson R A B 346 40 24.0 22.0 46.0 Stevenson D A B 166 40 20.5 23.0 43.5 44.8 32.8 Gut R A B 8.67 10 22.0 21.6 43.5 Gut D A B 177 10 18.0 22.0 40.0 41.8 29.8 25. Observations in the main scheme with a repeating instrument. — A 7-inch Berger repeating theodolite used on its own tripod and pro- tected from sun and wind by an umbrella will give the required accu- racy with from two to four sets of six repetitions each on each angle. This is the type of repeating theodolite recommended for tertiary triangulation in any region in which the convenience of transporta- tion of the instrument is an important consideration. With any re- peating theodolite make the observations in sets of six repetitions each, unless two sets of three repetitions each will give the desired accuracy. For each angle measured follow each set of repetitions upon an angle with the telescope in the direct position immediately by a similar set on the expleinent of the angle with the telescope in the reverse position. It is not necessary to reverse the telescope dur- ing any set of repetitions. If a theodolite of the type recommended above is used, the minimum number of sets of six repetitions on each angle shall be two, one directly on the angle and one on its exple- meiit, and the maximum number shall be four, two each on the angle and its explement, unless it shall be found that under the particular conditions encountered a larger number of sets is necessary to secure the required degree of accuracy. With any repeating theodolite measure only the single angles between adjacent lines of the main scheme and the angle necessary to close the horizon. In the compara- tively rare case in which the failure of adjacent signals to show at the same time prevents carrying out this programme make as near an approach to it as possible and then takt the remaining signals in another series together with some one, and only one, of the signals 14 COAST AND GEODETIC SURVEY. observed in the first series, and measure in the new series only the single angles between adjacent signals and the angle necessary to close the horizon. With this scheme of observing no local adjustment is necessary, except to distribute each horizon closure uniformly among the angles measured in that series. If the region is one in which there is no great inconvenience in transporting a heavier in- strument, and a 10-inch Gambey repeating theodolite or an equiva- lent instrument is used on triangulation of this class, it will probably be found that tM'o sets of three repetitions each, one directly on the angle and one on its explement, will be sufficient to secure the re- quired accuracy. 26. Example of record is given below. From this the resulting directions should be written in the List of Directions (Form 24A) without any other abstract. In this " Example of record " it is sug- gested that the usual practice of reading one repetition on the first measurement of each angle be changed to reading three repetitions in each case. By the suggested practice a value of the angle correct to within 10 inches is secured, which will check the reading of the min- utes on the repetitions. The reading of one repetition on one vernier does not give a sufficiently accurate check. Use the reading on three repetitions as check only. 27. Horizontal angles. — station : Dab. Island ; Luzon. Observer : Date : February 7, 1906. Instrument : B. & B. 7-inch theodolite No. 134. Objects ob- served. Time h. m. Tel. D. orR. 33 Angle. A. .B. Mean of Ver- niers. Arc passed over. Angle, mean D. and R. a. m. o ■ " " " o ' " „ „ „ Pet - Dog 8.00 3 •266 00 55 00 20 00 20 do.o D 6 173 .58 40 40 40.0 40.0 88 59 46.7 (Dog - Pet) E 6 00 10 20 15.0 25.0 44.2 46.5-0.7 44.8 Dog - Bat 3 127 30 30 40 E 6 2.5') 01 10 20 15.0 00.0 42 30 10.0 D 6 00 20 20 20.0 55.0 09.2 09.6-0.7 08.9 Bat - Kow 3 82 4J 20 30 D 6 165 26 30 40 35.0 15.0 27 34 22.5 E 6 GO 00 10 05.0 30.0 25.0 23. 7-0. 8 22. 9 Kow - Bol 3 113 02 10 20 R 6 226 04 20 30 25.0 20.0 37 40 43.3 D 6 00 10 20 15.0 10.0 41.7 42.5-0.8 41.7 Bol - Pet 3 129 16 20 20 D 6 259 30 30 30 30.0 15.0 163 15 02.6 R 6 00 10 20 15.0 15.0 j 00 02.5 02.6-0.8 01,7 360 03. 8 00. GENEKAL INSTKUCTIONS FOB FIELD WOKK. 15 28. Observations on intersection stations. — ^An intersection station is a station of which the position is determined by intersections from stations of the main scheme, or supplementary stations, and which is not occupied. The direction method of observation should be used in observations upon intersection stations even if the theodolite is a repeater. Each series of observations on intersection stations should contain some one line, and only one, of the main scheme (or a line used in fixing the position of a supplementary station) . Such a series of observations should commence with the selected line of the main scheme, with" the telescope in the direct position, and with the circle reading approximately zero. The intersection stations should then be observed in order of azimuth and the first half of the series closed with a reading on the line of the main scheme. The telescope should then be reversed and the same process repeated in the reverse order, beginning and ending as before with the selected line of the main scheme. One such observation is all that is required on each intersec- tion station. It is important to observe at least three lines to each intersection station in order to secure a check, but a possible inter- section station should not be neglected simply because only two lines to it can be secured. Example of record is given below. From this the resulting direc- tions should be written in the List of Directions (Form 24A) with- out other abstract. 29. Horizontal angles.- Station : Rat. Island : Negros. Observer : Date : January 10, 1902. Instrument : 7-lnch theodolite No. 157. Objects observed. Time. Tel. D.or . R. Angle. A. B. Mean of ver- niers. Cor- rec- tion. Direction. Re- marks. a. in-. o , " // // " o / // Tree 9.60 D 00 00 50 -f- 5 R 180 00 10 00 00 00 00 Bell tower, Dion D 21 18 so 20 R 201 18 40 30 30 21 18 35 on D R 176 13 10 30 20 -HO 176 13 30 Frog D 209 59 60 40 E 29 59 60 60 60 -t- 5 209 69 56 L. tang. Padang Id. D 232 18 * R 62 18 232 18 Peak 17 D 241 11 00 40 R 61 10 60 40 48 -1- 5 241 10 63 Tree D 359 59 50 40 10.32 R 179 69 60 50 .50 00 00 30. Observations on indefinite and temporary objects. — Observations may sometimes be made upon indefinite objects, as, for example, mountain or hill tops which are comparatively flat or wooded or upon 16 COAST AND GEODETIC SURVEY. points Avhich are temporary in nature and not marked upon the ground, such as flags in trees, etc., which are to be used as hydro- graphic and topographic signals. The direction method of observa- tion shall be used in all such cases, even if the instrument is a repeater. A single pointing is all that is necessary in such cases, though a second pointing after reversing" the telescope may be taken if it is considered desirable as a check. The method of observation should be that out- lined in the preceding paragraph for observations on intersection stations. Each pointing upon an indefinite object should be clearly marked " indefinite," and all topographic and hydrographic points not permanently marked upon the ground should be marked in the record with a circle and a dot. For observations of these two classes a graphic treatment will frequently be all that is desired, and hence the necessity of distinguishing them from other points, for which a complete computation must be made. In selecting indefinite points to be observed it should be borne in mind that an approximate deter- mination of the i^osition of a prominent mountain or hill too far inland to be included in the fringe of topography along the coast or an island too far out from the coast is frequently of considerable geographic value. 31. Hills or mountains determined, whether indefinite or not, must have their identity established in the record either by means of sketches, estimated distances, or descriptive notes, so that they may be plotted or computed without likelihood of confusion. Any charac- teristic features of hills, mountains, church spires, or other objects which would render them good landmarks must be clearly noted, as such notes are valuable in chart construction and in hydrographic Avork. The location of prominent objects and the determination of elevations of mountains are to be considered an important part of the work essential to the completeness of the survey. If needed for the improvement of the charts, important mountains and other objects are to be determined even when lievond the limits of the area to be surveyed. In the case of imperfectly known regions, tangents to points and islands outside of region to be surveyed should be taken. 32. Value of intersection stations. — In selecting intersection stations it should be kept in mind that the geographic value of a triangulation depends upon the number of points determined, the size of the area over which they are distributed, and the permanence with which they are marked. The geographic value of a triangulation is lost for a given area when stations can not be recovered within that area. The chance of permanency is increased by increasing the number of sta- tions as well as Ijy thorough marking. These and other considera- tions should lead to the determination as intersection stations of many artificial objects of a permanent character, such as light-houses, church spires, cupolas, towers, large chimneys, well-defined natural GENERAL INSTRUCTIONS FOR FIELD WORK. 17 objects, as sharp mountain peaks, prominent pinnacle rocks along the coast, and others easily identified and likely to be of value; should lead occasionally to the determination of specially marked stations es- tablished for this particular purpose, and should frequently lead to the permanent marking upon the ground of topographic or hydro- graphic stations and their determination as intersection stations. The practice of permanently marking such hydrographic points as are in commanding positions, on promontories, for example, and which are so situated that the station is not likely to disappear if permanently marked (on firm ground not likely to be washed away or on rocks), and determining their positions as intersection stations will fre- quently obviate the necessity which would otherwise exist for new triangulation when a later hydrographic survey is made. It is especially desirable to increase the area effectively covered for geo- graphic purposes by selecting intersection stations which are outside the area covered by the main scheme. 33. Marking stations. — Every station, whether it is in the main scheme or is a supplementary or intersection station, which is not in itself a permanent mark, as are lighthouses, church spires, cupolas, towers, large chimneys, sharp peaks, etc., shall be marked in a perma- nent manner. 34. The marking of stations by simple drill holes in rock is not sufficient, for the reason that after a time such holes are not distin- guishable from natural marks, especially in coral rock. Every drill hole should have a distinctive mark around it, as, for instance, a triangle or cross or arrows cut pointing to it. 35. Permanent reference marks, usually three, should be estab- lished at each marked station, and should be referred to it accurately by distance (tape measure) and true direction (theodolite angle), so that the position could be recovered from any one of these refer- ence marks. At least one reference mark of a permanent character shall be established not less than 10 meters from each station of the main scheme, and be accurately referred to it- by distance and azimuth, the object being to have at least one reference mark in a position such that it is not likely to be destroyed by any occur- rence that may destroy the station mark itself. Reference marks shall be of a different character from the center mark and shall preferably be established on fence or property lines and always in a locality chosen to avoid disturbance by cultivation, erosion, and building. At all stations at which digging is feasible it is desirable to establish both underground and surface reference marks which are not in contact with each other. Wood is not to be used in permanent marks. Metal is to be used with considerable caution wherever it might excite cupidity, as permanence rather than prominence is desired. 53695—08 2 18 ' COAST AND GEODETIC SURVEY. 36. For marking stations, cement (mixed with sand), drain tile (to be filled with cement), and glass bottles (for vinderground marks), are useful materials; the center mark may be embedded in the cement. 37. When former stations are recovered that are found to be in- sufficiently marked or described, or the marks partially effaced, special care will be taken to permanently re-mark the station, sending to the office a special report with an amended description. 38. Description of stations. — Descriptions shall be furnished of all marked stations. For each station which is in itself a mark, as are lighthouses, church spires, cupolas, towers, large chimneys, objects valuable for future hydrographic signals, sharp peaks, etc., either a description must be furnished or the records, list of directions, and lists of positions must be made to show clearly in connection with each point by special words or phrases, if necessary, the exact point of the structure or object to which the horizontal and vertical meas- ures refer. Every land section corner connected with the triangula- tion must be fully described. The purpose of the description is to enable one who is unfamiliar with the locality to find the exact point determined as the station and to know positively that he has found it. Nothing should be put into the description that does not serve this lourpose. A sketch should accompanjr the description, but should not be used as a substitute for words. All essential facts which can be stated in words should be so stated, even though they are also shown in the sketch. 39. In the case of light-houses and lights, the height of the light above the base of the structure and above mean sea level should be given in the description. 40. Volumes of descriptions of stations should in general have the description written on left-hand page, and the sketch on the right- hand page facing it, and should have an alphabetically arranged index on the first jsages. 41. The description should give the following in a , concise and systematic manner : Locality (general and particular). HoLc T)est reached. * Ho w marked. Distances (tape) and directions (theodolite) from center to refer- ence marks, and directions or magnetic bearings to prominent objects in the vicinity, tangents to points, islands, etc. 42. The height abo^e the station mark of the top of the signal pole, and of any other part of the signal likely to be used in observing vertical angles, should be measured and stated in the " Description of stations " in meters and centimeters. GENERAL INSTRUCTIONS FOR FIELD WORK. 19 43. The sketch should be drawn approximately to scale and with the north point toward the top of the page, and a meridian drawn upon the sketch. 44. It should show, if possible, all objects referred to, or directions to them when the distance is too great for them to appear on the sketch. The topography of the sketch should be drawn in black, directions and distances in red. 45. Land section corners and other survey marks. — Whenever it is feasible to do so without incuring undue expense the section corners established by the Land Survey, and survey marks of any kind found upon the ground, including provincial boundary monuments, shall be connected with the triangulation, either by direct measurement of a distance and direction from a triangulation station or by using them as intersection stations. 46. It will insure the permanence of a station if it is related by direct measures or otherwise to neighboring cadastral features, and a station located close to a line fence is less liable to disturbance than one situated out in an open field. 47. Poor seeing. — Observations either in the main scheme or on intersection stations in triangulation of this class may be taken under any atmospheric conditions when the object to be pointed upon is visible and no delay is to be made to secure good seeing before ob- serving. If the seeing is very poor it may be necessary to increase the number of observations on angles in the main scheme in order to secure the required accuracy. The decision in regard to the necessity of each increase should be based upon the triangle closures which are secured with such poor seeing rather than upon the appearance of the signals or even upon the range of the observations. 48. Field computations. — The field computations for the main scheme and supplementary stations are to be carried to tenths of sec- onds in the angles and azimuths, to hundredths of seconds in the lati- tudes and longitudes, and to six places in the longarithms. The field computations for intersection stations, for indefinite objects and all vertical measures should be carried out to a sufficient number of deci- mal places to give two uncertain figures in each result. The computa- tion of the horizontal measurements to and including the lists of directions for all stations and objects and the computation of the triangle sides of the main scheme should be kept up as closely as pos- sible as the work progresses, to enable the observer to know that the observations are of the required degree of accuracy and completeness. No least square adjustments are to be made in the field. All of the computation, taking of means, etc., which is made in the record books and in the lists of directions, should be so thoroughly checked by some person, other than the one by whom it was originally done, as to ren- der an examination in the office unnecessary. The initials of the per- 20 COAST AND GEODETIC SUKVEY. sons making and checking the computations in the record books and the lists of directions should be signed to the record as the computation and checking progresses. Pointings upon indefinite objects should be carefully examined, graphically or otherwise, the objects identified, and the identification clearly indicated in the records and computa- tions. It is important to indicate clearly what lines are to objects on which no pointing- was secured from a second station, as well as to indicate by common names or symbols which lines are to the same object. This must be done as the field work progresses. 49. In laying out the triangle side computation, the names of the stations should be written in the triangle in a clockwise direction, and the order of triangles should be such as to give two or more results for the side to be used as a base for going ahead. All triangles upon any one point should follow in consecutive order. For a system of quadrilaterals the triangles in the fig- ure 1, with A B as a base and C D the line for going ahead, would be written as follows, starting with C B A, the strongest triangle : C B A, D B A, D B C, D A C. 50. The mean of the several results of the length of any line is to be used as a base for going ahead and in the position computation, except in figures in which R^ is very much smaller than E, and in such cases, the result corresponding to Rj, i. e., through the two strongest triangles is to be used for going ahead and in the position computation. 51. In the position computation, the position of any point should be computed only from the two lines in any one selected triangle, using the angles in that triangle and the mean length of sides. In any triangle, C B A, the line from B is computed on the left page of the form and from A on the right page. With the triangle side and position computation written as above (from left to right) the angles at B and A are always, respectively, -|- and — , and no sketch is necessary to write up the position computation. The factors for the position computation in the latitudes from 1° to 17° are given in Appendix No. 4, Report of 1901, and in the latitudes 18° to 72° in Appendix No. 9, Report for 1894. 52. Where connection is made with a new base the new length is to be used in going ahead. 53. Beduction to center of observations at eccentric station. — Use Form 382, on the back of which may be found the directions neces- sary for computing. A sketch showing relative position of the center and eccentric station with direction to one or more stations must be entered in the record. GENERAL INSTRUCTIONS FOR FIELD WORK. 21 54. Spherical excess. — The spherical excess or ratio of the area of the triangle to the area of the sphere becomes appreciable only when the sides are from 4 to 5 miles in length. One-third of the computed excess is deducted from each angle of the triangle, and the difference between the resulting sum of the angles and 180° is the error to be distributed. The formula for the spherical excess E is E^^ah sin C m, in which a, h are the triangle sides and G the included angle. The values of m are tabulated for every 30' of latitude, and are printed in Appendix No. 9, Report for 1894, page 291. 55. Triangulation records. — Do not duplicate volumes of horizontal angles. Do not make an abstract of angles. Make a complete list of directions on Form 24A, in accordance withrthe instructions on the back of that form. The local adjustment corrections (to close horizon only) are to be written in the Horizontal Angle Record, and the List of Directions is to be made from that record directly. 56. Base, azimuth, and vertical angle observations may be recorded in Horizontal Angle Record. Base and azimuth observations are to be duplicated on computing paper and attached to List of Directions. 57. Descriptions of stations, including sketches, are to be completely duplicated in similar volumes. 58. All records of observations should contain an alphabetical in- dex of stations occupied. When stations are occupied more than once, each record should have a cross reference to page and volume. Records should also contain a preface giving briefly number, make, and size of instrument, with direction and manner of graduation and method of observation, and any information necessary to a complete understanding of the record. 22 COAST AND GEODETIC SURVEY. 59. Table for determining relative strength of figures in triangulation.- lOo 12° 14° 16° 18° 20° 22° 24° 26° 28° 30° 36° 40° 46° 60° 55° 60° 66° 70° 75° so° 86° 90° 10 428 35S ^ 12 369 296 263 14 315 253 214 187 16 284 226 187 162 143 IS 262 204 168 143 126 113 20 246 189 163 130 113 100 91 22 232 177 142 119 103 91 81 74 24 221 167 134 111 96 83 74 67 61 26 213 160 126 104 89 77 08 61 56 61 2S 206 153 120 99 83 72 T53 57 61 47 43 30 199 148 115 94 79 68 59 53 48 43 40 33 35 L^8 137 100 85 71 60 52 46 41 37 33 27 23 40 179 129 99 79 65 64 47 41 36 32 29 23 19 16 45 172 124 93 74 60 60 43 37 32 28 25 20 16 13 11 50 1()7 119 89 70 67 47 39 34 29 26 23 18 14 11 9 8 55 162 115 86 07 64 44 37 32 27 24 21 16 12 10 8 7 5 60 159 112 83 64 61 42 36 30 25 22 19 14 11 9 7 5 4 4' 66 156 109 80 62 49 40 33 28 24 21 18 13 10 7 ^e 6 4 3 2 70 162 106 78 60 48 38 32 27 23 19 17 12 9 7 5 4 3 2 2 1 75 150 104 70 68 46 37 30 25 21 18 16 11 8 6 4 3 2 2 1 1 1 80 147 102 74 67 46 30 29 24 20 17 16 10 7 5 4 3 2 1 1 86 145 100 73 65 43 34 28 23 19 16 14 10 7 5 3 2 2 1 90 143 98 71 64 42 33 27 22 19 16 13 9 6 4 3 2 1 95 140 96 70 63 41 32 26 22 18 15 13 9 6 4 3 2 100 138 95 68 51 40 31 25 21 17 14 12 8 6 4 3 2 1; 105 136 93 67 60 39 30 25 20 17 14 12 8 5 4 2 2 1' 110 134 91 66 49 38 30 24 19 16 13 11 7 6 3 2 2 l' 1 115 132 89 04 48 37 29 23 19 16 13 11 7 6 3 2 2 1 120 129 88 62 46 36 28 22 18 15 U 10 7 5 3 2 1 125 127 SB 61 45 35 27 22 18 14 12 10 7 5 4 3 2 130 125 84 59 44 34 20 21 17 14 12 10 7 5 ' 3 ' 136 122 82 58 43 33 26 21 17 14 12 10 7 6 4 140 119 80 56 42 32 26 20 17 14 i; 10 8 6 145 116 77 55 41 32 25 21 17 16 13 11 9 150 112 76 54 40 32 26 21 18 16 16 13 152 111 75 53 40 32 26 22 19 17 16 154 110 74 53 41 33 27 23 21 19 ' 1S6 108 74 54 42 34 28 26 22 168 107 74 64 43 36 30 27 160 107 74 66 46 38 33 162 107 76 69 48 42 164 109 79 63 64 166 113 86 71 JOS 122 98 170 143 i 60. In the table on the preceding page the values tabulated are f^\+jt -2 [c^^+'^A'^B+'^y in which d is the probable error of an observed direction. 'Sd is the number of directions observed in a figure, and Nc is the number of conditions to be satisfied in the figure. (See Wright's Adjustments of Observa- tions, second edition, pp. 237-238.) The summation indicated by 2 is to be taken for the triangles used in computing the value of the side in question from the side supposed to be absolutely known. 62. In the above formula the two terms t^t and 2 [(y\+ <^a<^b+<^^b] depend entirely upon the figures chosen and are inde- pendent of the accuracy with which the angles are measured. The product of these two terms is therefore a measure of the strength of the figTire with respect to length, in so far as the strength depends upon the selections of stations and of lines to be observed over. The strength table is therefore to be used, in connection with the values of — ^-"7 — — given hereafter, to decide during the progress of the reconnaissance which of the two or more possible figures is the strongest, and to determine whether a suiRciently strong scheme has been obtained to make it inadvisable to spend more time in recon- naissance. 63. To compare two alternative figures, either quadrilaterals or central point figures for example, with each other in so far as the strength with which the length is carried is concerned, proceed as follows : 1. For each figure take out the distance angles, to the nearest de- gree if possible, for the best and second best chains of triangles through the figure. These chains are to be selected at first by estima- tion, and the estimate to be checked later by the results of comparison. 2. For each triangle in each chain enter the table with the distance angles as the two arguments and take out the tabular value. 3. For each chain, the best and second best, through each figure, take the sum of the tabular values. 4. Multiply each sum by the factor ^^-^ for that figure. The quantity so obtained, namely, — ^-j -^'[(y^+cy^tyj+dy , will for con- venience be called Rj and Ea for the best and second best chains, respectively. ■ 24 COAST AND GEODETIC SURVEY. 6. The strength of the figure is dependent mainly upon the strength of the best chain through it, hence the smaller is Ri, the greater is the strength of the figure. The second best chain contributes some- what to the total strength, and the other weaker and progressively- less independent chains contribute still smaller amounts. In deciding between figures they shovild be classed according to their best chains, unless said best chains are verj;^ nearly of equal strength and the . second best chains differ greatly in the two cases. ]sr^ No 64. Some values of the quantity — j^ — . The starting line is supposed to be completely fixed. For a single triangle, —7 — =0.75. 10 — 4 For a completed quadrilateral, — 5-7^— ^0.60. For a quadrilateral with one station on the fixed line, unoccupied, 8—2 ^g-=0.75. P'or a quadrilateral with one station not on the fixed line, unoccu- 7 2 pied, -y— =0.71. 10 — 4 For a three-sided, central point figure, — r-x— =0.60. For a three-sided, central point figure with one station on the fixed '] 8 — 2 line, unoccupied, — 5— =0.75. For a three-sided, central point figure with one station not on the 7 2 fixed line, unoccupied, — ^ — =0.71. ]^4 5 For a four-sided, central point figure, — ^j^^O.64. For a four-sided, central point figure with one corner station on the 12 — 3 fixed Ime, unoccupied, — t^ — =0.75. For a four-sided, central point figure with one corner station not J J 3 on the fixed line, unoccupied, — t^j — =0.73. For a four-sided, central point figure with the central station not 10 2 on the fixed line, unoccujjied, — Yrf =0.80. 10 g For a five-sided, central point figure, -yo^ =0.67. For a five-sided, central point figure with a station on a fixed out- •-, T ■ n 16—4 side line, unoccupied, — j^^ — =0.75. GENERAL INSTRUCTIONS FOR FIELD WORK. 25 For a five-sided, central point figure with an outside station not on 1 K the fixed line, unoccupied, — p— =0.73. For a five-sided, central point figure with the central station not on the fixed line, unoccupied, - -.„ =0.85. 02 7 For a six-sided, central point figure, '' ^^ =0.68. For a six-sided, central point figure with one outside station on the 20 5 fixed line, unoccupied, ^^ =0.75. For a six-sided, central point figure with one outside station not on the fixed line, unoccupied, .„ =0.74. For a six-sided, central point figure with the central station not on 1(5 2 the fixed line, unoccupied, ..„ =0.88. For a four-sided, central point figure with one diagonal also -I f* y observed, -^ , — =0.56 For a four-sided, central point figure with one diagonal also observed, with the central station not on the fixed line, unoccupied, -^2-=0.67. 65. Examples of various triangulation figures, — The following foiir- teen figures are given to illustrate some of the principles involved in the selection of the strong figures and to illustrate the use of the Strength Table: 66. In every figure the line which is supposed to be fixed iii length, and the line of which the length is required, are heavy. Either of these two heavy lines may be considered to be the fixed line and the other the required line. Opposite each figure R^ and R^,, as given by the Strength Table, are shown. The smaller the value of R^ the greater the strength of the figure. R, need not be considered in com- paring two figures unless the two values of R^ are equal, or nearly so. 67. Compare figs. 2, 3, and 4. Fig. 2 is a square quadrilateral; fig. 3 is a rectangular quadrilateral, which is one-half as long in the direction of progress as it is wide; fig. 4 is a rectangular quadrilat- eral twice as long in the direction of jlrogress as it is wide. The comparison of the values of R^ in figs. 2 and 3 shows that shortening a rectangular quadrilateral in the direction of progress increases its strength. A comparison of figs. 2 and 4 shows that extending a rec- tangular quadrilateral in the direction of progress Aveakens it. 68. Fig. 5, like fig. 3, is short in the direction of progress. Such short quadrilaterals are in general very strong, even though badly dis- 26 COAST AKD GEODETIC STJEVEY. torted from the rectangular shape, but they are not economical in progression of work. 69. Fig. fi is badly distorted from a rectangular shape, but is still a moderately strong figure. The best pair of triangles for carrying the length through this figure are D S R and R S P. As a rule, one diagonal of the quadrilateral is common to the two triangles forming the best pair, and the other diagonal is common to the second best pair. In the unusual case illustrated in fig. 6 a side line of the quadrilateral is common to the second best pair of triangles. 70. Fig T is an example of a quadrilateral so much elongated, and therefore so weak, as not to be allowable in any class of triangulation. 71. Fig. 8 is the regular three-sided, central-point figure. It is extremely strong. 72. Fig. 9 is the regular four-sided, central-point figure. It is very much weaker than fig. 2, the corresponding quadrilateral. 73. Fig. 10 is the regular, five-sided, central-point figure. Note that it is much weaker than any of the quadrilaterals shown in figs. 2, 3, or 5. 74. Fig. 1] is a good example of a strong, quick expansion from a base. The expansion is in the ratio of 3 to 2. 75. Figs. 12 to 15 are given as a suggestion of the manner in which, in secondary and tertiary triangulation, a point (A), difficult or im- possible to occupy, may be used as a concluded point common to sev- eral figures. Figs. 13, 14, and 15 are all very strong figures, even though the occupied points are nearly in one straight line. ? — — 5 K /" Fig. 2. R, = 5 Same, any one station E, > < R2 = 5 not occupied. E^ / s l*L ==lo Same, any one outside Ri = ll station not occupied. Ra = 16 Same, central station Ri -^ 13 not occupied. Eg = 19 Fig. 11. El = 5 Ka-N,. 28-16 Nj ■ 28 ° GBNEKAIi INSTKUCTIONS FOB FIELD WORK. 29 '^ W riG. 12. El = 36 Unoccupied station >0 ^ Ej = 102 not on fixed line. Ri= 4 E(i = 20 Rl = 2 B2= 30 COAST AND GEODETIC SUBVEY. 76. Inclination correction tables for 25-meter tape lengths.— (For 50-nieter spans take one-half of the correction given in the table.) Correction for inclination = — (I — ■<,/ 1- — K') where I is inclined distance and h is difference of height. For I = 25 meters and h in feet, correction for inclination — — .00186 K' — .000000069 h*— . . The second term may be neglected for differences of height of 5 feet or less. Diflerence of elevation. Correc- tion. Difference of elevation. Correc- tion. Meters. Difference of elevation. Correc- tion. Difference of elevation. Correc- tion. Feet. Meters. Feet. Feet. Meters. Feet. Meters. 0.00 .0000 1.11 .0023 1.57 .0046 2.03 .0077 .16 .12 23 .58 46 .04 77 .17 1 .13 24 .69 47 .05 78 .28 1 .14 24 .60 48 .06 79 .29 2 .15 26 .61 48 .07 80 .36 2 .16 26 .62 49 .08 80 .37 3 .17 26 .63 49 .09 81 .43 3 .18 26 .64 60 .10 82 .44 4 .19 26 .65 51 .11 83 .49 4 .20 27 .66 51 .12 84 .50 5 .21 27 .67 52 .13 84 .64 6 .22 28 .68 62 .14 86 .56 6 .23 28 .69 53 .15 86 .69 6 .24 29 .70 54 .16 87 .60 7 .26 29 .71 54 .17 88 .63 7 .26 29 .72 55 .18 88 .64 8 .27 30 .73 56 .19 89 .67 8 .28 30 .74 66 .20 90 .68 9 .29 31 .75 67 .21 91 .71 9 .30 31 .76 68 .22 92 .72 .0010 .31 32 .77 68 .23 92 .75 10 .32 32 .78 59 .24 93 .76 11 .33 33 .79 60 .26 94 .78 11 .34 33 .80 60 .26 95 .79 12 .35 34 .81 61 .27 96 .81 12 .36 34 .82 62 .28 97 .82 13 .37 35 .83 62 .29 97 .85 13 .38 35 .84 63 .30 98 .86 14 .39 36 .85 64 .31 99 .88 14 .40 36 .86 64 .32 .0100 .89 16 .41 37 .87 65 .33 01 .91 15 .42 37 .88 66 .34 02 .92 16 .43 38 .89 66 .35 03 .94 16 .44 38 .90 67 .36 04 .96 17 .45 39 .91 68 .37 04 .96 17 .46 40 .92 68 .38 05 .97 18 .47 40 .93 69 .39 06 .99 18 .48 41 .94 70 .40 07 1.00 19 .49 41 .95 71 .41 08 .02 19 .60 42 .96 71 .42 09 .03 20 .61 42 .97 72 .43 10 .05 20 .62 43 .98 73 .44 11 .06 21 .63 44 .99 74 .45 12 .07 21 .64 44 2.00 74 .46 12 .08 22 .55 45 .01 76 .47 13 .10 22 .56 45 .02 76 .48 14 GENEEAL INSTKUCTIONS FOE FIELD WOEK. 31 Diflereoce of elevation. Correc- tion. Difference of elevation. Correc- tion. Difference of elevation. Correc- tion. Difference of elevation. Correc- tion. Feel. Meters. [ Feet. Meters. Feet. Meters. Feet. Meters. 2.49 .0115 2.87 .0153 3.25 .0197 3.63 . 0246 .50 16 .88 64 .26 98 .64 46 .51 17 .89 55 .27 99 .65 48 .52 18 .90 56 .28 .0200 .66 49 .53 19 .91 58 .29 01 .67 50 .54 20 .92 63 .30 03 .68 52 .55 21 .93 60 .31 04 .69 63 .66 22 .94 61 .32 05 .70 55 .57 23 .95 62 .33 06 .71 56 .58 24 .96 63 .34 07 .72 57 .59 25 .97 64 .35 09 .73 69 .60 26 .98 66 .36 10 .74 60 .61 27 .99 66 .37 11 .76 61 .62 28 3.00 67 .38 12 .76 63 .63 29 .01 68 .39 14 .77 64 .64 30 .02 70 .40 16 .78 66 .65 31 .03 71 .41 16 .79 67 .66 32 .04 72 .42 18 .80 69 .67 33 .06 73 .43 19 .81 70 .68 34 .06 74 .44 20 .82 71 .69 35 .07 75 .45 21 .S3 73 .70 36 .08 76 .46 23 .84 74 .71 37 .09 78 .47 24 .85 76 .72 38 .10 79 .48 25 .86 77 .73 39 .11 80 .49 27 .87 79 .74 40 .12 81 .50 28 .88 80 .75 41 .13 82 .51 29 .89 81 .76 42 .14 83 .52 30 .90 83 .77 43 .15 85 .63 32 .91 84 .78 44 .16 86 .54 33 .92 86 .79 45 .17 87 .55 34 .93 87 .80 46 .18 88 .56 36 .94 89 .81 47 .19 89 .57 37 .95 90 .82 48- .20 90 .58 38 .96 92 .83 49 .21 92 .59 40 .97 93 .84 50 .22 93 .60 41 .98 95 .85 51 .23 94 .61 42 .99 96 .86 52 .24 95 .62 44 4.00 98 ELEVATIONS BY VERTICAL ANGLES. 77. Scheme of observations. — In connection with tertiary triangula- tion a complete scheme of vertical angle observations should be carried out, except in the cases stated below. This complete scheme should consist of a continuous series of vertical angle measures through the main scheme of the triangulation, observing over each line over which horizontal angles are observed (the observations over each line to be made in both directions if both ends of the line are occupied) , and of observations of vertical angles upon all supplementary and intersec- tion stations corresponding to the horizontal angles measured upon 32 COAST AND GEODETIC SURVEY. such stations. Connections should be made with elevations accurately determined by precise leveling, wye leveling or tidal observations as frequently as possible. When a triangulation is carried along a coast or tidal stream, at each station near the shore, where it is convenient to do so, a connection shotild be made with mean sea level and the connection recorded. Tide gauge marks should be included where available, but otherwise an observed vertical angle to the water's edge and an approximate distance to the point sighted upon may serve the purpose, with note as to height of tide or the time. If the plane table topography fixes the elevations in the region covered by the triangulation, that part of the observations of vertical angles upon supplementary and intersection stations which would merely ' furnish redeterminations of elevations fixed by the topographic sur- vey may be omitted, but the observations of vertical angles in the main scheme and upon supplementary and intersection points bsyond the limits of the plane table surveys should be made. If all intersec- tion points determined by the horizontal angles are already known to be fixed in elevation by topographic surveys, no observations of vertical angles are "necessary, even in the main scheme, but this case will, it is believed, occur but rarely, excej)t on the Atlantic and Gulf coasts. 78. Plane of reference. — All heights will be referred to mean sea level. 79. Method of observations. — In the main triangulation scheme, two measures, each consisting of one pointing with the telescope in the direct position and one pointing with it in the reversed position, on each day of occupation, is sufficient. For observations on inter- section stations and indefinite objects, one measure consisting of one jDointing in each of the two positions of the telescope is all that is necessary. Such observation should be taken on each intersection station or indefinite object from all the stations from which hori- zontal angles are measured to that station or object. 80. An essential for the accurate measurement of vertical angles, whether in triangulation or for astronomic positions, is that the vertical axis be truly vertical, or that the effect of errors of verti- cality be eliminated by the method of observation. The instrument should of course be leveled and placed in adjustment before begin- ning observations, but thereafter error in verticality of axis will be eliminated by the following system of observing, for each elevation to be determined : 81. System of observations.-— 1. Point on object, bring horizontal thread to position by telescope-clamp slow-motion screw. 2. Bring to the center of the vial the bubble attached to the ver- niers of the vertical circle, by means of the vernier slow-motion screw. 3. Read both verniers. GENERAL INSTEUCTIONS FOR FIELD WORK. 83 4, Turn the instrument 180° in azimuth and transit the telescope. Repeat 1, 2, and 3 in same order. For all important objects, determination of elevation should be obtained from at least three stations. 82. Record. — Observations should be recorded in the usual Double Zenith Distance records except in work where very few stations are occupied for vertical angles, in which case they may be recorded in Horizontal Angle record book, and listed in the table of contents. The actual circle readings are always to be recorded. 83. The D. Z. D. record is arranged for use with repeating vertical circle ; for observations as here proposed the columns headed " Rep's of DZD," Level, C, and D may be left blank. 84. In every case the record must clearly show the height of the instrument in meters and centimeters above the surface mark at the station occupied, and the exact point observed on at each distant sig- nal, with its height above the surface mark ; whenever the entire sig- nal is visible the ground should be observed and so noted. In observ- ing objects other than signals care should be taken to note in the record the exact point in each case, as, for mountain peaks, " ground " or " tops of trees;" for a church, " top of dome " or " top of tower," etc. When tops of trees are observed, the estimated height above ground should be noted in record, and the ground should be observed if practicable. 85. In the case of mountain and hilltops a small sketch showing the relation of the point determined to the outline of the surrounding elevations will be useful in representing the object on the chart, as well as to the observer in identifying the object from another station. 86. Computation of elevations irom observations of zenith distances made in connection with teritary triangulation. — Abstract all zenith distances on Form 29, bringing together all observations upon the same object from a given station, and taking the mean. If the ob- servations are taken on more than one day, give the mean result for each day the same weight, regardless of whether many or few observa- tions were made on that day. 87. In the record book and on Form 29 carry all angles to seconds only. 88. The value in the column headed " Object above station " is zero if the object pointed upon is the final mark for elevation, as, for example, the top of a chimney, top of a spire, etc. 89. Use the column headed " Reduction to line joining stations " only when the observations are reciprocal — that is, are made in both directions over the line in question. The quantity in this column is t — . . an angle of which the value m seconds is — ^ ^^^ y^ , m which s is the horizontal distance between stations. This formula represents, there- 53695—08 3 34 COAST AND GEODETIC SURVEY. fore, a vertical eccentric reduction which is to be applied as a correc- tion to the observed zenith distance to obtain the corrected zenith distance. Four places in the logarithms are all that are necessary in computing these values. 90. If observations are made in one direction only over a line, tlie above vertical eccentric reduction is not needed. Instead, the differ- ence t — 0, expressed in meters, is to be applied as a correction to the computed difference of elevation, as indicated on Form 29B. 91. For reciprocal observations, use Form 29A in computing dif- ferences of elevation and refraction. The necessary starting data is to be transferred from Form 29, t,^ and ^2 being the mean corrected zenith distance at the respective stations of the other station named. Compute by horizontal lines on the form. Carry all angles to seconds only and all logarithms to five decimal places only, except that 2 log s.and 9 — 2 log s should be carried to three decimal places oi\\.j. The quantity h., — A^ and the three terms composing it are each to be car- ried to centimeters only. The logarithm (0.5 — 7n) should be com- puted to four decimal places and (0.5 — m) to three decimal places. The weights, p, should be carried to two significant figures onh'. 92. For tertiary triangulation log tan | (Jj — ^1) may be taken out directly from logarithmic tables and written in the line marked T, and the two lines above may be left blank (those in Form 29 A desig- nated -J- (^2 — ^1) in sees, and log ditto). 93. The complete formula for the computation is: h,-K = s tan i (C2-C,) [l+i|^.+^|^], in which h^ and Aj are the elevations of station 2 and station 1, re- spectively, above mean sea level. ^2 and ^^ have already been defined, and s is the horizontal distance between the stations, reduced to sea level, p is strictly the radius of curvature of the earth's surface in the mean latitude of the stations and in the azimuth of the line ob- served ; but it is sufficiently accurate for all cases in which this for- mula is to be supplied to assume p constant and equal to 6,364,750 meters (log. 6.803781). Wkew the formula is used in connection with the tables referred to in the following paragraph, the unit for all the linear quantities must be in meters. 94. The second and third terms, respectively, may be found by multiplying the first term, s tan I (^2—^1) into the values shown in Tables A and B. Before using Table B it is necessary to derive ap- proximate values, to the nearest hundred meters, of the elevations above mean sea level of the stations concerned. The second and third terms always have the same sign as the first term. The lines marked h„ and h„ on Form 29A, are to be used in deriving by suc- cessive additions and substractions, starting with a station of known GENEKAL INSTRUCTIONS FOR FIELD WORK. 35 elevation, the approximate elevations which are to be considered the final results of the field computation. 95. The portion of Form 29A below the lines A^ and A, is not re- quired in the field computation, being for use in the offtce computation in deriving weights and computing the coefficient of refraction. 96. The resulting elevations will be in meters, and are to be re- duced to feet for use on topographic sheets or charts. The reduction to feet may be made by the conversion tables or by multiplying the elevation in meters by 3.28083. 97. For nonreciprocal observations use Form 29B for computing differences of elevation. The starting data is to be transferred from Form 29, including the quantity t-o. The quantity ^ is the mean ob- served zenith distance of the object sighted, no vertical eccentric reduction being made in the case of the nonreciprocal observations. 98. Carry the angles to seconds only and all logarithms to five decimal places only. The quantity s cot f and those on the next five lines below it and the weighted mean elevations of the stations ob- served are each to be carried to centimeters only. The azimuth, «., and the mean latitude, <^, of the line are needed to the nearest tenth of a degree only. 99. For tertiary triangulation log cot ^ may be taken out directly from logarithmic tables and written in the line marked T, and the three lines above may be left blank (those marked 90° — I, 90° — ^ in sees., and log ditto) . The complete formula for the computation is : I _ , (0.5 - m) s' , (l-w)/cot^C h^ — hi = scoiZ + 1 p in which h^ and A^ are the elevations above mean sea level, respec- tively, of the object sighted and the telescope. The other quantities concerned have already been defined. 100. For field computations the coefficient of refraction m may be assumed to be 0.071 and that log (0.5 — ?n) = 9.63246. As a matter of fact m varies for different regions and conditions, and the best available value in each case will be introduced in the office computa- tion. Log p may be taken from table of logarithms of curvature to the earth's surface. The log (second term) is the sum of log (0.5 — m) and 2 log s, minus log p. 101. The third term may be obtained from Table C (in which the side argument is s cot t, and the top argument is 0.5 — m) , provided s cot t, is not greater than 3,500 meters. For the rare cases in which s cot ^ is greater than 3,500 meters it will be necessary to compute the third term from the formula, and in making this computation the value of p may be assumed constant and equal to 6,364,750 meters (log 6.803781). 36 COAST AND GEODETIC SURVEY. 102. The resulting elevations will be in meters, and are to be re- duced to feet for use on topographic sheets or charts. The reduction to feet may be made by the conversion tables or by multiplying the elevation in meters by 3.28083. 103. The computation of weights at the foot of Form 29B may be omitted in the field computation. 104. Computation of elevations of indefinite objects. — If a zenith dis- tance of an indefinite object is observed, as, for example, of a moun- tain or hilltop whicli is comparatively flat or wooded, and the distances to this object are determined by graphic methods only, the computation of these elevations may be made with sufficient accuracy by using the tables shown on pages 338-341 of Appendix 7, Report for 1905. " The Plane Table Manual," provided the horizontal dis- tances are not greater than 20,000 meters. These tables are also pub- lished in separate form. For greater distances use Form 29B, and iji tlie computation carry the angles to whole seconds only, the loga- rithms to four decimal places only, and the elevations to meters only. 105. Table A.- Values of j^y-^ in units of the fifth decimal place, p being assumed equal to fi,3C4,750 meters. logs s- 12 p2 logs s2 12 p2 logs s2 12 p2 logs s'- 12 pS logs s'- 12 p= 9.8 4.30 0.1 4. .iC 0.3 4.82 0.9 5.08 3.0 5.34 4.32 0.1 4.58 0.3 4. .SI 1.0 5.10 3.3 5.36 10. .S ] 4.34 0.1 4.60 0.3 4.86 1.1 5.12 3.6 5. 38 11. s 4.36 0.1 4.62 0.4 4.88 1.2 5.14 3.9 ,5.40 13.0 4.38 0.1 4.64 0.4 4.90 1.3 5.16 4.3 5. 42 14.2 4.40 0.1 4.66 0.4 4.92 1.4 6.18 4.7 5.44 15.6 4.42 0.1 4.68 0.5 4.94 1.6 5.20 5.2 5.46 17.1 4.44 0.2 4.70 0.5 4.96 1.7 .5.22 5.7 5.48 18.8 4.46 0.2 4.72 0.6 4.98 1.9 5.24 6.2 5. .50 20.6 4.48 0.2 4.74 0.6 .'i. 00 2.1 5.26 6.8 4.50 0.2 4.76 0.7 .•i.02 2.3 5. 2.S 7.5 4.52 0.2 4.78 0.7 .S. 04 2.5 5.30 8.2 .1..S4 0.2 4.80 0.8 .^.06 2.7 5. 32 9.0 If s is less than 20,000 meters or log « less than 4.30, tlie second term depending on this table may be neglected as it will not exceed one centimeter. By the use of Table A in connection with Crelle's multiplication tables the second term may be secured with all the accuracy necessary in any case. Whenever h^ — h^ is less than 1,000 meters the factor 75-^ is needed ''' to five decimal places only, and the tabular values may be taken out to the nearest unit only. GENEEAL INSTEUCTIONS FOR FIELD WORK. 37 106. Table B, — Values of " ^^ ' in units of the fifth decimal place, p being assumed equal to 6,364,750 meters. /i2 + hi 2 hi + hi hi + A, »2 hi + h 2p hi+ hi 2 hi+ hi 2p hi + hi 2 fe + Ai 2p »! + hi 2 hi + hi ■2p Meters. Meters. Meters. Meters. Meters. 100 l.C) 1,300 20.4 2,600 39.3 3,700 .'■|8. 1 4,900 77.0 200 3.1 1,400 22.0 2,600 40.8 3,800 69.7 6,000 78.0 300 4.7 1,500 23.6 2,700 42.4 3,900 61.3 5,100 80.1 400 6.3 1,600 25.1 2,800 44.0 4,000 62.8 6,200 81.7 500 7.9 1,700 26.7 2,900 46.6 4,100 64.4 5,300 83.3 600 9.4 1,800 28.3 3,000 47.1 4,200 66.0 5,400 84.8 700 11.0 1,900 29.9 3,100 48.7 4,300 67.6 5,500 86.4 800 12.6 2,000 31.4 3,200 60.3 4,400 69.1 5,600 88.0 900 14.1 2,100 33.0 3,300 61.8 4,500 70.7 5,700 89.6 1,000 15.7 2,200 34.6 3,400 63.4 4,600 72.3 5,800 91.1 1,100 17.3 2,300 36.1 3,600 66.0 4,700 73.8 5,900 92.7 1,200 18.9 2,400 37.7 3,600 66.6 4,800 75.4 6,000 94.3 If neither h^ nor h, exceeds 500 meters the third term, depending on this table, may be neglected, as it will not exceed 1 centimeter. By the use of Table B in connection with Crelle's multiplication tables the third term may be secured with all the accuracy necessary in any case. Whenever h^ — \ is less than 1,000 meters the factor ^^^ ^ is needed to five decimal places only, and the tabular values may be taken out to the neares vmit only. 107. Table C. — The tabular values are — - — s^ cot^ C in meters, p being assumed constant and equal to 6,364,750 meters. 1 .? cot (,*. 0.5-m=0.45 or m=0.05. 0.5-m=0.40 or »i=0.10. s cot i- 0..5-m=0.45 or m=0.05. 0.6-m=0.40 or m=0.10. Meiers. 200 .01 .01 Meters. 1,900 ..■54 .61 300 .01 .01 2,000 .60 ..W 400 .02 .02 2,100 .06 .02 600 .04 .04 2,200 .72 .68 600 .05 .06 2,300 .79 .75 700 .07 .07 2,400 .86 .81 800 .10 .09 2, .500 .93 .88 900 .12 .11 2, 600 1.01 .96 1,000 .15 .14 2,700 1.09 1.03 1,100 .18 .17 2,800 1.17 1.11 1,200 .21 .20 2,900 1.26 1.19 1,300 .25 .24 3,000 1.34 1.27 1,400 .29 .28 3,100 1.43 1.36 1,,500 .34 .32 3,200 1..53 1.45 i;600 .38 .36 3,300 1.63 1.64 1,700 .43 .41 3,400 1.73 1.63 1,800 .48 .46 3, .500 1.83 1.73 38 COAST AND GEODETIC StTEVEY. If s cot ^ is less than 200 meters the correction may be neglected, as it will be much less than 1 centimeter. If s cot ^ is greater than 3,500 meters the formula may be used, and p may safely be assumed constant at the value shown at the head of the table. 108. — Table of logarithms of radius of curvature to the earth's surface for various latitudes and azimuths, based upon Clark's ellipsoid of rota- tion (18fifi), and for metric unit. Latitude. Azimuth. 5° 0" 7° 8° 9° Meridian 6. 80178 6. 80180 6. 80181 6. 80183 6. 80186 10 187 188 190 192 194 20 212 214 215 217 219 30 261 262 254 266 257 40 299 300 301 303 304 50 350 351 352 353 354 60 398 398 399 400 401 70 437 437 438 439 440 80 462 463 463 464 465 90 471 472 472 473 474 Latitude. Azimutti. 10° 11° 12° 13° 14° Meridian 6. 80188 6.80191 6.80194 6. 80197 6. 80201 10 197 200 202 206 209 20 222 224 227 230 233 30 260 262 264 267 270 40 306 308 310 313 315 50 366 358 359 361 364 60 403 404 406 407 409 70 441 442 443 444 446 80 466 467 468 469 470 90 474 475 476 477 478 Latitude. Azimuth. 15° 16° 17° 18° 19° Meridian 6. 80204 6. 80208 6.80213 6. 80217 6.80222 10 213 217 221 225 230 20 236 240 244 248 252 30 273 276 280 284 287 40 318 321 324 327 330 50 366 368 371 373 .376 00 411 413 415 417 419 i 70 447 449 451 453 45^ i 80 471 473 474 476 478 90 480 481 482 484 4S5 GENEKAL INSTRUCTIONS FOE FIELD WORK. 39 PRECISE LEVELING. 109. For general instructions for this work see Appendix 3, Report for 1903, " Precise leveling in the United States," pages 211 to 223. AZIMUTH. 110. General remarks. — Parties engaged in general coast surveys need make azimuth observations only when a good initial azimuth is not available, or when specially instructed. If the accuracy of the triangulation is kept up to the standard specified for tertiary tri- angulation, additional azimuth observations will not ordinarily be required. When called for, an azimuth may be measured at any convenient station of the triangulation, but preferably at some sta- tion at which the deflection of the plumb line is not large in the prime vertical, therefore avoiding, if possible, points having nearby moun- tain masses to the east or west. 111. Observation and computation of azimuth. — For example of obser- vation with repeating theodolite see page 389, Appendix No. 7, Coast and Geodetic Survey Report for 1897-98, " Determination of time, longitude, latitude, and azimuth." The computations should be on Form 380, following the form on page 387 of the above appendix ; the formula will be found explained on pages 381 to 384. If the weather conditions are unfavorable or there is difficulty in using the instru- ment for repetitions on the star, the observations may be made with- out repetitions, using a greater number of sets. Table of the values of m = — —■ — 5-b — for use in computing the curvature correction is given on the following pages. 112. If convenient, Polaris should be observed at or near elongation, to eliminate uncertainties in time. The chronometer correction may be obtained by comparison with the telegraphic time signals ; or, lack- ing these, the local time must be obtained by observation on sun or stars, or by comparison with standard chronometers. Whether ob- tained by comparison or observation the record must show clearly and completely each step in the derivation of the chronometer correction. 113. For other standard methods for observing and computing azi- muths see Appendix 7, referred to above. 40 114, COAST AND GEODETIC SURVEY. m = — -. — ^ sin \" t Qm im 2m 3" 4m 6m 6m 7m 8" s „ „ „ „ „ ,^ „ „ „ 0.00 1.96 7.85 17.67 31.42 49.09 70.68 96.20 126. 65 1 0.00 2.03 7.98 17.87 31.68 49.41 71.07 96.66 126. 17 2 0.00 2.10 8.12 18.07 31.94 49.74 71.47 97.12 126, 70 3 0.00 2.16 8.26 18.27 32.20 50.07 71.86 97.68 127.22 4 0.01 2.23 8.39 18.47 ZIAI 50.40 72.26 98.04 127.75 5 0.01 2.31 8.52 18.67 32.74 50.73 72.66 98.60 128.28 6 0.02 2.38 8.66 18.87 33.01 51.07 73.06 98.97 128.81 7 0.02 2.45 8.80 19.07 33.27 61.40 73.46 99.43 129.34 8 0.03 2.52 8.94 19.28 33.54 61.74 73.86 99.90 129.87 9 0.04 2.60 9.08 19.48 33.81 62.07 74.26 100.37 130.40 10 0.06 2.67 9.22 19.69 34.09 62.41 74.66 100.84 130.94 11 0.06 2.76 9.36 19.90 34.36 52.75 75.06 101.31 131.47 12 0.08 2.83 9.50 20.11 34.64 53.09 75.47 101.78 132.01 13 0.09 2.91 9.64 20.32 34.91 63.43 75.88 102.25 132.55 14 0.11 2.99 9.79 20.63 36.19 53.77 76.29 102. 72 133. 09 16 0.12 3.07 9.94 20.74 36.46 64.11 76.69 103.20 133.63 16 0.14 3.18 10.09 20.95 35.74 64.46 77.10 103.67 134. 17 17 0.16 3.23 10.24 21.16 36.02 64.80 77.51 104. 16 134. 71 18 0.18 3.32 10.39 21.38 36.30 66.16 77.93 104.63 135.25 19 0.20 3.40 10.64 21.60 36.58 55.50 78.34 105. 10 136.80 20 0.22 3.49 10.69 21.82 36.87 66.84 78.75 105.68 136.34 21 0.24 3.68 10.84 22.03 37.16 66.19 79.16 106.06 136.88 22 0.26 3.67 11.00 22.25 37.44 56.55 79.68 106.66 137.43 23 0.28 3.76 11.15 22.47 37.72 56.90 80.00 107.03 137.98 24 0.31 3.86 11.31 22.70 38.01 67.25 80.42 107.51 138.53 25 0.34 3.94 11.47 22.92 38.30 67.60 80.84 107.99 139.08 26 0.37 4.03 11.63 23.14 38.59 57.96 81.26 108.48 139.63 27 0.40 4.12 11.79 23.37 38.88 68.32 81.68 108.97 140. 18 28 0.43 4.22 11.96 23.60 39.17 68.68 82.10 109.46 140.74 29 0.46 4.32 12.11 23.82 39.46 69.03 82.52 109.95 141.29 30 0.49 4.42 12.27 24.05 39.76 69.40 82.95 110. 44 141.86 31 0.52 4.52 12.43 24.28 40.06 69.76 83.38 110.93 142.40 32 0.66 4.62 12.60 24.51 40.35 60.11 83.81 111.43 142.96 33 0.69 4.72 12.76 24.74 40.66 60.47 84.23 111.92 143.52 34 0.63 4.82 12.93 24.98 40.95 60.84 84.66 112.41 144.08 35 0.67 4.92 13.10 25.21 41.25 61.20 • 85.09 112.90 144.64 36 0.71 6.03 13.27 25.45 41.55 61.67 86.52 113. 40 145. 20 37 0.75 5.13 13.44 25.68 41.85 61.94 86.96 113. 90 146. 76 38 0.79 6.24 13.62 25.92 42.15 62.31 86.39 114. 40 146.33 39 0.83 6.34 13.79 26.16 42.46 02.68 86.82 114.90 146.89 40 0.87 5.45 13.96 26.40 42.76 63.05 87.26 116.40 147. 46 41 0.91 6.66 14.13 26.64 43.06 63.42 87.70 116.90 148.03 42 0.96 6.67 14.31 26.88 43.37 63.79 88.14 116. 40 148. 60 43 1.01 6.78 14.49 27.12 43.68 64.16 88.57 116.90 149.17 44 1.06 6.90 14.67 27.37 43.99 64. 64- 89.01 117.41 149.74 45 1.10 6.01 14.86 27.61 44.30 64.91 89.46 117.92 150.31 46 1.16 6.13 15.03 27.86 44.61 65.29 89.89 118.43 150.88 47 1.20 6.24 16.21 28.10 44.92 66.67 90.33 118. 94 151.45 48 1.26 6.36 16.39 28.36 46.24 66.05 90.78 119. 45 162. (u 49 1.31 6.48 15.57 28.60 45.56 66.43 91.23 119.96 152.61 50 1.36 6.60 15.76 28.85 46.87 66.81 91.68 120.47 153. 19 51 1.42 6.72 15.95 29.10 46.18 67.19 92.12 120.98 153.77 52 1.48 6.84 16.14 29.36 46.60 67.58 92.67 121.49 164.35 53 1.53 6.96 16.32 29.61 46.82 67.96 93.02 122. 01 154.93 54 1.69 7.09 16.61 29.86 47.14 68.35 93.47 122.53 166.51 55 1.66 7.21 10.70 30.12 47.46 68.73 93.92 123.05 166.09 56 1.71 7.34 16.89 30.38 47.79 69.12 94.38 123.67 156.67 67 1.77 7.46 17.08 30.64 48.11 69.51 94.83 124. 09 167.25 58 1.83 7.60 17.28 30.90 48.43 69.90 95.29 124. 61 167.84 59 1.89 7.72 17.47 31.16 48.76 70.29 96.74 126. 13 158. 43 GENERAL INSTETJCTIONS POK FIELD WORK. _ 2 sin^ i t 41 sin 1" ( 9m 10" 11m 12" 13" ■ 14" 16" 16" s 159.02 196.32 237.54 282.68 // 331. 74 384.74 441.63 502.46 1 159. 61 196.97 238.26 283. 47 332.59 386 65 442.62 503.50 2 160. 20 197. 63 238.98 284. 26 333.44 386.56 443. 60 604.55 3 160. 80 198.28 239. 70 286.04 334.29 387.48 444. 68 505.60 4 161. 39 198.94 240.42 286.83 335. 15 388. 40 445.66 606. 65 5 161. 98 199.60 241.14 286.62 336.00 389.32 446.66 607.70 6 162. 68 200.26 241. 87 287. 41 336.86 390.24 447. 54 608. 76 7 163. 17 200.92 242.60 288.20 337. 72 391. 16 448. 53 509. 81 8 163. 77 201.59 243.33 289.00 338. 58 392.09 449. 61 610.88 9 164. 37 202.25 244.06 289. 79 339. 44 393. 01 450.60 511. 92 10 164.97 202.92 244. 79 290.58 340.30 393. 94 461.50 512. 98 11 165. 67 203.68 245. 52 291.38 341. 16 394. 86 462. 49 514. 03 B 166. 17 204. 25 246.26 292. 18 342.02 395. 79 463.48 515. 09 13 166.77 204.92 246. 98 292.98 342.88 396.72 464.48 616. 16 14 167. 37 206. 69 247.72 293.78 343. 75 397.65 455. 47 617.21 15 167.97 206.26 248.46 294.68 344. 62 398'. 58- 466. 47 518. 27 16 168. 68 206.93 249. 19 296 38 345.49 399. 62 467. 47 619. 34 17 169. 19 207.60 249. 93 296.18 346.36 400.46 458. 47 620. 40 18 169.80 208.27 260.67 296.99 347. 23 401. 38 459. 47 521. 47 19 170. 41 208.94 251. 41 297.79 348. 10 402. 32 460. 47 522. 63 20 171. 02 209.62 262. 15 298. 60 348.97 403. 26 461. 47 623. 60 21 171.63 210. 30 262.89 299. 40 349. 84 404. 20 462. 48 624, 67 22 172. 24 210. 98 253.63 300.21 350. 71 406, 14 463. 48 525, 74 23 172. 85 211.66 254. 37 301. 02 351.68 406.08 464. 48 626. 81 24 173. 47 212. 34 256. 12 301. 83 362. 46 407.02 465.49 627. 89 25 174. 08 213.02 265. 87 302. 64 353. 34 407.96 466.50 628.96 26 174. 70 213. 70 266.62 303.46 364.22 408.90 467.51 530.03 27 176. 32 214. 38 267. 37 304.27 356. 10 409. 84 468.62 531. 11 28 176. 94 215. 07 268. 12 306.09 355.98 410.79 469. 63 632. 18 29 176.66 215. 75 258. 87 306.90 356.80 411. 73 470. 64 633.26 30 177. 18 216. 44 259.62 306.72 357. 74 412.68 471.55 534. 33 31 177.80 217. 12 260.37 307. 64 358.62 413. 63 472. 67 635. 41 32 178. 43 217. 81 261. 12 308. 36 369. 51 414. 59 473.68 636. 50 33 179. 05 218.60 261.88 309.18 360. 39 416. 54 474,60 537. 68 34 179.68 219. 19 262.64 310.00 361. 28 416. 49 475. 62 538, 67 35 180.30 219. 88 263.39 310. 82 362. 17 417. 44 476. 64 639. 75 36 180.93 220. 58 264. 15 311.65 363. 07 418. 40 477. 66 640. 83 37 181.66 221.27 264. 91 312. 47 363. 96 419. 35 478.67 541. 91 38 182. 19 221.97 265.68 313. 30 364. 85 420.31 479. 70 543.00 39 182. 82 222. 66 266. 44 314. 12 365. 75 421.27 480.72 544, 09 40 183.46 223.36 267. 20 314.95 366.64 422.23 481. 74 646. 18 41 184.09 224.06 267. 96 316. 78 367. 53 423.19 482. 77 .546. 21 42 184. 72 224. 76 268. 73 316.61 368.42 424.15 483. 79 547. 36 43 186. 36 225.46 269. 49 317. 44 369. 31 426.11 484. 82 548. 45 44 185.99 226. 16 270. 26 318. 27 370. 21 426.07 486. 85 549. 65 46 186.63 226.86 271. 02 319. 10 371. 11 427.04 486.88 ,550.64 46 187. 27 227. 67 271. 79 319. 94 372. 01 428.01 487.91 561. 73 47 187.91 228. 27 272.58 320.78 372. 91 428.97 488.94 562. 83 iS 188.56 228.98 273. 34 321. 62 373. 82 429.93 489. 97 553. 93 49 189. 19 229.68 274. 11 322. 46 374. 72 430. 90 491. 01 555. 03 50 189. 83 230. 39 274. 88 .323.29 376. 62 431. 87 492.06 656.13 51 190. 47 231. 10 276.66 324.13 376. 52 432.84 493. 08 567.24 62 191. 12 231. 81 276. 43 324. 97 377. 43 433. 82 494. 12 558. 34 63 191. 76 232. 52 277. 20 325. 81 378. 34 434. 79 495. 16 559. 44 64 192. 41 233. 24 277.98 326. 66 379.26 435. 76 496. 19 560.66 55 193.06 233.96 278. 76 327. 50 380. 17 436.73 497. 23 561.65 56 193. 71 234. 67 279.66 328. 35 381. 08 437. 71 498, 28 562.76 67 194. 36 235. 38 280. 33 329.19 381.99 438.69 499. 32 663. 87 68 195. 01 236. 10 281. 12 330. 04 382. 90 439.67 600. 37 664, 98 69 195. 66 236. 82 281.90 330. 89 383. 82 440. 65 601, 41 666. 08 42 COAST AND GEODETIC SIJEVEY. TOPOGRAPHY. 115. Use of tVe plate table. — Full details regarding topographic sur- veys with the plane table will be found in "A Plane Table Manual," Appendix No. 7, Coast and Geodetic Survey Report for 1905, Avhich majr be obtained bound separately. 116. Control of topography. — The most satisfactory way of making detailed topographic surveys is to first complete and compute a sys- tematic triangulation, and plot the points determined on a projec- tion. This will always be feasible in charting new regions, on ac- count of both time and expense, and the topography will sometimes have to be executed at the same time as the triangidation. 117. Where topography is carried on simultaneously with triangu- lation and other work, if practicable, the triangulation will be kept sufficiently in advance so that the distance (not necessarily the geographic positions) may be computed and plotted on the sheet before filling in the topography. In all cases where this will cause too great a delay or is not practicable from other causes, the topog- rapher must check the distances on his sheet by the computed dis- tances as soon as they are available, and where there are important discrepancies must correct the error by examining the portion af- fected. All triangulation signals must be cut in with the plane table and shown on the topographic sheet; those falling off the limits should have direction lines drawn on the sheet. All traverse lines run must, if practicable, be checked by closing circuits, and small errors adjusted; if large errors appear the lines must be rerun. If proper care is exercised it will be possible to fit a projection to the plane table sheet by the triangulation points, so that there will be little or no error in the resulting chart. 118. The magnetic meridian should be marked on the sheet in the field. 119. In order to utilize all the available force at the beginning of a season, it may be advantageous to make a plane-table survey of a har- bor, and fill in the hydrography on this, the signals to be determined by triangulation later. 120. "When former triangulation stations are searched for and not found, or when stations are recovered which are insufficiently marked or described, report should be made, and if practicable the deficiency in marking or description should be remedied. 121. Scale. — For all general coast topography in new regions, unless otherwise specified, a scale of ^oItd wiH be used. Larger scales, as ttJou aiicl in exceptional cases -g-^^-^), are to be used for special har- bor surveys where the amount of detail or the importance of the locality warrants, but smaller scales than ^uItj-jj will not be used unless specially authorized. GENEEAL INSTEtrcTIONS FOB FIELD WORK. 43 122. Contour intervals for ordinary coast topography should be either 20, 50, or 100 feet ; 40-foot intervals should not be used. The choice of intervals should depend on the nature of the country and the scale of the sheet. 123. laying out sheets. — Plane table sheets should, in general, be laid out to run parallel with the coast and to cover as long a length of coast line as convenient, and to include the signals necessary for its control. Sheets containing small detached fragments of topog- raphy should be avoided as far as possible; this can sometimes be done by placing a subplan on an adjacent sheet. Where the geog- raphy permits, sheets should preferably be laid out with the meridian. 124. Table of dimensions of standard topographic sheet, 30 inches by 62 inches, expressed in nautical and statute miles, for different scales : Scale. Nautical miles. Statute miles. Width. Length. Width. Length. 2.06 4.U 8.22 16.45 41.12 3.66 7.13 14.26 28.51 71.27 2.37 4.74 9.47 18.94 47.35 4.10 8.21 16.41 32.83 82.07 125. The features to be included in ordinary coast topography are the following: 126. The careful location of the shore line, including the ordinary high-water line and the low-water line so far as it may be determined or estimated without waiting for low tide. 127. Rivers and streams for a reasonable distance back from the coast, according to their importance ; large rivers should be surveyed to the limit of the detailed topography, while small and unimportant creeks need be shown only a short distance; navigable rivers should be surveyed to the head of tide water or ship navigation. 128. Offlying reefs and rocks, including elevations of all prominent rocks and islets. Offlying reefs should be designated as bare, awash, or covered at high or low water, as the case may be. 129. Towns, settlements, roads, and important trails within a rea- sonable distance of the coast, with designation on the sheet of promi- nent buildings or other objects useful as landmarks. Prominent buildings of solid construction should be located individually ; "clus- ters of unimportant houses may be sketched or generalized. Build- ings of masonry or other permanent construction should be shown solid black, and native houses of light material open. In a town the built-up area should be generalized, and buildings not individu- ally located except those of special prominence. Important high- ways near to or parallel with the coast and not more than 3 or 4 miles distant should be surveyed. 44 COAST AND GEODETIC SURVEY. 130. A'atiiral objecty along the shore that may be useful in future hydrographic work whould be located with care and so named or described that they may be identified without diiRculty. This is par- ticularly important along a coast line where a hydrographic survey is to follow, and will warrant the special marking, where practicable, of points to be used in the hydrography. 131. The location and elevation of hills or mountains within the limits of the sheet, so far as may be obtained from the vicinity of the coast. 132. The nature of the coast line and of the low-water line, as sand, coral rock, mangrove, etc., must be indicated by symbols, and the gen- eral vegetation along the shore must be shown. 133. Features not fully surveyed, as the extension of a stream be- yond the limits actually run, may be indicated by broken line or appropriate note. 134. The plane of reference for elevations is mean high water, except- ing for the Philippine Islands, where mean sea level is the plane adopted. Elevations are stated in feet. All elevations given either by figures or contours should represent the elevation of the ground ; when for any reason the elevation of the top of trees or vegetation is given, note to that effect should be added, with estimate of height above ground. 135. Elevations may be read from the hypsograph (see Appendix Xo. 4, Report for 1902), or they may be scaled from a graphic dia- gram, or computed by " Table of factors for computing differences in elevation " and " Table of corrections for curvature and refrac- tion " (pp. 338 and 339 of Plane Table Manual, and also printed separately). 136. Contours. — The general shape of the elevated regions within the limits of the survey should be shown by contours drawn to accord with determined elevations, tangents to the slopes, and the ajjpearance of the country. AYhere the topographer is not able to get a reason- ably definite idea of the region the contours should be omitted or shown Ijy broken line to indicate the general trend of the country. Where detailed information can not be shown, the general nature of the region should be stated by descriptive notes on the sheet. 137. Interior elevation. — Under the head of triangulation, provision is made for determining important elevations visible from the coast and beyond the limits of the plane table sheet. Information indi- cating the relation of these elevations to the surrounding country is of value, especially on small-scale general charts. A'Vliile it is im- possible to obtain correct detailed information without going over the country, yet it is suggested that by plotting on a small scale (as on a piece of a general chart) the points determined a sketch may be made showing the trend of the ridges and the low areas as far as GENERAL INSTEUCTIONS FOR FIELD WOKK. 45 visible. This will to some extent avoid the false idea which is given of a mountainous country by showing on the chart only detached summits. 138. Use of sextant and theodolite in topography. — While the plane table is the most valuable instrument for topography, the surveyor should not regard himself as restricted to its use. Where located signals are in sight and the shore is lined with swamp or mangrove, and in other situations presenting no suitable locations for the table, the sextant may be used to advantage in filling in topography by locating each principal feature by two or preferably three sextant angles, with additional angles to tangents of points and other objects. A continuous sketch should be made in a sketchbook, with the angles written opposite the corresponding points on the sketch. Also cases may arise where the topography may be obtained advantageously with a theodolite traverse line (the transit and stadia method). 139. When any of these auxiliary methods are used the work should be plotted and combined by the topographer and added to the general topographic sheet, and the Descriptive Report should state what por- tions are so surveyed. 140. Approximate locations. — If from any triangulation or plane- table station breakers or other indications of offlying dangers not previously located are noticed, directions should at once be determined, and also, if practicable, vertical angles. From the latter and the ele- vation of the instrument approximate distances may be computed, which will aid in identifying the objects from other stations. An- other method quite useful in rapidly getting approximate locations of objects so as to permit of their future identification is to take cuts on them from a plane-table station and then from another station near by. Of course such locations are to be confirmed by good inter- sections from other points. 141. Plans of towns and local maps, if available, should be obtained. These, if verified and found sufficiently reliable, may be used for filling in details, especially of towns, but not for the position of important objects. Copies of maps of value obtained should be for- Avarded to the Office. 142. Symbols and lettering. — The standard topographic symbols are to be followed : 143. The high-water line, being one of the most important features on the sheet, should be drawn with sufficient strength to make it clearly distinguishable. The use of a full line for defining the limits of vegetation outside of the high-water line, or the limits between marsh and fast land, should be avoided. 144. Time need not be taken for the elaborate covering of a sheet with topographic vegetation symbols; vegetation symbols and sand- ing should be put on sparsely and rapidly, and it will sometimes suf- 46 COAST AND GEODETIC SUBVEY. fice to show only the limit of extent of any vegetation feature by sym- bols, with words in the center to show the area covered. Cultivation features should be generalized, and ordinarily attempt need not be made to indicate tlie extent of individual fields. Words may be used to indicate vegetation features for which there is no special symbol. 145. The field topographic sheet is a survey record ; it should show all useful information jDlainlj^ neatly, and correctly, but time that can be more usefully employed should not be expended in endeavoring to make it a handsome drawing. 146. Valuable information, useful notes, etc., should not be omitted for fear of marring the appearance of the sheet ; nor should the topog- rapher hesitate to place the necessary information on the sheet because he is not expert at lettering. 147. In lettering topographic and hydrographic sheets, names ap- plying to the land should be in vertical letters, and names applying to the water, including objects covered at high water, should be in slanting letteis. All geographic names are to be in black ink, and names solely for survejdng use, as of signals and stations, are to be in red ink. 148. Cai-e must be taken not to confuse the symbols for sunken rocks (a simple cross), rocks awash (three lines crossing), and rocks above high tide (heavy dot or shape) . Brief notes are desirable clearly indi- cating the nature of important reefs and rocks, as " awash at low water," " awash at high water," " coral 'heads bare at low water," " breakers at low water," etc. 149. The following remarks apply particularly to Philippine topo- graphic sheets. The cocoanut palm being usually a distinctive feature on the coast, should be shown by the special symbol. Mangrove grow- ing in the water should be limited by a xeiy light line to preserve the detail and correct position, and yet to represent it differently from the strong black line used for the high-water line. Sometimes where there is mangrove the solid shore may not be seen, and it may not be practicable to loCate it. The ordinary coral reef symbol should be used only to represent the limit of reefs bare or awash at low water, and should not be used to represent reefs covered to some depth at low tide. "When not developed by the soundings the limits of sub- merged reefs should be indicated by the sunken rock symbol. Rice paddies may be represented conventionally by small irregular quad- rilaterals bounded by slightly irregular lines and a little grassing. 150. Inking of sheets should, if possible, be done by the topographer himself, and as soon as practicable after the field work is completed on each sheet. Intervals of delay in field operations may be utilized advantageously for this purpose. 151. Accuracy, neatness, and clearness are necessary in inking sheets ; beyond this fine drafting is not essential. GENERAL INSTRUCTIONS FOR FIELD WORK. 47 152. When for any reason an uninked sheet is transmitted to the office, the greatest care must be exei'cised by the chief of party that every feature, fact, and name is clearly and distinctly shown. The topographer must also make it a point to see and verify the sheet at some time after it is inked, examining every detail. 153. It is particularly important in such case that small detached rocks along the shore, and other features that might be mistaken for accidental markings, should be made clear, and in general such ob- jects should be inked by the topographer. 154. The elevations of summits should be distinctly marked on the sheet, and care must be taken that they are not rubbed or lost before inking. 155. Triangulation stations should be marked by small black circles inclosed in red triangles, with names in red ink, but in no case should this symbol be permitted to obscure an essential topographic feature ; for instance, in case of an offshore rock or islet used as a triangula- tion station, the rock or islet should not be obscured by the station symbol, but the latter may be omitted if necessary, and an explanatory note may be added as to the station. 156. Plane-table positions should be marked on the sheet with small red circles when the positions are recoverable and likely to be of future value ; otherwise such positions should not be inked. 157. Titles should not be inked on original sheets in the field, but ■ must be furnished plainly written in pencil on the sheet, or on pajaer pinned to the sheet. The information for the title must include the general locality, special locality, names of persons making survey and of chief of party, date (months and year), and scale. In the Philippines the stamped title form should be filled in ink on the sheet or on a sli23 pinned to the sheet. 158. In preparing and inking original sheets, north shall be taken as the top, and titles, names, numbers, and symbols shall be put on normal to the meridian regardless of the direction of the borders of the sheet, except where it is desirable that names be lettered to con- form to geographic features. In such cases the names shall be inked so as to be read when looking north. Names should by their direc- tion and proximity clearly indicate the object designated. 159. Photographs or tracings of sheets. — When there is reason to be- lieve that the mode of forwarding a sheet is not secure, it should, if practicable, be photographed, or if photographic facilities are not available an outline tracing of the more important features of the original sheet may be made. Otherwise, no tracing of an original sheet should be made in the field. Bromide enlargements from photographs of sheets should not be made except at Washington, unless specially ordered. 48 COAST AND GEODETIC SURVEY. 160. List of plane-table positions. — Before transmitting topographic sheets to the office, chiefs of parties will prepare a list of the promi- nent objects on the sheet that have been determined by the plane table, namely, spires, chimneys, cupolas, flagstafPs, trees, etc., and such natural objects as sharp, well-defined mountain peaks, rock cliffs, and other objects that might be recovered and utilized, and liarticularly such objects as will be useful in hydrographic work; and indicate the position of each object listed by scaling the D. M. and D. P. from the sheet in the following form, giving the height, if determined : PUmr-ldhJc iiomtUjiia. Object and description. Cupola, Harrison's house Cupola, Blackwell'sbarn chimney, square house, Smith'i North chimney, Rodger's house Episcopal church spire Murray Mountain Latitude. A-l 21 •12 22 42 2.5 42 26 42 25 42 27 D. M. Longi- tude. D. P. Height. Meters. 'O ' Meters. Feet. 356 72 40 608 146 845 72 39 724 138 632 72 37 395 167 981 72 38 1,028 125 68 72 40 875 250 426 72 46 126 3,266 Top. Weather vane. Top. Top. Top of cro.^s. The north peak. 161. This list should be written on the sheet itself or be attached to the descriptive report. The exact position of the objects referred to should, of course, be distinctly indicated on the sheet. Where space permits, the more important objects, and especially those land- marks Avhich should appear on the chart, should be named directly on the sheet itself, either close to the object or by reference letter and note elsewhere on the sheet. Brief legends descriptive of important landmarks may also, where practicable, be conveniently placed on the sheet. HYDROGRAPHY. 162. Data to start survey. — When the information is available from previous work, the following will be furnished with the instructions from the office, and the chief of party should at once examine the in- formation to see that it is complete and understood: Projections on which have been plotted triangulation points, shore line, and all objects or features located by plane table or otherwise which may be useful in the hydrography ; list of geographic positions ; descriptions of stations; tidal plane of reference; description and relation of tidal bench marks; copies of previous charts or surveys; information as to dangers reported or other special features to be examined, and, in the case of continuous surveys along the coast in a new region, a copy of the progress sketch of the previous season. In regions where sur- GENERAL INSTRUCTIONS FOR FIELD WORK. 49 vey work has not previously been done the triangulation may have to be made and the tide plane determined by the party charged with the hydrography, and the projection will then be made in the field. In some cases it may be desirable to carry on the hydrography simul- taneously with the triangulation or topography in order to save time or utilize the services of all of the party at the beginning of a season. In such cases preliminary locations of the signals may be plotted graphically on the boat sheet, but all the work should be planned with the view of ultimate control by the triangulation, and the more important stations should be carefully marked. 163. The lists of geographic positions and descriptions of stations furnished to field parties must be returned to the office upon the com- pletion of the work." When former stations are recovered that are found to be insufficiently marked or described, or the marks partially effaced, or the witness marks gone, the defects should be remedied and an amended description forwarded. Stations should not be reported as lost unless an exhaustive search has been made. When building signals over stations care should be taken not to disturb the station marks. 164. Shore line. — When there is reason to suppose that the shore line has changed materially since a previous survey the important features should, if practicable, be located in connection with the hydrography, either with the plane table or by determining prominent points by sextant angles (preferably three at each point) and sketching in the intermediate shore line. Shore line so located should be drawn in broken line. The same course should be followed when the hydrog- raphy precedes the topography and it is impracticable at the time to obtain the complete topographic information desirable. 165. Scale. — Unless otherwise directed, inshore hydrography should be plotted on scale not less than jo.Vsu; ^id must be done in sufficient detail to fully develop recommended sailing lines, approaches, chan- nels, and anchorage areas and remove doubt as to dangers. Anchor- age, harbors, and channels may sometimes require scales of yj^xjo- or even ^ hi- Offshore hydrography may usually be plotted conven- iently and economically on smaller scales, as 15,^50, bb utss, so.Ub; or TTiTrStio- Where there are no dangers or details either of the last two may be sufficient for charting purposes. 166. Location of signals. — It is desirable that in advance of the hydrographic development a reconnaissance be made, the best loca- tions for signals chosen, and the whole work systematically planned. 167. If the hydrographic work is to be based on triangulation and topographic points previously determined, these should be first recov- ered, as far as practicable, and if necessary additional points located from them. 53695—08 i 50 COAST AND GEODETIC SURVEY/ 168. Suitable triangulation methods, instruments, and records should be used for. the extension of the triangulation beyond the lim- its already executed and to supply the place of points lost. The sextant should not be used for this purpose, nor for the location of important hydrographic signals, or of permanent objects, such as light-houses, beacons, buildings, and other useful landmarks. 169. When in the course of the hydrographic work it is desirable to locate new signals by sextant, three angles should be taken, if prac- ticable. Such angles may be recorded in the sounding-record book, preferably on separate pages at the beginning, and on the first page of the sounding-record book should be an index giving name and page of each signal whose determination is recorded in the volume. 170. In some cases it may be necessary to locate a subordinate sig- nal or object by angles from several positions of the boat, the latter determined from other signals. Where recourse to this device is necessary at least three positions should be used as a check. 171. The officer in charge should make sure while yet on the ground that the position of every signal or object used in the hydrography is determined with sufficient accuracy for the scale of the projection, and this must be tested by actually plotting or computing in the field. 172. Great care must be taken that ample information for the cor- rect plotting of every hydrographic signal accompanies the record. A list of such as depend on plane-table locations and a list of such as depend on sextant angles should be given in the Description of Sta- tions or in the Descriptive Report. 173. In connection with the triangulation and plane-table work along the coast in a new region special attention must be given to determining suitable objects for hydrographic work, each of which should be described and marked when necessary, so as to be available for future use. 1.74. A signal erected exactly over an old station should bear the name of that station. If for any reason a signal is located near, but not exactly at a previous station, it must have a distinguishing name, or may be given the old name followed by " No. 2 " or the year. 175. Names of signals. — For convenience short words of not more than three or four letters should be used for names of hydrographic signals. 176. List of permanent positions determined. — Before transmitting hydrographic records or sheets to the office chiefs of parties Avill pre- pare a list of prominent objects or positions of a permanent char- acter that may be useful in future work that have been determined in connection with the hydrographic work. Indicate the position of each object listed by scaling the D. M. and D. P. from the sheet, in the form given under Plane Table Positions (paragraph 160). This GENEEAL INSTRUCTIONS FOR FIELD WORK. 51 list should be written on the sheet itself, or be attached to the De- scriptive Report. 177. Character of signals. — It will materially facilitate hydro- graphic work to have a sufficient number of conspicuous signals which may be readily picked up by the sextant observers. For convenience as well as economy natural objects, such as bowlders, cliffs, and lone trees, and artificial objects, s'uch as towers, flagstaffs, light-houses, gables of buildings, etc., should be used as signals when available. 178. A good form of hydrographic signal is a tripod with slats across two of its sides, or a pole with banners of cloth stretched be- tween cross pieces so that the banners will show in different direc- tions. Driftwood, small trees, and other material on the ground, and in the tropics bamboo poles fastened with wire or rattan, may be used economically. Signals near each other or similarly situated should be varied in form or color to avoid likelihood of confusion. The directions from which they will be viewed should, of course, be considered in building signals. Natural as well as artificial objects may readily be made conspicuous by whitewash. Against a dark background white signals show best, against the sky black is preferable. 179. A tripod made of lengths of iron pipe, wired together through T joints at the top, makes a simple signal that will stand in a mod- erate depth of water, and if wrapped with cloth and with flags set in top may be seen at a good distance. In exposed situations in the water such signals may be made more secure by pumping the legs into the bottom by means of a water jet; long poles and saplings ha\e also in this manner been pumped in on ocean bars and have withstood storms. 180. Indefinite objects, such as tops of round hills and centers of islands, should never be used for critical or inshore hydrography, but for offshore hydrography it is sometimes necessary to use the summits of mountains which have been determined by triangulation ; of course, for this purpose, definite and conspicuous points are to be selected as far as practicable. 181. When sounding from boats, it may sometimes be desirable to use the foremast of the vessel as a signal; in such case the vessel should be anchored with a short scope, and her position determined both on flood and ebb current and, if necessary, when swinging to wind, and the time of determination recorded. The sounding rec- ord should show whether the ship swings to ebb or flood tide. 182. Plan of development. — The plan of development should be carefully considered in advance, so as to properly cover the whole area, with suitable allowance for the relative importance of the dif- ferent parts ; the closeness of development should vary from a maxi- 52 ' COAST AND GEODETIC SURVEY. miun in channels and anchorages having depth near the draft of the vessels to be accommodated to a minimum on extensive flats of much less depth and in clear areas of much greater depth. The soundings upon the chart in addition to indicating to the mariner dangers, channels, and anchorages, may also enable him by casts of the lead to recognize his position, and this requires a sufficient development of the material and relief of the bottom within the limits of ordi- nary sounding depths. 183. In working on the general scale along a coast in new regions, closer development should be made of all parts where vessels are likely to be compelled to approach land, as in possible anchorages or off promontories, even though present conditions do not warrant special large scale surveys. The lines should also be closer off pro- jecting pojnts of land or reefs. 184. The order of the development of the hydrography should de- pend on the most economical management of the party. Much valu- able time may be lost in sending boats to sound a long distance from the ship or headquarters, and whenever circumstances -permit the anchorage or the shore quarters should be shifted to keep near the working ground. 185. Systems of sounding lines. — Systems of parallel lines cover an area most evenly and economically, and zigzag lines should, in gen- eral, not be used. The development should usually be by straight lines perpendicular to the general trend of the coast, though the direction will dejiend someAvhat on currents, wind, and vessel. When thei'e is a strong current in a thoroughfare or river, lines run normal to the channel, owing to the irregular progress of the boat over the bottom, will not afford reliable means for plotting soundings unless position angles are observed at each sounding. Under such circiunstances the greater part of the development should be made by lines run Avith or against the current. A close system of lines in one direction will cover an area more uniformly than the same distance run of lines crossed perpendicular to each other. However, cross lines furnish a valuable check on the accuracy of the work, and it is there- fore recommended that for inshore hydrography the first system of parallel lines be crossed by lines approximately perpendicular and spaced several times as far apart as the first system. Outside of the 10-fathom curve, very few, if any, cross lines should be run. When, however, a system of lines extends seaward for a considerable dis- tance beyond where it can be checked by observations on fixed objects a few cross lines should be run to enable the draftsman to detect gross errors. 186. The si:)acing of lines will have to depend largely on tlie character and relief of the bottom and the importance of the region. Tn general coast Avork Avith flat and sandy bottom and Avithout indi- GENEEAL INSTETTCTIONS FOR FIELD WORK. 53 cation of danger, inshore lines may be spaced 200 to 300 meters apart, but this interval should be diminished for uneven, rocky bottom; and in important anchorages and channels lines as close as 50 meters may be required. Between the 10 and 100 fathom curves about four, or even less, lines to the mile should be sufficient in regions where there are no indications of dangers. 187. For the sake of economy care must be taken not to extend the close inshore system of development into open and deep areas where it is unnecessary; otherwise a serious loss of time and energy may result. The system of lines must be varied to suit the conditions. Ordinarily the close inshore work will be done with launch or boat, and the more open offshore work with ship, the latter system slightly overlapping the limit of the former. 188. The 100-fathom curve should be developed if within a rea- sonable distance of the coast. Outside of the 100-fathom cnrA-e it will add to the completeness and general value of the chart of an abrupt coast if lines are run out to a distance of about 10 miles from the shore, spacing the lines 4 or 5 miles apart, but soundings outside of the 100 fathom curve need not be made unless facilities and condi- tions are favorable. 189. Sounding interval. — The interval between soundings should depend on the nature of the bottom and the depth of the water. In depths of critical importance to navigation it should be made as short as is consistent with good work, and it should always be less than the interval between lines. Generally in moderate depths of water more soundings will be taken than can be plotted on the sheet. 190. Time interval. — The time interval should usually be uniform, the recorder indicating the time by the order " sound " to the leads- man. For very irregular bottom the time soundings should be aban- doned and the leadsman should sound as rapidly as possible. Under normal conditions and with a single leadsman the following time intervals have been found to meet the requirements: Depths under 2 fathoms 15-seconcl interval. Depths from 2 to 4 fathoms 20-second Interval. Depths from 4 to 7 fathoms 30-second Interval. Depths from 7 to 10 fathoms 40-second Interval. Depths from 10 to 15 fathoms 1-minute interval. 191. Sounding speed. — The speed of the boat should be varied as may be necessary for efficient and economical work. It may be increased in veiy shoal water when soundings can be made rapidly, and also in deeper open water where a close interval is unnecessary. But it should never be so great as to interfere with getting correct soundings. It is impossible to obtain up-and-down casts when the vessel is rim- ning at high speed, especially with the current. About 5 knots should be considered as the maximum speed for sounding with a hand lead under favorable conditions. 54 COAST AND GEODETIC StTKVEY. 92. Precautions in case of danger indications. — "When the bottom is rocky, or when detached rocks are known or suspected to exist, the precautions in sounding should be much increased. 193. In all cases of shoals, suspicious soundings, and indications of dangers whatever additional work is necessary to thoroughly develop the bottom and to determine the least depth of water must be done regardless of any prearranged system of lines. It must not be assumed that the regular lines of soundings show the least depth. A sounding showing even veiy little less than the average depth should be regarded as the indication of a possible shoal, much more so when two such shoaler soundings are found on contiguous lines, and in such case very careful investigation should be made of the vicinity to obtain the least depth. 194. Depth curves. — A valuable test of the sufficiency of data from a hydrographic survey is to ascertain if the curves can be drawn for all depths without leaving doubt as to their directions anywhere. It is extremely important that the depth curves should be sketched and studied as the work jDrogresses, as peculiarities in these curves will point out possible dangers that should be examined or indicate defects in the record. 195. Additional development. — All channels, sailing lines, and anchorages should be sounded thoroughly and dragged if necessary ; additional lines in the direction of the axis of the channel or of the sailing lines should be run if they are not parallel with the system of sounding lines adopted for the general development. Leading lines should not be recommended without actual test by running lines of soundings over them. 196. Ranges for running lines. — Sounding lines are ordinarily run on compass courses. Ranges of natural objects on shore should be i:)icked up when ^practicable, and will be especially useful when cur- rent or wind is of uniform strength. Usually, however, it will not be desirable to delay the work to select ranges or for the purpose of getting the boat in the exact position to start a proposed line, and this must not be done unless there is special reason for it. When essential to select a range, the angle between some signal and the line proposed to be run may be taken off the sheet with a protractor, and with the sextant set to this angle search made for suitable objects ashore in the direction of the line. 197. Running lines by compass. — A proposed system of parallel lines spaced as desired may be laid out in pencil on the boat sheet. In following a jDencil line by compass, when a position plots off' to one side, the course should not be changed before a new position is secured. To take up a new line angles are observed and plotted before reaching the beginning of the line. This affords a means of estimating the distance to be run and the time for turning the vessel on to the new GENERAL INSTRUCTIONS FOR FIELD WORK. 55 course. When properly headed the angles are observed for the first position on the new line. 198. In close development with parallel lines, soundings should not be taken between the last position on one line and the first position on the next line. 199. In cases of exposed shoals with breakers it may be imprac- ticable to do more than run a line just outside of the breakers. 200. Special development of reefs, shoals, bars, and channels. — In surveying a reef with a single high point or surface a buoy is gen- erally placed on the highest point and radial lines run from this ; but this may give an imperfect idea of the shape of the reef, as the lines diverge rapidly from each other. New lines should be introduced, therefore, between the first radial lines as they recede from the buoy, or preferably the area in question should be developed by a system of close parallel lines and cross lines. 201. If the reef has more than one high point, several buoys placed upon them will give the means of laying out upon a diagram and of executing by sounding a regular plan of work which will show the peculiarities of the reef, multiplying the soundings where the slopes are steep or the irregularities great. It is very desirable to visit rocks and shoals at extreme low water, when an examination may show how near the surface any portion approaches. 202. In harbors, lines should be run alongside quays and wharves to show what water can be taken to them ; a boat pulling with the oars just clear of the sides will give about the position that the keel of an ordinary vessel will occupy; the positions of such soundings should pref erabljr be obtained by actual measurement rather than by sextant angles. 203. When convenient, shoals and flats bare at low water may be sounded over at or near high water and the heights above the plane of reference given just as are the depths below that plane. These heights will be plotted on the chart as " minus soundings," that is, the heights in figures will be plotted with the minus sign before each. In general, whenever a sounding is less than the amount of the tide reduction at the same moment, the difference should be plotted as a minus sounding. All minus soundings are, of course, to be included within the low-water line. 204. Locating reefs in heavy weather. — On a field of work exposed to the sea, reefs and shoals may be discovered, located, or verified dur- ing heavy weather by occupying two or more stations, and with an instrument cutting in the breakers, or by cutting them in from a ves- sel. The depths can be ascertained during fair weather. 205. When the survey of a shoal or rock is finished, care must be taken to note upon the spot all useful ranges, bearings, and marks which lead over it or close to it on every side. 56 COAST AND GEODETIC SURVEY. 206. Examination for adequate development.— The development of channels having moderate depths in the fairway, and that of bars, if there are any, which obstruct the fairway, are of the utmost impor- tance and should receive the close personal attention of the chief of party. After the lines are plotted and the curves drawn in, he should carefully trace out each channel and assure himself that no soundings are wanting to show exactly how much water can be carried through- out its whole extent, and extra lines should be run where there is the least room for doubt. Should he find indications of a bar, a further examination must be made to develop its form and extent and to make sure of having found the least depths upon it. 207. Dragging for dangers should be resorted to in cases of important channels and anchorages where obstructions have been reported and not found, or where the nature of the bottom and surroundings indi- cates a likelihood of dangers which might be missed in the ordinary sounding lines. Even in the closest development with the sounding lead pinnacle rocks may be missed, and a thorough sweeping of a doubtful area is necessary to prove that it is clear. Experience indi- cates that this precaution is well warranted in important areas. 208. The long-wire drag or sweep is the most effective means for this purpose. It is described in the Coast and Geodetic Survey Re- port for 1907, Appendix No. 7. 209. In plotting this system of drag work the colors of the lines should be applied in such manner as to bring out the areas dragged in the clearest manner; the same color should be used for both sides of a dragged strip, and the colors for overlapping strips should be varied as may be desirable. The boats may be distinguished by the letter colors as on other hydrographic sheets, but not by the colors of the lines. 210. A drag made of pipe and intended for use with a surveying vessel is described in Appendix No. 6, Coast and Geodetic Survey Report for 1903. The sweeping bar consists of a 2^-inch galvanized- iron pipe suspended beneath the vessel by vertical piping and suit- ably guyed. As the length of the bar can not much exceed the beam of the vessel, only a limited width can be swept in each course, and it is therefore difficult with this method to completely cover every por- tion of an area. Soundings are made in addition to the dragging, and the sounding record must clearly show the lines run with drag set, and the depth at which set, with reduction to plane of reference. On the sheet the dragged lines are to be indicated by colored lines, using different colors for different reduced depths of drag. 211. When strikes are made they should be entered on the left- hand page of sounding record, and it should be stated clearly whether or not the indication was due to contact with the bottom or to struc- tural weakness of the sweep, Th chief of party should personally GENERAL INSTRUCTIONS FOR FIELD WORK. 57 verify and initial such entries and see that the spots found are suffi- ciently examined, 212. Where special apparatus is not available a drag of some sort should be improvised to search for an important reported obstruc- tion which can not be found by the lead. Two pulling boats may be used, and the principle of the'wire drag should be followed in keep- ing the drag taut by means of weights at each end and the boats towing on courses somewhat divergent. Wire should preferably be used or in its absence rope or light chain, or an iron pipe or bar suspended horizontally may be towed beneath a launch or between two boats. 213. It is obvious that there is no need of dragging over an area in which dangers of less depth than that sought are already located. 214. All records of dragging operations should be kept in sounding record books, and the work clearly explained. 215. In some localities it may be practicable to use divers to ad- vantage in searching for reported dangers, especially in regions such as the Philippines where the natives are expert divers. 216. Position angles. — For locating position of sounding boat the two methods generally used are by theodolite angles on the boat from two stations ashore, and by sextant angles from the boat on three shore signals, or a combination of the two. The former is the most precise, but is not well adapted to surveys of extended areas. 217. The second method is employed in nearly all the coast work, the principles involved being the same as in the location of a plane table in topographic work by the three-point problem. The strength of a determination of position depends directly on the relative posi- tions of the three fixed points and the position sought. There are usually a number of objects from which to select in taking the sex- tant angles, and good judgment is required in making this selection ; some positions of the objects with respect to the observer give strong conditions and some very weak conditions for the angles. 218. A single angle between two fixed points gives the locus of a position as somewhere in the circumference of a circle passing through the two fixed points and the position sought ; this circle may be designated as the position circle. Two angles measured between three fixed points determine the position as at the intersection of three such position circles, passing through each two of the points respectively. The strength of the position depends in part on the angle at which these circles intersect, as they approach tangency the position becomes weak, until the limiting case is reached and the posi- tion is on the circumference of a circle passing through the three fixed points, hereinafter referred to as the " great circle." In this case the three position circles coincide and the position is indeter- 58 COAST AND GEODETIC SUEVEY. rainable and can be plotted only as somewhere on the circumference of the great circle. 219. Whenever the distance between any two of the fixed points is small as compared with the distance from them to the observer, the corresponding position circle will be poorly defined, and the position will be weak even though the intersection with the other circles is good. 220. Based on the above two general principles, the following should be observed in selecting objects for angles. 221. Avoid any selection in which the position sought is on or near the great circle passing through the three fixed points. This is com- monly called a " revolver " and is to be constantly guarded against. In case there is nq choice of signals and a " revolver " is expected, as may sometimes occur inshore near the end of a line, a third angle should, if practicable, be taken to a point of land or other defined object. 222. Avoid a selection in which two of the fixed points are close together as compared with their distance from the observer. 223. A strong position will be obtained with the three objects nearly in line or with the central object nearer than the others and no angle less than 30°. 224. Small angles should generally be avoided as they in most cases give weak positions and also are apt to be inconvenient to plot. 225. There is one case, however, in which a small angle will give a strong position, and that is when two of the objects are nearly in line and not close together and the third object is so located as to give a good angle of intersection with them. The limiting case is where the position sought is in range with two of the objects. But a single angle need then be observed, or a second angle on a fourth ob- ject may be taken as a check. A range should be taken when there is opportunity, but the range points should not be close together. 226. As slight errors in angles affect a position more with distant signals than when near objects are observed, preference should always be given to the latter, other conditions being favorable. The uncer- tainties of plotting due to paper and instruments also make it prefer- able to use near objects. Thus for inshore hydrography it is desir- able that signals on the adjacent shore be used, and not very distant signals, as for instance, those on the opposite side of a bay. 227. When the central object is very close and the other two objects distant, the whole angle between the latter should be observed if jaracticable, or the two separate angles should be taken from the same spot, to avoid the error in position that will otherwise result from angles taken by observers at points slightly apart. 228. If practicable, avoid angles between signals having consider- able difference of elevation, when either is near the observer. GENERAL INSTRUCTIONS FOR FIELD WORK. 59 229. If, in running the sounding line, both angles change slowly the position will be weak. In plotting it should be noted that the position is strong if a slight movement of the center of the protractor throws the arms away from one or more points, and that the position is weak if such movement does not appreciably disturb the relation of the arms to the three points. 230. The time interval between positions will depend on the scale and the character of the hydrography, but on large scale work should seldom exceed three or four minutes. For convenience in plotting and spacing soundings, positions should ordinarily be taken on the full minute, and when possible at uniform intervals. Position angles should, however, be observed when there are sudden changes of depth, and at all changes of course and of speed, and at changes in the time interval between soundings. 231. Where the change of course is considerable, positions should be taken both before the change and after the boat is on the new course, and in such case the track of the sounding boat should be plotted as a curve and not as a sharp angle. 232. In addition to the position at the beginning of the line, posi- tion angles should again be observed when the boat gains full head- way (to be noted in the record) in order to avoid the serious errors in spacing sovmdings on the plotted sheet as a result of the variable speed of the boat. The same holds true when the speed is slowed down on the approach to shoal water at the end of a line, that is, position angles should be taken when the boat is slowed down as well as at the end of the line. The irregular and improbable appearance of depth curves sometimes seen on plotted sheets near the shore may be due to fg,ilure to take account of the changes in speed of the boat near the beginning and end of sounding lines. 233. Positions may conveniently be recorded in the following form, the signals being named from right to left : >y Bet 70° 40' Cat Dog 41° 14' 234. The position number is to be placed immediately to the left of the time at which position was taken, being careful that there is no uncertainty as to which time is referred to. It is important that the time recorded should be that at which the position and sounding Avere actually taken; discrepancies in the hydrography will result from lack of care in this respect. 235. A range is indicated by zeros Avith a line drawn through them, thus: '■y Bet 61° 27' Cat Dog 00 60 COAST AND GEODETIC SURVEY. 236. Buoys and other aids to navigation within the field of work should be determined by special sextant angles. If found to be out of position or unfavorably located, this should be promptly reported, as well as any recommendations as to desirable positions for aids to navigation. 237. The method of locating positions by two theodolites ashore should be used when extreme accuracy is demanded, as in harbor improvement surveys. Although not often employed in general coast work, it may be convenient in some cases. For instance, the signal at the masthead of a vessel may sometimes be distinguished at a greater distance otfshore than the shore stations can be seen from the vessel. The two theodolites are set up at suitably situated tri- angulation stations. All the directions are referred to a known direc- tion as zero, which it will be convenient in plotting to have to the left of any position of the vessel, when the theodolite is graduated clockwise. This zero should be verified, say at the beginning of each page of the record, bj' recording a pointing on the reference object. 238. A time ball or flag is shown from the vessel each time a posi- tion is required, and the instant it is dropped the direction of the foremast of the vessel will be observed at each station, and the time recorded at the two stations and on board. Or observations may be made by time only, in which case occasional signals should be made when practicable for the comparison of clocks. The clocks should be set to agree and compared at the end of the day. 239. Positions for offshore hydrography. — For the survey of an im- portant bank offshore out of sight of objects on land, a sextant triangulation should be carried out from the shore to locate several buoys or beacons placed on the bank to serve as signals during the hydrographic development. For the intermediate stations between shore and bank sailboats may prove convenient, as they can be readily shifted from point to point in a scheme which requires several figures to make the connection. If this method is impracticable, the log and compass can be depended on for moderate distances, and a position on the bank determined by the adjustment of outward runs from a known position combined with that of return runs to a similar position in sight of land. The relative positions of the other signals on the bank can then be determined by courses ajid log distances, as well as sextant angles. When the signals are short distances apart, a run between any two by compass and log should be immediately repeated in the reverse direction to eliminate the effect of current and other sources of error. For long distances the two runs should begin and end respectively with the same phase of tide. The record should be complete as to the compass deviations, log corrections, cur- rents, wind, and apparent drift. The lines should be adjusted and GENERAL, INSTRUCTIONS l^OB FIELD WORK. 61 the soundings plotted by the field party, giving the best practicable interpretation to the work. For long distances offshore the determi- nation of positions by dead reckoning will be supplemented by astronomic observations. Details as to navigational records for this purpose are given in Sigsbee's Deep-Sea Sounding and Dredging (Coast and Geodetic Survey, 1880), and the record forms therein described can be supplied. The complete computa-tions of positions must be shown. 240. A reconnaissance of a bank offshore, where signals can not be seen from a boat, may be made by anchoring the ship and sound- ing with a boat, obtaining the distance from the ship by measuring the vertical angle from the water line to the masthead and taking bearings on the boat with the ship's compass. The height of the mast above the water furnishes a vertical base for plotting the distance of the boat. 241. Soundings with lead and line. — The leadsman should be trained to estimate the probable depth for the next sounding in order that he may pay out an adequate amount of spare line; too much may be more objectionable than too little, as a strong current may catch the bight of the line and thus produce a false sounding. The effort should be to have the lead draw the line taut before it reaches the bottom ; also to have the lead reach the bottom as the leadsman gets over it, or just before the line becomes plumb. The leadsman should then quickly lift the lead off the bottom, and as it touches again read the depth. This is an important precaution for the purpose of straightening the line and keeping the lead vertical; otherAvise a strong current might carry the bight of the line and increase the apparent depth, or the lead is liable to fall over, with the same effect. When there is a swell or the surface of the water is agitated the leadsman should be careful to make an allowance for the height of the waves, so that the reading of the lead line will give the depth from the mean surface. 242. Soundings to depths of about 20 fathoms may conveniently be taken with hand lead and line. 243. Soundings with vessel under way. — When working in moderate depths, and yet beyond those in which it is practicable to sound with a hand lead, there is considerable saving of time and of wear on machinery by using methods which permit the soundings to be taken without stopping the vessel. 244. Trolley rig. — A satisfactory and often used method is that of dropping the lead near the bow and reading the depth as the lead line comes vertical under the leadsman stationed on the quarter deck. With a sounding lead of from 50 to 100 pounds up-and-down sound- ings can thus be obtained rapidly in depths up to 40 fathoms, with speeds up to 4^ knots, without stopping. Various methods are used 62 COAST AND GEODETIC SURVEY. for carrying the lead forward and automatically releasing it. A troUey wire may be rigged along one side of the vessel, with a grade downward toward the bow. The lead suspended from a traveler hung from two grooved wheels is then caried forward, where a pro- jecting bolt on the traveler strikes a rubber surface on a boom, push- ing back the catch holding the lead and releasing it. The lead drops to the bottom, and the traveler is hauled aft again. Another device is described and illustrated in Wharton's Hydrographical Surveying. 245. Deflection scale. — A system of sounding under way with sound- ing machine and wire has been used in moderate depths. An iron weight of 30 to 60 pounds, attached to sounding wire, is employed, the amount of wire out read on a registering sheave, and the angle of deflection from the vertical- of the wire noted on a horizontal scale projecting from the deck. Soundings are made rapidly without stopping, the weight being lifted only a short dis- tance off the bottom and not brought to the surface. The weight dragging neai' the bottom will develop the presence of shoal spots be- tween the soundings. An occasional sample of bottom may be brought to the surface. The correction for deflection of the wire is — 1 (/ — cos a) where I is the inclined length of wire and a is the angle of deflection from tlie vertical, supposing the wire to be straight. This method of sounding has been used to advantage only in moder- ate depths and at moderate speeds. In greater depths the angle of deflection will become too great, and the curvature of the wire will introduce difficulties in the correction. 246. A modification of this method has been used in greater depths, up to 50 fathoms. Soundings were taken when the headway of the vessel was reduced sufficiently to keep small the correction for in- clination of wire. As soon as the angle is reduced to the desired limit it is read, the lead is dropped, and the instant it strikes bottom the registry dial is read and the reading recorded. The advantage of this over up-and-down soundings is that less reversing of machinery is required, and that the vessel, retaining some headway, is under better control and the proposed sounding lines can be more easily followed. 247. In machine sounding in moderate depths where vertical casts are obtained there may be some saving in simply lifting the lead a short distance off the bottom and going ahead without reeling in, except where a samjjle of bottom is desired. 248. Pressure tubes. — Pressure tubes are successfully used for sound- ing from a vessel under way, and where there is a large area ranging in depth from l.T to 90 fathoms they should be employed. Appli- ances employing the overflow device, where the depth of the sound- ing is indicated on a scale which measures the height of the water trapped in the outer glass tube, have proved convenient and reliable GENERAL INSTRUCTIONS FOR FIELD WORK. 63 for surveying work. Special precautions must be taken when using these to see that the orifice for draining the tube is clear and that all the water in the tube is shaken out before closing the valve for the next sounding. Glass tubes having their interior surfaces spirally ground also give reliable results. After a sounding has been made with one of the latter kind and the depth read off by means of the scale, the tube should be well dried. To effect this one end of the tube is placed in the mouth and air sucked through. Also it should be occasionally dipped in fresh water to prevent an accu- mulation of salt. To avoid delay a number of tubes should be kept at hand and used in rotation. Tubes lined with a chemical prepara- tion can only be used for a single sounding, except when it is known that there is an increase in depth in successive soundings. The chemically prepared tubes as supplied by the trade are usually of very small bore and, consequently, liable to obstruction by some minute objects. 249. Appliances which include springs and pistons in connection with pressure tubes are not considered sufficiently accurate for sur- veying work. All forms of pressure tubes should receive a prelim- inary test for accuracy before being employed. 249. Sounding machines. — The Cosmos hand sounding machine may be used successfully for up-and-down soundings to depths of 400 fath- oms, using No. 21 steel wire Brown and Sharp gauge and about a 25-pound lead. When sounding under way in moderate depths, a stranded steel wire will be more durable and should be employed. Other small sounding machines may be used when available, such as the Kelvin navigational machine, or the Tanner machines. In all cases it is preferable to use a separate registering sheave, such as the Tanner, for reading the length of wire out, instead of the dial on the reeling drum which is subject to correction depending on the amount of wire on the drum. The ordinary sounding record books may be used for work with these machines; the time required to reach bottom should be recorded for the deeper soundings as a useful check. For description of the Sigsbee deep-sea sounding machine and explanation of its use, reference should be made to Tanner's Deep-Sea Explora- tion (United States Commission of Fish and Fisheries, 1897), and to Sigsbee's Deep-Sea Sounding and Dredging (Coast and Geodetic Survey, 1880). A special form of record, " Soundings with wire," is now available and should be used for deep-sea sounding. 250. Sounding records. — All sounding records must be complete and intelligible, and the chief of party must personally see that the record is being kept in a systematic and careful manner. Many things which are jDerfectly clear to an observer, having the work fresh in his mem- ory, may not be so to a stranger, hence the necessity of making com- plete notes with each day's work, and recording everything essential 64 COAST AND GEODETIC SURVEY. to a fomplete understanding of the record. All uncertainties and doubtful places should be carefully investigated before leaving the field. 251. Sounding record volumes must, as far as practicable, be kept separate for each hydrographic sheet, and numbered in separate series. It is inconvenient in plotting and filing records to have in one volume soundings that go on different sheets ; to avoid this, where pro- jections are not furnished, the scheme of sheets should be planned in advance as far as circumstances Avill permit. 252. Identification letters and numbers. — In order to aid in the iden- tification of sounding records, hydrographic projections sent from the office will be designated by a temporary number, and those made in the field should be assigned a letter, and these field numbers or letters, marked plainly in pencil, should form part of all sounding records, descriptive reports, etc., pertaining to each sheet respectively. 253. At Manila, Philippine sheet numbers will be assigned to each field jiarty at the beginning of the season, and the records and reports must be systematically marked in ink with the corresponding sheet numbers. 254. Information notes. — xVt the beginning of each day's work enter in the sounding book the time the party left the vessel, or the vessel left the anchorage ; the distance to the field ; the fact that the sex- tants, clock, and lead lines have been examined and were correct, or the corrections, if any; the names of the observers, recorder, and leadsmen, and should any of these be relieved during the day a note should be made in the column of remarks at the time it occurs. If there are two observers, state which takes the right and which the left angle. Should there be any correction or fact recorded later, which should be known before commencing the plotting of the day's AYork, a note calling attention to it should be inserted at the begin- ning of the day's record. 255. At the close of the day's work note again the examination of • sextants, clock, and lead lines, and their corrections, if any, the time of returning to the vessel, and the distance from the working ground. 256. In the division of work between the two observers it will be well for one to supervise the steering of the boat and the plotting, and the other to watch the correctness of the leadsman and the recorder. 257. Standard time is to be used in all records, and so noted in the column " Time " at the beginning of each day. 258. Any information that will be of value in plotting the sheet or in explaining the hydrography should be noted in the remark col- umn, as, for instance, the force and direction of the wind, the state of the sea whether rough or smooth, the force and direction of the GENERAL INSTRUCTIONS FOR FIELD WORK. 65 current, the bearing and estimated distance of any object passed by the boat and which is or should be plotted on the projection, and the time of crossing the range of two well-defined objects. The time of changes in wind or current should be noted, as well as eddies, tide rips and their trend, whirlpools, etc. When, owing to surf, or other dangers, a sounding line can not be run to the shore, explanation should be given in the record. 259. Special care should be taken that sounding records are com- plete in the following respects : (a) In remark column the relation of beginning and end of line to some object should be given approximately, as " line begins about 300 meters N. 30° E. from A Tree;" "line ends 25 meters from reef, N. from © Eun." (h) The course should be noted at beginning of each line, and when changed the time of change and direction of the course should be indicated, as C. C. N. 56° E. In the new form of sounding record the ship's or boat's head as read by compass should be entered in the left column on the right page, and the course intended to be made good should be written in the right column. (c) A reference mark should be made against every sounding or time to which any note refers. (d) When stops are made, the " ahead " time should be noted, as well as any change of speed. (e) For every line beginning or ending near the shore, the esti- mated distance in meters to the shore, reef, or breakers must be stated, and for every important object passed in a sounding line, as rock awash, breakers, buoy, etc., the estimated distance and bearing must be noted, or when not otherwise determined an additional sextant angle should be taken to it from two or more positions. 260. Courses, bearings, and directions should, so far as practicable, be stated in degrees rather than in points, and whenever there is a possibility of confusion a statement should be added as to whether magnetic or true ; the use of degrees in preference to points is recom- mended for all purposes on surveying vessels. 261. Name of tide gauge to be used in reduction should be entered at the heading of each day's work. 262. The first page of a volume of soundings should contain an index of signals determined and an index of currents noted; also a special reference to any other important information contained in that volume, giving in each case the page reference. 263. Duplication. — Sounding records should not be duplicated, ex- cept when specially directed, or when there is considered to be an unusual risk in forwarding records. A good security against loss 53695—08 5 66 COAST AND GEODETIC STJBVEY. Avill be to plot the sheet as far as practicable, and to forward the rec- ords and sheet at different times: 264. Depth units. — Soundings will in general be recorded in fathoms and even feet; only in exceptional cases neea fractions of feet be recorded. Deep-sea soundings need be recorded in even fathoms only. Leadsmen should be trained to call soundings in the same terms in which they are to be recorded. 265. " No bottom " soundings are not satisfactory, and where prac- ticable the depth should be obtained. They are quite objectionable in harbor surveys. 266. Character of bottom. — The sounding record should show the character of the bottom at the top of each page and at each change rei^orted by the leadsman, by the usual abbreviations used on the charts, which are as follows: M, mud; S, sand; G, gravel; Sh, shells; P, pebbles; Sp, specks; CI, clay; St, stones; Co, coral; Oz, ooze; bk, black ; wh, white ; rd, red ; yl, yellow ; gy, gray ; bu, blue ; dk, dark ; It, light; gn, green; br, brown; hrd, hard; sft, soft; fne, fine; crs, coarse; rky, rocky; stk, sticky; brk, broken; Irg, large; sml, small; stf, stiff. 267. It is particularly important that information as to the bottom be given for harbors and anchorages. The information given by the sounding lead may be somewhat superficial, and when convenient a useful check is furnished by the actual experience in anchoring, and the material brought up by the anchor, which should be noted. 268. In the record of soundings, one line should be omitted after the sounding on which a position was taken, and about four lines between the end of one line of soundings and the beginning of the next line. 269. The times of soundings and positions should be carefully re- corded, as they are used in spacing the soundings. The time the boat starts or stops is required, although the angles may be taken earlier or later. When under way, if no sounding is taken on the position, leave that part of the line blank in the record. 270. Corrections. — Erasures should not be made in records. Mis- takes discovered may be crossed out and corrected by writing above or to one side, with explanation, if any. Full explanation must be written in the record if any work is rejected. 271. The recorder should promptly call attention to any unusual sounding; if it is confirmed it should be marked " O. K." 272. The success of the hydrographic work depends directly on the correctness and clearness of the record ; the recorder must make sure that he hears and records every fact properly and that the record is complete, and must not hesitate to ask for repetition when necessary. He should call back the figures as entered. GENERAL INSTKX7CTIONS FOE FIELD WORK. 67 273. To save space in plotting upon the sheet, each day's work is known by a letter. The vessel and each boat should have a separate series, distinguishing them by using capitals of one color for the ves- sel and loWer-case' letters of another color for each boat, these dis- tinctions to be preserved in the books, on the sheets, and in the table of statistics. For convenience of reference the letters used in each book should be given on the outside of the covers in the upper colors. 274. When the alphabet has been exhausted for day letters, use double letters or primes, as AA or A'. Red, blue, and green are the best colors to use; black should not be used, as this would obscure the soundings. 275. When a sounding machine of any kind is used the record should clearly state the kind of machine, manner of making sound- ing, and correction to machine or registering dial, and how correction was obtained. 276. Reduction of soundings. — The reducers, or tide corrections to be applied to the soundings, will be taken out in the tide record for the hours during which soundings were made, with correct sign, by sub- tracting the tide guage reading from the plane of reference reading. 277. The reducers, or tide corrections, will be entered in the sound- ing record to tenths of feet. The correction for the lead line, also to tenths of feet, must be applied at the same time as the tide reduction, but the lead line correction may be omitted if not exceeding one-half of 1 per cent of the depth. The reduced soundings will be entered to the nearest foot, ordinarily omitting decimals. For soundings over 50 fathoms the tide reduction may be omitted unless the range of tide exceeds 2 per cent of the depth. In plotting or verifying the hydro- graphic sheet the draftsman may, when necessary for important crit- ical soundings, make the reduction so as to take account of the frac- tion of a foot. In taking out the tide reducers and in verifying the reduction of soundings uncertainties of two or three tenths of a foot may be neglected. 278. The record must show, by initials at the end, by whom reducers were entered and soundings reduced, and by whom each of these op- orations was checked. 279. Plane of reference. — The plane of reference adopted for the reduction of soundings and the publication of the charts of the Coast and Geodetic Survey is as follows : 280. For the Atlantic and Gulf coasts of the United States and Porto Rico, the mean of the low waters. 281. For the Pacific coast of the United States, Alaska, the Ha- waiian Islands, and the Philippine Islands, the mean of the lower low waters (except for Puget Sound, where the plane is 2 feet lower, and for Wrangell Narrows, 3 feet lower than the mean of the lower low waters). 68 COAST AND GEODETIC STJEVEY. 282. For the derivation of the above planes, see under " Tidal observations." 283. Plotting hydrographlc sheets. — On boat sheet or field hydro- graphic sheet positions should be plotted, and sufficient soundings should be plotted to keep general track of the work and to see that the area is properly covered. As the work progresses soundings on cross- ings, and all soundings showing unusual or dangerous depths at critical places, should be approximately plotted even without tidal reduction, so that immediate examination may be made, before leav- ing the field of work, of doubtful or discordant points and of spots that give indication of danger to navigation. 284. Depth curves drawn on the boat sheet or tracing of smooth sheet as the work progresses afford a valuable means of detecting doubtful spots and inconsistencies in the work. 285. As promptly as possible field parties should complete smooth hydrographlc sheets, plotting the positions in ink. When owing to the depth or other cause soundings are so widely placed that all can appear on the sheet, they should be plotted in the field, otherwise they should be omitted. In no case should soundings be plotted in ink by the field party. 286. Necessary details on completed sheet. — Every original hydro- graphic sheet when completed must contain the following: (a) Projection in black ink, fine full lines, the latitudes and longi- tudes on each end of each parallel and meridian; a note at bottom giving the latitude and longitude, with seconds in meters, of some one triangulation station. (b) Triangulation, plane table, and such other points as may have been determined or established by the hydrographlc party must be plotted, each with its distinctive sign and name. Use red ink for topographic and blue ink for hydrographlc positions of signals. Large buildings and prominent landmarks determined in connection with the hydrography should be indicated on the hydrographlc sheet and designated by appropriate legend ; if necessary, a reference letter may be used and the legend placed where there is more room. (c) The shore line must be drawn on the sheet in a continuous black line if it has been surveyed by a topographic party ; if sketched in by a hydrographlc party, it is to be indicated by a broken line. The high-water line and all information outside of it should be trans- feried from the topographic sheet ; the low-water line and other features outside of high-water line should, however, be left in pencil until the hydrography is plotted, when the information should be combined, in general giving greater weight to low- water line as devel- oped by the soundings. The low-water line should be indicated by dotted line, as far as determined. The area between high and low water should not be sanded. GENERAL INSTRUCTIONS FOR FIELD WORK. 69 (d) The soundings on the finished sheet should be in pencil, with the positions, letters, and numbers, in colored ink. Minus sound- ings, -which represent the heights above the plane of reference of areas bare at low water, should be given with the minus sign and inclosed within the dotted low-water line. (e) Rocks, reefs, coral, and shell banks, sunken or awash, must be marked with the proper signs. Where the least depth over a submerged rock is obtained, the depth should be shown, with the word " Eock " or " Rk." Do not use symbol for sunken rock in such case. (/) The positions of all buoys, light-vessels, etc., must be given with their proper signs. (g) Bottom characteristics should be noted on the sheet at moder- ate intervals, to give the information contained in the record; it is particularly important that sufficient information be given in an- chorages and channels, and a good test of the sufficiency of the data in such areas would be to draw pencil curves on the sheet to see if the areas of various materials are defined; this would also be valu- able later in selecting the bottom characteristics to be placed on the chart. The standard abbreviations are to be used. (See paragraph 266.) (A) The limits of grass, kelp, etc., must be indicated. If the bot- tom is grassy, it is to be so written. Kelp must be marked with its proper sign. («') The names of islands, points, rocks, reefs, shoals, banks, chan- nels, creeks, etc., must be given on the sheet. Care must be taken to obtain these names correctly. Names should, as far as practi- cable, be placed on the land area, leaving the water area clear. Let- tering should not be allowed to obscure soundings. (j) All ranges, bearings for dangers, etc., and sailing lines on courses or ranges should be given and drawn as follows : The range in black lines broken with long dashes; the bearings in black dotted lines; and the sailing lines in black lines broken with short dashes, with the positions of the objects for ranges and bearings determined, marked, and named, and the names of the objects and the purpose of the range or bearing written along its line. (k) Current stations and tidal stations must be plotted in position. (l) Titles should not be inked on original sheets by the field party, but must be furnished plainly written in pencil on the sheet or on a sheet of paper pinned to the sheet. The information for the title must include the general locality, special locality, names of persons actually in charge of sounding, and of chief of party, vessel, dates of beginning and ending, and scale. In the Philippines the stamped title form should be filled in ink on the sheet or on a slip pinned to the sheet. The title of a hydrographic sheet must clearly indicate 70 COAST AND GEODETIC STJEVEY.' the limits of the hydrography, and the same title must be given on the record books pertaining to it. 287. Table of statistics. — (m) A table of statistics should be made as the sheet is plotted and transmitted with the sheet. This table may be written on computing paper and should be in the following form : Statistics sheet Xo. Date, 1903. Letter. Vol. Posi- tions. Sound- ings. Miles, statute. Vessels. a 1 164 1,309 24.8 Launch. 7,4SS 53,981 950.8 (h) There must be a note stating the unit for soundings (fathoms or feet) and the plane of reference. Also a tidal note, giving the location of the gauge, and if there was more than one tide gauge, for what parts of the sheet each was used ; also the following information : Plane of reference, reading on gauge. Lowest tide observed, reading on gauge. Highest tide observed, reading on gauge. 288. Depth curves. — (o) The depth curves must be drawn on the sheet, and each curve should include the outer soundings of the depth represented by the curve. When curves run so close together as to confuse the sheet, the less important, or those representing greater depths, may be dropped. Curves must not be completely drawn where the information is insuificient, but parts of curves or curves with broken line may be put in. The field party should leave the curves in pencil. When the sheet is verified the curves will be inked will full colored lines, according to the following scheme : 6-foot or 1-fathom curve Green. 12-foot or 2-fatbom curve Red. 18-foot or 3-fatliom curve Blue. 24-foot or 4-fathom curve Yellow. 30-foot or 5-fatliom curve Red. 36-foot or 6-fathom curve Green. 60-foot or lO-fathom curve Yellow. 120-foot or 20-fatliom curve Blue. 300-foot or 50-fathom curve Red. 600-foot or 100-fathom curve Green. 1,200-foot or 200-fathom curve Yellow. 6,000-foot or 1,000-fatbom curve Blue. GENEEAL INSTETJCTIONS FOR FIELD WORK. 71 The attention of all engaged in hydrographic work is especially called to the value of depth curves in interpreting and examining the results of the field work, and such curves should be drawn and carefully studied on boat sheets, as well as on finished sheets. The depth curves will often indicate areas of possibly shoaler depths which should be further examined. Also abnormal and improbable curves are a strong evidence of probable uncertainties or inaccuracies in the hydrographic STirvey. Depth curves correspond to contours on land, and in nature are therefore generally of graceful sweeping form, free from sudden changes in direction and from corners ; ordi- narily they can not cross or abruptly run into each other; on approaching they tend toward parallelism; any departure from probable natural conditions is an indication of error either in field Avork or in plotting. A study of the characteristic bottom forms in any region is of value in the interpretation of hydrography, as such forms are apt to repeat themselves under similar conditions. (p) The sheet should be signed by the draftsman and by the offi- cer responsible for the hydrography, if it is completely finished under his direction. 289. Comparison with previous surveys. — In plotting comparison should be made with the results of previous surveys, and with charts covering the same region, if available, especially as to all dangers or less depths shown on previous surveys. 290. All remarks, comments, etc., in sounding records should be carefully noted in plotting; abrupt changes in depth should be veri- fied by checking tide reduction, etc. ; boat sheets and descriptive re- ports should be examined and compared to see that all essential information is on the smooth sheet. 291. All important doubtful points which arise in the plotting of the. sheet, due either to uncertainty in the record or insufficiency or imperfection of development, should be called attention to by note on the face of the sheet, with any explanation needed. 292. Ciiaracter of drafting. — The drafting work on the finished hydrographic sheet requires accuracy, neatness, and legibility, and of course good judgment and knowledge of the work, but it does not re- quire an expert draftsman, and the plotting should never be deferred because of lack of skill with the pen. 293. Marking positions. — As each position is plotted on the sheet a point should be pricked through to show its exact position, and this point should be marked with a light dot of colored ink (small circles should not be made). 294. Each position must be nvimbered and the number placed just below and to the right or left of the position; the position numbers must be small and so placed as not to interfere with the soundings. 72 COAST AND GEODETIC SURVEY. 295. The letter of the day's work must be placed at the beginning and end of each line, at about every fifth position on the line, and at the point of any decided change of direction in line. 296 The color of the position, day letter, and number must be the same as the color given the vessel or boat in the sounding record. 297. Distinctness of important features. — It is important in plotting hydrographic sheets that the more important features, such as rocks and least depths on shoals, shall be perfectly clear and distinct, and great care must be taken not to obscure them by attempting to plot all of the numerous soundings that may have been taken for the de- velopment of such a feature. If for any reason an important feature is not clear on the finished sheet, or is so shown that there is a likeli- hood of its being overlooked, a note should be added calling attention to it. 298. Selection of soundings. — ^Where the number of soundings taken is greater than can be plotted on the sheet, they will not be plotted in the field ; those showing greatest and least depths and changes of slope must be shown, the selection being such that a cross section could be drawn from it showing all important features; in general as many soundings should be plotted as is consistent with clearness ; in no case should a mere mechanical selection be made, as, for instance, every third or every fourth sounding. 299. Enlarged scale for complicated areas. — It is sometimes difficult to properly plot the soundings to show the development of a com- plicated area on the scale of the general hydrographic sheet. In such cases an enlargement of the plotted positions may be made and the soundings plotted on the enlargement, which may appear on the sheet as a subplan. The enlargement should be to some even decimal scale, and the scale should be stated on the plan. The curves at the margin of the subplan should be reduced and transferred to the main sheet to make sure that the work is consistent. 300. Overlap of sheets. — For adjacent hydrographic sheets the curves and soundings should be common for a narrow strip, and this overlap should be accordant on the two sheets. 301. Dangers and stage of tide. — Definite information should be given on the sheet as to dangers which show at various stages of the tide, as so many feet above low water, awash at low water, awash at high water, breaks at half tide, breaks in heavy weather only, and the like. "Awash " should always be qualified by the stage of tide at which it occurs, and the mere use of the symbol for rock awash will not be sufficient for any important danger. 302. Tide rips should be indicated on the sheet by words, qualified as heavy, moderate, or light. GENEBAL INSTETJCTIONS FOR FIELD WORK. 73 303. Depth units. — All soundings will be plotted in feet only, except that on offshore sheets in deep water fathoms may be used. The whole of any one sheet must be in one unit. 304. On sheets plotted in feet no fraction of feet will be shown, fractions of less than 0.8 being omitted, and those of 0.8 or more being written as the next whole foot, except that in critical places on navigable bars and in channels fractions (|, ^, and |) may be shown where important, and also except that on outlying dangers all frac- tions shall be omitted and the next lower foot shall be given. 305. On sheets plotted in fathoms, quarter fathoms may be used up to 7 fathoms, and half fathoms from 7 to 10 fathoms. 306. In converting fractions the following Avill be observed : When plotting in even feet omit all fractions of less than 0.8, and those of 0.8 or more write as the next whole foot; when plotting in quarters take 0.1=0, 0.2=^, 0.3=i, 0.4=4, 0.5=i, 0.6=^, 0.7=f, 0.8=f, 0.9=1; when plotting in halves, take 0.1 to 0.3 as 0, 0.4 to 0.7 as ^, and 0.8 to 1 as 1 ; when converting from feet to fathoms and quarters, take less than 1 foot as 0, 1 foot and less than 2.5 feet as ^ fathom, 2.5 feet and less than 4 feet as J fathom, 4 feet and less than 5.5 feet as f fathom, and 5.5 feet and over as 1 fathom ; when converting from feet to fathoms, for less than 4.5 feet drop the fraction, for 4.5 feet and over take the next whole fathom. 307. Defining reef limits. — The limits of reefs as located by the hydrography should be fully marked on the sheets in the field. The danger limit of rocky bottom having some depth of water, but which can not be investigated in detail, should be indicated by the sunken rock symbol. The coral-reef symbol should be used to indicate the extent of coral reefs either bare or awash at low water. 308. Blunders and omissions. — Where from any reason but a single angle is available (as when a blunder has been made in reading one angle) a line of position may be plotted by setting the angle on a pro- tractor and plotting several points in the vicinity of the work. The boat must have been at some place on the line drawn through these points, and its location can be fixed by the intersection of this line with the course made good, or by plotting on it the distance from either the preceding or succeeding position according to the time interval. If two angles have been observed, but without a common object, the two lines of positions may be plotted separately and their intersection will be the position of the boat. 309. Blunders in angles or record may sometimes be detected by estimating the position from time and course and testing the angles with the protractor. No arbitrary deviation from the record should be made, however, unless it is reasonable and supported by other evi- dence. Such cases, or rejection of any portion of the record, should 74 COAST AND GEODETIC SURVEY. be noted in the column of remarks with reason therefor, and this statement signed. 310. North the top of sheet. — In plotting and inking original sheets, north shall be taken as the top, and titles, names, soundings, and sig- nals shall be put on normal to the meridian, regardless of the direc- tion of the borders of the sheet, except where it is desirable that names be lettered to conform to geographic features. In such cases the names shall be inked so as to be read when looking north. Names should by their direction and proximity clearly indicate the object designated. 311. Very large sheets should be avoided in plotting hydrography, being inconvenient to handle both in office and field. The standard size of topographic sheet is 31 inches by 53 inches. Somewhat larger sheets may sometimes be necessary for hydrography, but they should not exceed 42 inches by 60 inches. 312. For smooth hydrographic sheets, Watman's paper is furnished mounted, of size 31 inches by 53 inches, "\^^len larger sheets are required backed drawing paper of the best available quality should be used. 313. Fragmentary sheets for small pieces of work should be avoided ; such information may often be placed as a subplan on an- other sheet in the vicinity, separated by a border and with subtitle. 314. For boat sheets a good quality of mounted paper should be used, and a paper with brownish tint has been found very satisfac- tory. For steamer work, where the finished sheet is kept plotted up to date, and may be well cared for, the use of a boat sheet may be avoided, which will save duplication of plotting. 315. Thin transparent celluloid has been used advantageously for boat sheets ; one side of this material should have a dull finish so that it may be written upon with a pencil. The celluloid is laid over the smooth sheet and the signals marked. In the boat the celluloid is used over a sheet of paper ; at the end of the day's work the celluloid may again be laid over the smooth sheet and adjusted in position l)y the determined points, and the positions pricked through, ordinarily saving replotting. 316. The boat sheet, if one was used, should always be forwarded to the office, to assist in the final plotting. 317. The distances that will be included on a sheet of given size and scale may readily be obtained from the following table of scale equivalents, by dividing the length or width of the sheet by the length of 1 mile on the given scale. For instance, a sheet 42 inches by 60 inches on scale jo^^yo will include an area 11.5 by 16.5 nautical miles. GENBKAl, INSTRUCTIONS FOE FIELD WOKK. 75 Scale. Nautical mile. Statute mile. Inches. Centi- meters. Incites. Centi- meters. btjSo 14.693 37.06 12. 672 32.19 Tntw 7.296 18.53 6.336 16.09 Tsicp 4.864 12.36 4.224 10.73 uCT^nn 3.648 9.27 3.168 8.05 snW 2.432 G.18 2.112 5.36 toJdj: 1.824 4.63 1.684 4.02 BirJuij 1.459 3.71 1.267 3.22 QOffiro 1.216 3.09 1.066 2.68 aBOTTO 0.912 2.82 0.792 2.01 iiijj'bbif 0.730 1.85 0.634 1.61 2iI0\fiTD 0.365 0.93 0.317 0.80 ?5nBTHJ 0.182 0.46 0.158 0.40 tbbShiht 0.073 0.18 0.063 0.16 318. Manipulation of protractor. — In plotting positions it is well for the sake of rapidity to have a uniform practice in placing the pro- tractor. It is usually preferable to place the central arm on the central object, with the right and left arms about equally distant from the corresponding objects; keeping the central object on, push the instrument up, reducing the distances on either side equally until all three arms are on. 319. For plotting angles Avhere the three-arm protractor can not advantageously be used, either because the angles can not be set off or the positions fall under the frame, the Court celluloid protractor should be used. This is more convenient than using tracing paper. 320. Spacing soundings. — In plotting soundings the space between the plotted positions should be divided (using the convenient stand- ard spacing dividers) according to elapsed time, and the soundings placed at positions indicated by their time. Where there is any distinction, the more reliable system of lines should be plotted first. The center of the figures is the position of the sounding. 321. Where considerable change of course is made and soundings are continued with the vessel or boat under way, allowance must be made in plotting for the curve made in turning and the fact that there is an appreciable interval before the vessel is on the new course. 322. The following are some of the errors and blunders which occur in hydrographic work, and which should be carefully guarded against in the field work and in plotting: Errors of sextant, lead line, and clock ; reversal of angles in record ; confusion of numbers of simi- lar sound, as seven and eleven; failure to note variations of speed and course ; sextants read 5 to 10 degrees out ; error in plane of ref- erence or tide reduction ; tide gauge not well located for hydrography ; confusion of signals ; blunders of leadsmen in calling depths ; uncer- tainty in soundings at sudden changes in depth. 76 COAST AND GEODETIC STJEVEY. 323. Lead lines. — For hand lead lines hard-laid sash cord has been found a good material. To avoid large corrections to soundings it is desirable and convenient to have the lead line as nearly correct as practicable. The following method has been found to give a fairly constant lead line: First, each lead line should have its own sized lead, and not be subject to different tensions from leads of dif- ferent weights; second, before marking, let the line, with lead at- tached, drag after the vessel for several hours a day for two or three days, and afterwards keep the line soaked in salt water until marked ; third, mark the fathoms with line suspended vertically with lead swinging free at the bottom, laying off the marks with a steel tape; the intermediate marks can be put in with line extended on the deck, averaging the spaces. 324. Verification. — The lead line must be verified at the beginning and end of each day's work, and the corrections recorded in the sounding record or a statement entered that lead line is correct. In verifying the line care should be taken to apply a pull equal to that of the lead in water. 325. Permanent marks may be placed on a deck or a wharf with copper tacks, and the verification of lead line can then be quickly accomplished. 326. The record in the sounding book of the comparison of lead lines should be so explicit as to avoid any possibility of error in ap- j>lying the correction to soundings, and the following form is rec- ommended : Mark on lead line =M. True length on tape or standard = L. Correction to soundings =L-M. Ifm. 2fm. 3fm. 5.8 ft. 11.9 ft. 18. 1 ft. -0.2 ft. -0.1ft. +0.1 ft. 327. The minus sign indicates that the lead line is too short, so that the depths obtained with it appear too large, and the correction to the soundings is subtractive. The plus sign indicates that the lead line is too long, so that the depths obtained with it appear too small, and the correction to the soundings is additive. 328. The lead-line correction may be neglected if not exceeding one- half per cent. 329. Lead lines are marked as follows: 1 fathom. — A piece of leather with one strip. 2 fathoms. — A piece of leather with two strips. 3 fathom.s. — A piece of leather with three strips. 4- fathoms. — A piece of leather with four strips. fathoms.- -White rag. GENERAL INSTRXJCTIONS FOR FIELD WORK. 77 6 fathoms. — A piece of leather with one strip. 7 fathoms. — Eed rag. 8 fathoms. — A piece of leather with three strips. 9 fathoms. — A piece of leather with four strips. 10 fathoms. — A piece of leather with a hole in it. 11 fathoms. — A piece of leather with one strip. 12 fathoms. — A piece of leather with two strips. 13 fathoms. — Blue rag. H fathoms. — A piece of leather with four strips. 15 fathoTns. — Same as 5. 16 fathoms — A piece of leather with one strip. 17 fathoms. — Same as 7. 18 fathom,s. — A piece of leather Avith three strips. 19 fathom,s. — A piece of leather with four strips. W fathoms. — Two knots. 330. Up to 5 fathoms the line should be marked with small white cord for every foot, the half-fathom mark being distinguished by a cord with a knot, and this designation for half fathoms should con- tinue to 10 fathoms. 331. Sounding poles instead of lines may be used in shoal dejDths; when the bottom is soft a disk should be placed on the bottom of the pole. 332. Sextant glasses. — A sufficient supply of spare sextant glasses should be kept on hand. When the glasses become unserviceable they should be returned to the office. Sextant mirrors are expensive and precautions should be taken against their being lost, broken, or scratched. 333. In case of emergency sextant glasses may be resilvered in the field by the following method: The necessary requisites are tin foil and mercury. Lay the tin foil, which should' exceed the surface of the glass by a quarter of an inch on each side, on a smooth pad of paper; rub it smooth with the finger; add a drop of mercury about the size of a small shot, which rub gentlj' over the tin foil until it spreads itself and shows a silvered surface ; gently add sufficient mer- cury to cover the leaf, so that its surface is fluid. Prepare a slip of clean tissue paper the size of the tin foil. Brush the surface of the mercury gently to free it from dross. Take the glass, previously well cleaned, in the left hand and the paper in the right. Lay the paper on the mercury and the glass on it. Pressing gently on the glass, withdraw the paper. Turn the glass on its face, and leave it on an inclined plane to allow the mercury to flow off, which is accelerated by laying a strip of tin foil, as a conductor, to its lower edge. The edges may be removed after twelve hours, and in twenty- four hours give it a coat of varnish made from alcohol and red seal- ing wax. Spare sextant glasses are now furnished with each sex- 78 COAST AND GEODETIC SXJEVEY. tant. ^ATien they become unserviceable they can be returned by mail to the office to be resilvered. 334. The mercury-tin amalgam while less readily affected chemi- cally, is more liable to mechanical injury than silver, and caution is therefore necessary in handling the sextant glasses. 335. Dangers previously reported. — Existing charts and publications must be carefully compared with the development of the field work. Should a rock or shoal previously indicated on a chart or mentioned in a publication not be found during the progress of the work, the locality must be so carefully searched and the records must be so com- plete as to show beyond doubt that the rock or shoal does not exist. It must be specially mentioned in the descriptive report, and in this re- port must be given, if possible, the evidence of any one who may be deemed an authority in the matter. No rock or shoal which has found a place on the publications is removed unless it is proved beyond any doubt that such rock or shoal no longer exists. 336. Information must be obtained from all available sources. Pilots, fishermen, shipmasters, boatmen, and others living in the vicinity, or acquainted with the locality, must be consulted, and every place credited with a rock or shoal, even if only by rurnor, must be examined. 337. Blank areas on charts. — Surveying vessels when proceeding to or from the field of work should take opportunity, when it will not materially delay more important duties or interfere with their in- structions, to obtain occasional soundings in areas on the charts where no information is at present given, particularly in the ordi- nary tracks of vessels. 338. Eanges for compass deviations. — Report should be made of ranges of prominent and easily distinguished objects that would be suitable and useful foi* the purposes of determining the compass devi- ations of vessels, in the vicinity of important harbors or anchorages (similar to the ranges for San Francisco Bay given in the Pacific Coast Pilot, and for Manila Bay in the Philippine Islands Sailing Directions). TIDE OBSERVATIONS. 339. Purposes. — Tide observations are made in connection with hydrographic work in order to furnish data for computing the plane of reference, for reducing the soundings to that plane, and for use in making predictions and giving tidal information for the tide tables, coast pilots, and charts, published by the Survey. Tide observations also furnish determinations of mean sea level for use in connection with precise leveling, and give information valuable for other engi- neering and scientific purposes. One or more tide gauges must be maintained in connection with all hydrographic work. GENEBAL INSTRUCTIONS FOR FIELD WORK. 79 340. Location of gauge. — In connection with hydrographic work, it will in general be desirable to set up an automatic tide gauge at some central point, and continue it there throughout the season ; sub- sidiary tide staffs are also to be established in the immediate vicinity of the work, as may be necessary. 341. In selecting a site for a tide gauge, existing facilities and the accessibility of the location to the observer must generally be taken into account. It will be convenient to place an automatic tide gauge on a wharf if a substantial one exists. It is important, however, that the location shall be such that there is free communication with the sea, shelter from storm waves, and deep water close to the position at the low tides. For use in connection with the hydrography on the outer coast, it is advisable to avoid a location for a gauge well inside of a river mouth or shallow estuary, or in any body of water having only a narrow connection with the sea. 342. Diflferences in neighboring areas. — In straits connecting two areas having tides of different ranges and epochs of occurrence, it will usually be found that there is a portion of the strait in which the tide varies rapidly from place to place. For instance, with in a single mile at Hell Gate, East Eiver, New York, the time of tide changes about an hour, and the mean range varies about 1 foot. Similarly, in the channel north of Vancouver Island, British Columbia, there is a difference of about 2 hours in the time of tide, and of about 5 feet in the mean range, within a few miles. There may be an appre- ciable difference in both time and height of tide on the different sides of the same island in an archipelago, for it often happens that rapid changes occur in the tides and currents of such groups. Sometimes the occurrence of a shoal near one end of a rather small detached island will cause the tides to differ considerably on opposite sides of the island, depending upon the location of the shoal with reference to the approach of the tide wave. 343. In large shallow bays, in broad stretches of rivers, or along low sandy shores where the water is shoal changes in thf direction and force of the wind often make considerable variation in the level of the water surface, and two stations differently exposed to the wind may be affected unequally. Instances have been known where lines of soundings have failed to cross by several feet on this account. 344. As a general rule, whenever the soundings fail to cross satis- factorily, owing to the foregoing conditions, establish a tide staff nearer the work, if practicable. 345. Temporary tide staff. — For surveys of off-shore bars and ex- posed channel approaches, where close soundings will be required for which the record of an inshore tide staff will not be sufficient, a tem- porary tide staff should be established, by pumping down a scantling or otherwise. s 80 COAST AND GEODETIC SURVEY. 346. There are four types of tide gauges, known as tide staffs, box gauges, pressure gauges, and automatic gauges. 347. Tide staff. — This is the simplest form of gauge, and must be well secured in a vertical position to a pile or other suitable support. 348. Tide staffs must be graduated to feet and tenths, not to feet and inches, on account of the convenience of the use of the decimal scale in reducing the observations. The numbers and graduation marks should be cut into the wood. The staff should be numbered from below upward, and is properly set when its zero is lightly below the lowest tide likely to occur at the station. With ranges of less than 10 feet, a properly set staff will rarely require more than one figure to express the whole feet of any reading, and the saving in labor of tabu- lations and all subsequent discussions of the observations by having one less figure in each value is considerable. The length of the tide staff should be sufficient to measure the heights of the highest and lowest tides known to occur at the port. 349. Multiple staffs. — Where the range of tide is too great to be measured by a single staff a succession of staff's may sometimes be used along shores with gentle sloi^es. A field glass will usually en- able the observer to read the outer staff. 350. Glass tube. — When the water is rough it is a great advantage to have a glass tube, jDartially closed at the ends by notched corks, secured to the face of the staff. Some floating object should be intro- duced into the tube in order to give definition to the water line Avithin, such as a ball cut out of cork and scorched to blacken it or a little colored oil. 351. Temporary bench. — A temporary bench mark should always be made as close as practicable to a tide staff, so as to enable one to reset the staff in case of accident from storm or other cause, but per- manent benches must also be established to prebcrve the plane of ref- erence. 352. Movable staff. — It Avill sometimes be preferable to use a mov- able tide staff, which may be taken out and cleaned or repainted or removed for use in another locality. A guide board is secured in a vertical position to a pile or otherwise and pieces nailed to it so as to form grooves in which to slide the tide staff, exposing only its gradua- tions. A metal plate is screwed to the top of the guide board, form- ing a stable support for a metal shoulder which is fastened upon the back of the movable staff. This metal plate serves as a temporary bench mark, whose relation to the zero of the graduation must be stated. 353. Box gauge. — This form may sometimes be employed, when the swell renders the use of a simple staff inconvenient or when more accurate readings are required. This gauge consists of a long vertical box, within which a float rises and falls with the tides. GENEEAL INSTEUCTIONS EOR EIELD WORK. 81 354. The float is a copper cylinder about 8^ inches in diameter and 3 inches high. A lead weight is soldered on the bottom or shot intro- duced to give it steadiness in the water, and the top has either a socket or ring in its center, according to the form of gauge. 355. The box should be an inch or so larger inside than the diameter of the float. It must be made tight in all its joints and at the bottom. Where the water has a great deal of sediment the bottom should be slanting. The water is admitted through a single opening in one side of the box near the lowest part where the bottom slants, say a quarter of an inch wide and 4 inches long in a vertical direction. On the outside of the box a board is arranged as a gate or valve to slide in grooves so as to regulate the opening from above. The water inside the box should be at the same average level as that on the out- side, but the short-period waves should just be barely perceptible within. The farther the box extends below the surface of the water the larger may be the openings, as the amplitude of wind waves de- creases rapidly downward. 356. Comparison with staff. — A simple tide staff should be located near a box gauge, and the readings of the two gauges should be fre- quently recorded, to prove that the difference is nearly constant. The line of flotation may be changed by a very small leak in the float, or the entrance of water into the box may be diminished by sand or marine growth, so that the box gauge may not give the true range of tide, and this can only be found out by comparison with staff gauge. 357. Reading indicators. — ^A convenient form of box gauge where the range of tide is not too great is to have a light wooden rod fixed in a socket on top of float and steadied by crosspieces at the upper end of the float box, so as to rise and fall in a vertical line. The rod should be numbered from above downward, zero being at the upper end. The top of the box, or of a board or a piece of sheet metal placed at a convenient height for the eye, is used as the reading point, 358. In another form of box gauge one end of a small flexible wire is attached to a ring in the top of the float, while the wire passes over one or more pulleys and terminates with a counterpoise. An index on the wire passes over a fixed scale, which may for convenience be either vertical or horizontal. 359. The scale of a box gauge is usually that of nature, but some- times it is desirable to either increase or diminish the scale, which can be done by passing wires over drums of different sizes or by means of movable pulleys. 360. Reference to bench. — To refer any form of box gauge to bench mark, proceed as follows: Make simultaneous readings on the box gauge and the staff guage, which must always be used as a check upon the correctness of the indications of the box gauge. The dif- 53695—08 6 82 COAST AND GEODETIC STJEVEY. ference between the two gauges ought to be constant, but owing to the difficulty of reading the staff gauge closely, sensible variations are likely to occur in these differences, and the mean should be taken of a number of such readings on both rising and falling tides. Then B=S-\-D where Z?= elevation of B. M. above zero of box gauge. /S=elevation of B. M. above zero of staff gauge. Z?=box gauge reading=staff gauge reading. 361. The expression " bench mark above zero of gauge " when ap- plied to a box gauge means the plane of the water at the time the gauge reads zero, and not the elevation of any portion of the actual scale of the gauge. In the record book all three of these quantities, B, S, and Z>, should be given, so as to avoid the possibility that a single value might be misunderstood. 362. Pressure gauge — Manometer. — When at work off the coast, a pressure gauge may sometimes be used to advantage on shoals where it is too deep to set up a tide staff. One of the simplest forms of pressure gauge is a German patent, known as the " Manometer." It consists of an ordinary bicycle pump secured to a stand and connected with an air chamber; to this chamber there are attached two steam- pressure gauges of the Bourdon type and some flexible hose. One of the steam gauges retains its dial, indicating pounds of pressure, while the other has a special dial showing depth of water, either in meters or in feet. Both the upper and the lower end of the rubber hose has a nonreturn valve, the lower one being covered with a metal guard. The hose comes in sections, with air-tight couplings, for adjusting its length according to the depth of water at the station. 363. In order to use the manometer proceed as follows: Fill the air chamber with fresh water, through the filling hole in the side, up to the height of the hol^, so that the nonreturn valve in the chamber is half covered. Couple up the tubing and test the connections ; a con- venient way of doing so is to screw on the spare pressure gauge in place of the lower nonreturn valve at the end of the tube, and then pump up. Both gauges show the same pressure, and any leakage will be indicated by a fall of pressure. This leak may be located, so far as the tube is concerned, by passing it through a tub of water. 'Wlien the connections are satisfactory, replace the nonreturn valve on the end of the hose, and the apparatus is ready for use. 364. Attach a heavy weight to the metal guard of the lower valve, to serve as an anchor to retain the valve in its place at the bottom of the sea. A line must be attached to this weight to lower and raise it without straining the hose, and this line should be secured to tlie tubing about once every 5 fathoms of line. When the weight is rest- ing on the bottom, pump up until the gauge ceases to rise, which B GENEEAL INSTRUCTIONS FOE FIELD WOEK. 83 will be when the pressure is equal to that due to the height of water above the lower valve. Any excess of pressure within the tube is equalized by the air bubbling out through the lower valve. A record is then made of the time and readings of both dials on the pressure gauges. Similar readings may be made every half hour, every hour, or at such intervals as may be adopted. 365. Rubber-bag gauge. — Another form of pressure gauge, which was formerly used by this Survey, consists of a strong rubber bag, holding about 6 gallons, connected with a flexible, air-tight tube, having an inside bore about one-fourth-inch, and made in sections, like garden hose. The upper end of the tube has a stopcock, and a steam gauge. The bag is incased in an iron box, which is nearly water-tight, so as to exclude the influence of short-period waves, the same as for a box gauge. The iron box containing the inflated bag is lowered to the bottom, the gauge on board the vessel being read at intervals, the rise and fall of tide being, indicated by change of pressure. 366. Portable aero-mercurial tide gauge. — The distinctive features of this form of pressure gauge are as follows : Fig. 16 shows the driving jar A, which is a closed cylindrical metal vessel 12 centimeters high and 12 centimeters in diameter. By the tube a water enters the vessel and com- /f- ^ presses the inclosed air. The vessel A communicates with the recording ap- paratus by means of an air-tight tube Z, preferably lead, the internal diame llA ter of which may be 3 millimeters. '^ The tube L is tightly connected with the arm B of the U-shaped system, fiq. le. which is again connected by means of a small tube to the arm C. These two arms are partially filled with mercury, and the pressure of the inclosed air is balanced by the pres- sure due to the difference in the heights of the two mercury columns. The change of pressure caused by the change of water level above the diving jar appears then as the motion of the mercury column in tube C. To record this motion tube C is provided with a float, which can be connected with any suitable recording device, such as a standard automatic tide gauge. The two arms of the U system can be made of either glass or iron, and the difference of their diameters will be deter- mined by the range of tide at any particular station, and the scale desired for the record; equal diameters producing a natural scale; B greater an enlarged scale; B smaller a diminished scale. 84 COAST AND GEODETIC SURVEY. 367. For temporary service the diving jar is provided with a heavy lead disk Z>, and allowed to rest on the bottom. But for permanent service it should be securely fixed to some substantial structure like a pier, at some depth below the lowest tide. 368. Automatic or self-registeriiig gauge. — This gauge traces on paper a curve, the record being called a marigram. 369. The gauge has two clocks, the motor clock (Fig. 18, No. 1), which revolves the main cylinder (Figs. 18 and 19, No. 4) once in twelve hours, and the time clock (Fig. 18, No. 2) which mechanically marks each hour upon the marigram. 370. There are three rollers; the supply roller (Fig. 19, No. 3), a round brass rod with flanges at each end, one of which is removable for putting in the paper ; the main cylinder, a hollow drum made of brass tubing about 1 foot in circumference, with twelve sharpened steel pins set at equal intervals around the cylinder near each end, designed to prevent the paper from slipping over the smooth surface of the cylinder as it is revolved once in twelve hours by the motor clock; and third, the receiring roller (Fig. 18, No. 5), a hollow tube of sheet brass, with a small slit running its entire length, and two flanges, one of which is removable, so that the completed tide roll may be removed. 371'. The axle of the hour hand of the motor clock extends through the back of the case, and has a toothed or carrier wheel upon it (Fig. 19, No. 6). The main cylinder has a hinged carrier arm attached to its axis, which can be thrown into or out of the teeth of the carrier wheel (Fig. 19, No. 6), thus making or breaking the connection be- tween the motor clock and the gauge. 372. The pencil screw (Fig. 19, No. 8) is made of phosphor bronze, about five-eighths inch in diameter, with one square thread with a pitch of one thread per inch ; it carries a nut bearing the pencil arm (Figs. 18 and 19, No. 9). The threads at the ends of the pencil screw are turned down so as to prevent jamming of the nut if tides should occur too high or too low to be recorded by the gauge, and spiral springs are attached to each side of the nut so as to make it return to the screw as soon as the tide reverses. Securely fastened to the pencil screw, at its end farthest from the clocks, is the counterpoise pulley (Fig. 19, No. 10), and to this is clamped by means of two nuts the -float pulley (Fig. 19, No. 11). Both these pulleys have right-handed threads cut in their faces, 16 to the inch, to prevent the wire or cord from winding on itself. The key wrench for turning the clamp nuts is kept below the float pulley. 373. Attached to the frame, near the counterpoise pulley is the sliding grooved pulley (Fig. 19, No. 12), fixed on a long axle so that it can slide in and out along its bearings and thus hold the cord as it unwinds opposite the proper thread of the counterpoise pulley. GENEElL INSTRUCTIONS BOS FIELD WOEK. 85 fc_^ ■ 1 ^^ 1 ^BRU r g p flBfis^^ii^l i ' 00 ^^^^HHI^I m Ml |^n| C l^nl Ol a P Hr (D ^M 1 ll__.^.:;/ ,.:..:. . 86 COAST AND GEODETIC SURVEY. GENERAL INSTRUCTIONS FOR FIELD WOIIK. 87 374. On the axis of the receiving roller is a pulley (Figs. 18 and 19, No. 13) provided with a pawl and ratchet. On this is wound the cord attached to the tension weight to roll up the paper on the receiving roller. The tension weight is the smaller one of the two weights furnished with the gauge. 375. The datum -pencil rod of brass (Fig. 18, No. 14) runs parallel to the main cylinder and on the side opposite to the pencil screw, upon which the datum pencil (Figs. 18 and 19, No. 15) may be moved to any desired position and clamped. 376. Upon projections of the frames supporting this rod there is a metal strip (Fig. 19, No. 16) on which is graduated four different scales, one on each edge. When in use that scale is turned toward the main cylinder, which corresponds to the float pulley on the gauge. At the side of the wooden base on the gauge there is a piece of metal cut out something like a broad, two-pronged fork (Fig. 19, No. 17), the outer edges of which are straight and designed for carrying the scale reading to the tide curve. 377. The hour hreak device is as follows: The time clock is like an ordinary striking clock, but instead of striking a bell it moves a lever (Fig. 19, No. 18, and Fig. 20, No. 2) connected with a trip- ping rod (Fig. 18, No. 19, and Fig. 19, No. 5), which is supported on rocker arms about 2 inches above the metal scale. The pencil arm is jointed and provided with a hook which passes under the trip- ping rod. On the even hour the striking mechanism of the clock moves the lever outside, which in turn rocks the tripping rod. The latter engages the pencil hook and pulls the pencil arm, causing the pencil to make a short stroke parallel to th-e edges of the paper. 378. The float is a copper cylinder, 8^ inches in diameter, and weighted so as to float with about one-third of its height above the surface. It is connected with the float pulley by means of a soft phosphor bronze wire. No. 23, American wire gauge. The float moves vertically with the tide in a float box. The float box must ■ be vertical and well braced ; it should be long enough to amply cover the range of tide, and should be made. of 2-inch planks, and be 10 inches clear and dressed inside. The openings should be small, so Fig. 20. 88 COAST AND GEODETIC SURVEY. as to avoid wave effect on the record. If the gauge is to be main- tained many months, the box should be covered with sheet copper, and the bottom of the box be made of copper in shape of an in- verted pyramid, and the only opening a hole about one-eighth inch in diameter in the center of the bottom. The advantage of this bottom is that the opening in copper is not likely to become clogged with marine growth, and that the opening may be cleared readily by lowering a stiff wire, weighted, into the float box. 379. The top of the float box should be covered, to prevent anything being dropped into the water; this cover is best made of two pieces with a narrow slit in the cen- ter for the float wire. 380. The automatic tide gauge is inclosed in a wooden case or cover about 15 by 24-J inches at the base and 12J inches high, which at the sta- tion is placed upon a table or shelf about 30 inches high, on a wharf where the water is deep enough for the lowest tides. 381. The tide house (Fig. 21). — A rough house is usu- ally constructed to protect the gauge from the weather and from being interfered with. This structure may be about 4 by 5 feet at the base and 7 feet high at the eaves, with a door and a window. It should be well bolted down to the wharf. It is often well to have the float box outside the house, especially at permanent sta- tions, as it permits of easy re- yjj, 2j ^ newal of the box without hav- ing to remove the roof from the house ; in this case a sloping cover must be provided for the box, which may be hinged and padlocked to permit of easy access to the float, while protecting it from being interfered with. If reasonably protected from molestation, an automatic tide gauge may be operated for a limited time without a house, but protected by a box with a hinged cover and padlock and a canvas cover. 382. To prevent freezing.— To prevent the formation of ice in the float box during cold weather and the consequent stoppage of the ;3 ,3^=,:^.=:=:— CXO GENERAL INSTEXJCTIONS FOE FIELD WOEK. 89 gauge, petroleum has been frequently employed. A column of oil in the float tube of two or three feet in height would suffice for ordi- nardy latitudes, and one of 4 or 5 feet in height, it is believed, would suffice for stations in Alaska. If possible the gauge should be located where the depth at very low tides would be about 2 feet more than the length of the column of oil required. An iron pipe makes the best float tube, and the lower end of this should rest firmly on the bottom, and thence upward be securely fastened to the spile of a wharf with iron bands. To admit the water a half-inch hole is drilled a foot or 18 inches from the bottom. The supply of petroleum should be sufficient to equal the column of water between the drill hole and the plane of the lowest tides. If a gauge has been working with ordi- nary sea water only, the introduction of petroleum in the float tube changes the line of flotation of the float and also the relation of the curve to the datum line. But this will not cause any inaccuracy of record if the observer furnishes readings of the staff, to which the curve can be referred. In locations where there is an extensive land drainage, such as near the mouth of a large river, the difference be- tween the specific gravity of the water at the end of the flood and at the end of the ebb may be sufficient to cause a sensible periodic oscil- lation in the height of the surface of the petroleum with reference to that of the sea outside the tube, thus introducing a periodic variation which is not tidal and which it would be difficult to get rid of. There- fore, petroleum can not be employed where there is much change in the specific gravity of the water. 383. Installation of an automatic tide gauge. — ^When an automatic tide gauge is to be established at a station, a fixed or portable tide staff should be set up first, in order that a rough estimate of the mean range of tide and of the reading for mean sea level may be ob- tained from a few observations. The half sum of mean high and mean low water for any four consecutive tides may be taken as mean sea level for the purpose of setting the automatic gauge while the difference between mean high and mean low water may be regarded as the range, although it may be more than 25 per cent too large or too small for the true mean value. 384. Selection of float pulley. — A float pulley should be selected according to the following table : Peet. Feet. When mean range of tide ia between — and 2, use scale ^. 2 and 5, use scale |. 5 and 8, use scale ^V- 8 and 12, use scale ^^. 12 and 1 4, use scale jV- 14 and 20, use scale jj. 20 and 28, use scale ^^. 90 COAST AND GEODETIC SURVEY. 385. The above table is not to be taken as absolute, but merely as a rough guide for the selection of a float pulley for any given station. At some places where the table would clearly give a particular scale for normal tides, yet, owing to a greater range produced by some winds, it may be necessary to use a smaller scale. It is, however, better to use as large a scale as possible. 386. Counterpoise weight. — To put on counterpoise weight. Two or more fixed pulleys are placed overhead in such positions as to carry the counterpoise to one side of the house or otherwise out of the way (fig. 21). The counterpoise, which is the larger one of the two weights provided with the gauge, is attached to a movable pul- ley and placed upon a block of wood or other substance to raise it an inch or two above the floor. Pass the end of the varnished fish line, furnished for the purpose, through the small drill hole near the inner edge of the counterpoise pulley and tie a knot at the end. The other end of the line is then passed under the sliding-grooved pulley on the machine, thence over the fixed pulleys overhead, down through the movable pulley on the weight and up again to the ceiling where the end is fastened. After removing the block from under the weight it should hang just a little above the floor. The line will be wound upon the counterpoise pulley by the descent of the float to the water. 387. Float wire. — To put on the float wire. The length of wire re- quired for the float, in feet, may be obtained by the formula : L=9 C+D where L=number of feet of wire required for float. C= circumference of float pulley used, in feet; this is always one-twelfth of the denominator of the fraction expressing the scale, for instance, with scale of ^, C= i%=-^. D= distance from float pulley to mean sea level in feet, meas- ured by the route the wire must take to the float in cases where the float is not vertically beneath the float pulley. 388. Unclamp the float pulley by turning the nuts a very little, as too much slack will cause it to jam. Undo the end of the wire on the spool, holding a finger on the coils to prevent its springing off the spool, pass the wire through the small drill hole near the outer edge of the float pulley, twisting it a few times around itself to make it fast. Now revolve the float pulley, holding the spool of wire with -the other hand and keeping sufficient tension to wind the wire smoothly in the threads cut in the pulley. The number of feet of wire which may be wound upon the float pulley is 18 C, and if this is less than L, wind the pulley entirely full, clamp it tightly to the counterpoise pulley, and measuring off L — 18 C feet of additional wire, cut it off and fasten the end to the float. Some one must keep GENERAL INSTRUCTIONS FOR FIELD WORK. 91 tension on the wire or hold a finger on the float pulley while this is being done, or the Mare will spring off the pulley. If L is less than 10 ri T 18 C, find Q , which is the number of threads of the float pulley to be left unwound when the wire is cut, and attached to the float. Place the float in the float box, letting it down gently until all the slack wire is used, and then hold the float pulley with friction enough to prevent any hasty descent of the float to the water. At the be- ginning of unwinding the wire from the float pulley by the descent of the float, take care that the counterpoise line enters the thread of its pulley, after which it will take care of itself. 389. Placing paper and roll. — To place the paper on the gauge, take the supply roller, which is the one with the solid rod, from the gauge, remove one of the flanges, insert the rod into the central hole in the roll of blank paper furnished for the gauge, and replace the flange. Put the roller with its paper back into the gauge, taking care that the loose end may pass from below inward toward the main cylinder, there being no difference on which side the movable flange happens to come. Set the tension weight on the gauge or floor, unwinding the cord by taking it off over the groove of the pulley, if necessary ; dis- connect the main cylinder from the motor clock, pass the paper over the main cylinder and about three-quarters of an inch into the slit in the receiving roller. With one hand revolve the roller a turn or two so that the paper will pass from the main cylinder to the outer or farthest side of the roller; a moderate pressure of a finger at its jniddle j^oint at the start will cause the paper to adjust itself squarely as it winds. Connect the motor clock with the main cylinder, restore the tension weight to its hanging position, and rewind its cord upon the pulley, holding the flange of the receiving roller while winding, to avoid disturbing the paper. 390. Adjustment of pencil arm. — The pencil arm should now be adjusted so as to bring the pencil about the middle of the paper at mean sea level. It is not necessary to wait imtil the tide is actually at its mean level, for knowing the difference between that level and the actual stage of tide, the pencil may be set accordingly by the metal scale provided with the gauge. To adjust the pencil arm, press on the counterpoise pulley with one finger, to prevent the weight from turn- ing it, a finger of the same hand resting upon the last coil of wire on the float pulley to prevent the wire from springing out of the grooves ; then with the other hand slightly unclamp the two set screws within the float pulley. Next revolve the counterpoise pulley until the pencil is in the desired place, when the float pulley is firmly reclamped. 391. Taking record off gauge. — The paper furnished with the gauge is about 12^f inches wide and 66 feet long, being sufficient for a month of record. For long series it is usually preferable to remove the rec- 92 COAST AND GEODETIC SUEVEY. ord on the last day of each month, at a time intermediate between high and low water, but when only a few months of observations are obtained, the paper ought to remain until it is nearly exhaused, when it may be removed without any regard to the day of the month. . 392. To take off the paper, place the tension weight on thei gauge or floor, unwrapping enough of the tension weight cord for this pur- pose without revolving the pulley. Then disconnect the main cylinder and motor clock and wind up the remaining paper on the receiving- roller. Take this roller out of its bearings, remove one flange, when the marigram can be drawn off. The record will now be on the out- side of the paper, and it should be rewound so that the curve will be on the inside and the last date of record in the center of the roll. 393. Care of automatic tide gauge. — The tide gauge is to be visited once a day ; both clocks and tension-weight cord are to be wound. The tide staff is to be read ; in a choppy sea the mean of the readings for the crest and trough of the wave is to be taken. The clocks are to be corrected after comparison with standard time, obtained from tele- graph time signals, comf)arisoa with ship's chronometer, or other means. If the tide keeper has no means of getting correct time, a well-regulated watch should be rated and left with him, the watch to be compared with chronometers whenever there is opportunity, and record made of comparisons so as to check its rate. (See paragraph 818.) 394. Chiefs of parties must assure themselves that the tide observer has a clear and intelligent comprehension of his duties before intrust- ing him with the care of the gauge. 395. On some of the older forms of gauges, it should be noted that in addition to keeping the hour-marking clock as nearly correct as practicable, and well regulated, the lever on the back of this clock, which trips the hour-break attachment, should also be kept correct, so that the hour mark corresponds to the even hour by the clock. 396. Whenever anything unusual happens to the gauge, such as disturbances caused by storms, stoppage of the gauge, change of adjustment of pencil or staff, etc., an explanatory statement should be written on the tide roll itself; in fact, this roll should be a complete record in itself, as notes made elsewhere are liable to be lost sight of. 397. Operating' troubles. — Some of the difficulties likely to be met in the operation of an automatic gauge are : 398. Breaking of float wire; which is nearly always due to sharp bends or kinks in the wire when it is slack; a new wire should be put in, rather than to splice the old one. 399. Vibration of pencil on record; this is due to too large open- ings in the tide box and if serious may require rejiair of box. 400. Failure of gauge to record full range of tide, as shown by comparison with staff readings; this is due to clogging of openings GENERAL INSTEUCTIONS FOE FIELD WOEK. 93 in float box, and is likely to render the record worthless, and cfdls for immediate examination of box and clearing of openings. 401. Jamming of paper rolls, due to paper not running true ; r&lls should be examined to see that they are in their right position, and paper rewound if necessary. 402. Failure of hour-marking device; remedied by examining and oiling the tripping mechanism, if necessary taking it to a clock maker. Sometimes this trouble is entirely due to having the pencil project either too far out or not far enough through its holder. (See paragraph 816.) 403. Instructions for running an automatic gauge. — On the blank paper, both at the beginning and end of each marigram, write and fill out a note similar to the following: Station Latitude ° ' ". Longitude ° '_! " The time used is Tidal record from to Marigram No Tide Gauge No Scale Observer Chief of iJarty 404. The marigrams are to be numbered in order at each station. 405. The time. — Standard time should always be used in tide obser- vations and records, and so stated on the marigram or other records. 406. The time clock should be kept correct to the nearest minute by daily comparison with some standard timepiece or time signal. If the location is such that this is not possible recourse must be had to time observations. 407. Standard time = Apparent time -|- Equation of time -|- L = S, v\fhere L and S are the longitude in hours of the local and standard time meridians, respectively, being positive for the west longitudes and negative for the east longitudes. 408. Comparative staff readings. — In connection with every auto- matic gauge there should be a fixed tide staff, to be well secured in a vertical position, where it can be conveniently read in the vicinity of the gauge. This staff is to be set originally with its zero slightly below the lowest tide likely to occur at the station. 409. Notes on the marigram. — A general form for making staff read- ings and making time comparisons is as follows : Thursday, April 25, 1007 ; correct time, 10.22 a. m. Time clock 10.20, corrected. Stafl', 2.4.5 feet. If the clock is correct to the nearest minute when comparison is made, write " clock correct " instead of " corrected " in the note. A note may be written upon any convenient portion of the marigram paper, but it should always be connected with the exact position of the recording pencil of the gauge by an arrow or other mark. 94 COAST AND GEODETIC SURVEY. 410. Gaps in record. — If any portion of the record is lost from any cause, always move the paper forward a few inches before starting, the gauge again. This can easily be done by throwing out the carrier arm from the toothed carrier wheel, so as to disconnect the motor clock and gauge, but the hand must be held on one of the rollers before disconnecting the motor clock or the paper will be suddenly jerked forward by the tension weight. A lead pencil or other short stick will enable one to remove the carrier arm from the notched wheel when it is out of reach. 411. A marigram should contain a complete statement in itself of all that may help one to understand the record, for explanations given in letters or in a book are liable to become permanently separated from it, and thus ranj be of no practical use when wanted. 412. The datum line. — The fixed pencil which traces the datum line should be set to about the middle of the paper, so that any errors due to hygrometric changes in the paper between the times of making and reading the record may be rendered as small as possible. 413. Adjustment of hour brake. — The adjustment for a satisfactory working of the hour brake device is rather delicate; the pencil must be clamped at such a height in the pencil arm that the attached hook will just clear the tripping rod. As the pencil point wears away it will require readjustment. Any good lead pencil which is not too hard will answer, a No. 2 Faber pencil being usually used. The tripping rod may be sprung over the end of the hook when it is desired to lift the pencil from the paper. 414. Cleaning the pencil screw. — The pencil screw must be frequently cleaned, otherwise the pencil arm may be raised from the paper on a falling tide, or pressed so firmly against the main cylinder on a rising tide as to tear the paper, either accident causing a loss of record. A small rag moistened with gasoline passed around the pencil screw and drawn back and forth several times will keep it clean. Never put oil on the pencil screw, as it soon catches dirt and makes it worse than it was before. 415. Winding the clocks and weights. — Although these are eight-day clocks, it is usually better to wind them regularly twice a week on fixed days. The counterpoise weight will not require any attention for a long period of time if once properly wound and supported, for it reverses its motion with each change of tide, and never runs down. Bvit the tension weight, which winds the paper upon the receiving roller, must be regularly wound. By running the line up to the ceiling, so as to give a longer drop, it will require less frequent attention. 416. Preparing a marigram for tabulation. — The hour marks are numbered between the successive time notes, from 0" for midnight to 23" for 11 o'clock at night. Care must be taken to compare the GENERAL INSTRUCTIONS FOE FIELD WORK. 95 numbering of the hour marks with each successive time note before passing it. The time clock ought to be correct within a few minutes, or the hour break device becomes of little value. But, if it does happen that these hour marks are too fast or too slow by an appre- ciable amount, the error of the clock must be proportioned between the successive notes. If the clock is too fast the hour mark is made before the real hour, and the true time follows that shown on the record, while if the clock is too slow the hour mark is not made until after the true hour, so that the correct time is before that on the record. The hour begins the instant the mark leaves the curve, and no account need be taken of the length of the stroke. 417. In cases where the hour break device fails to work the hours may be obtained as follows: Prepare a strip of paper known as the " dividing scale,'''' somewhat longer than the greatest distance between the time notes on the marigram ; on one edge of the strip of paper lay off equal divisions about 1^^ inches long and number them accord- ing to the hours. Upon one edge of a piece of paper about 1^ inches square lay off an inch subdivided into twelfths of an inch, to serve as a time scale; make the alternate divisions longer than the intermediate ones, and number them from left to right, when the scale is on the upper edge of the paper, successively, 0, 10, 20, 30, 40, 60, and 60, the whole inch representing an hour on the curve. Place that division of this time scale which corresponds, to the minutes of the time note on the tip of the arrow or other mark which indicates the precise part of the curve to which the note applies, estimating between the five-minute spaces on the scale, and mark the whole hour from either end of the time scale, pref- erably using that which is nearest the time note; that is, if the minutes of the time note are less than 30 mark the preceding hour, and when greater than 30 mark the following hour, the idea being that the smaller the distance measured by the time scale the less the error will be, as the size of the hours may vary slightly from a va- riety of causes. Through the whole hour thus marked rule a straight line all across the paper perpendicular to the datum line. Having done this with each time note within the portion where the hour break device failed to work, take the dividing scale and adjust it obliquely between adjacent cross lines passing through the whole hour points so that the numbers of the hours on the scale agree with those of the vertical lines, and place weights to hold it in position. Lay a straight edge parallel to the datum line a little below it, and with a square moving along the straight edge, mark the places where the other edge of the square crosses the datum line while successively adjusted to each hour of the dividing scale. 418. Marking the high and low waters on a marigram. — Upon both the longer edges of a strip of paper about 6 inches long and 2 inches wide 96 COAST AND GEODETIC STJEVEY. lay off any convenient scale of equal parts, like quarter inches; join the corresponding divisions nearest to the middle of each edge by a heavy line, and number the subdivisions from this central line as zero, alike on both the right and left hand halves of the scale, making the numbers for both edges read erect when one edge is kept on top. This is known as the bisecting scale, and it is placed on the marigram so that its edge is parallel to the datum line while cutting off a small arc or portion of the curve at high or low water, and adjusted so that the curve passes through the divisions of the same number on each end of the scale, when the heavy central line will indicate the middle of the chord cut off. The point on the curve in the prolongation of the heavy central line of the bisecting scale is taken as the high or low water jDoint, although it may not be quite as high or low as some other part of the curve, as these extreme heights may be as much as an hour from the average time of tide. The point selected may be marked by a dot within a circle, or by a simple straight line passing through the curve at right angles to the datum line. Wlien the curve is I'ough, or differs much from a sine curve, this method does not give as satisfactory results as an estimation of the position by mere inspection. 419. Comparative readings. — Write the day of the month in the left- hand column of Form 137, allowing two lines for each date, and six- teen days on the first side of the form, the remainder of the month being put on the other side of the sheet. As a general rule the upper line is for morning tides, and the low line for afternoon tides, but in some localities there will frequently occur two tides of the same phase in the same half day, in which case the earlier tide is placed on the first line and the later one on the second line of the day, without re- gard to whether it is a. m. or p. m., the number of the hour itself being sufficient to avoid any confusion. Note that the broad or binding margin of the form is on the left hand when beginning to use it. Copy out the time and height of the staff readings given on the mari- gram into the appropriate date and line upon the form, and with the metal scale from the gauge, a paper scale made from it, or a glass scale furnished by the office, make the corresponding scale reading for each staff reading ; it does not matter which division of the scale is placed upon the datum line for these readings, but it is usually con- venient to use that whole foot division which will make the scale reading nearly like that of the staff. In making these scale readings the edge of the scale is made to pass through that portion of the curve which is marked as being the place to which the staff reading applies, and the height of the curve at that point is the scale reading which is to be compared with the staff readings, care being taken to keep the same division of the scale upon the datum line for each scale reading. The heights are usually read to hundredths GIUSrEEAL INSTRUCTIONS FOR FIELD WORK. 97 of feet, for comparison with the staff, but all the other tabulations are generally made only to tenths of feet. The column headed " Diff." is to be filled out by subtracting each scale reading from its corresponding staff height. The successive differences should be ap- proximately similar, so long as the adjustment of the gauge remains unchanged, and the mean difference for one or more marigrams is taken, provided they are all evidently about the same. The mean difference is added algebraically to the value of the scale which was placed upon the datum line when the scale readings were made, and the division of the scale corresponding to this sum, is marked with pencil or ink, so that the subsequent readings of the curve may be made with this marked division of the scale upon the datum line. The test of the correctness of the mark upon the scale is that when the scale is placed with this mark upon the datum line the scale and staff readings practically agree. 420. Tabulations of high and low waters. — ^Using the edge of the height scale as a straight edge, place it at right angles to the datum line, so that it passes through the hour preceding the high or low water as previously marked, and with the hour or time scale already mentioned, measure the minute from the edge of the height scale to the mark made to designate the place of high or low water. ^Iien the hour break device is in operation it must be borne in mind that the hour is at the instant that the mark leaves the curve, and has nothing to do with the length of the stroke made upon the paper. The days of the month having already been written in Form 137, enter the time of the high or low water in its appropriate column, using hours and minutes, the latter being usually rounded out to the nearest multiple of five, as this is about the limit of accuracy practicable with our gauges. Immediately after tabulating the time of a tide, record its height in feet and tenths, on the same line in its column on the form. In reading these heights, which are headed " Height on staff," place the marked division of tlie height scale on the datum line, and see that the edge of the scale is at right angles with that line. The readings so made will average about the same as if they had been made on the staff, and the plane of reference can thus be referred to bench mark. 421. Tabulation of hourly heights. — ^With the same marked division of the height scale which was placed upon the datum line for the high and low waters, read the height of the curve at every hour, and tabulate it in Form 362, following a mean curve when the hour marks are disturbed by seiches or other short period oscillations. This form has double columns, only the left hand one of which has the word " feet " as a heading, and is to be filled out, while the right hand part is left blank. The readings are to be made only to feet and tenths (never to hundredths). 53695—08 7 98 COAST AND GEODETIC SURVEY. 422. Lost record. — When any portion of the record is lost, unless it be so great as to break the series into entirely separate portions, the proper vacant space should be left for it in the tabulations, so that the missing tides may be interpolated either in the field or in the office after the tabulations have been received. 423. Length of series. — For reduction of soundings it is necessary to have at least one month of continuous tides, day and night, at the principal stations, in order to obtain the plane of reference. At subordinate or secondary stations, the staff should be read at hourly intervals, day and night, for forty-eight hours, at least, or longer if it can conveniently be done, simultaneously with the record at the principal station. This is for the purpose of obtaining a good comparison between the two stations, so that the plane of reference derived from the automatic gauge records may be referred to the other station. After satisfactory connection of the planes of refer- ence, hourly readings during the intervals when soundings are in progress will furnish the data required for reduction of soundings. AVhere a staff is located in lieu of an automatic gauge for any reason, and there is no established plane of reference in the vicinity, the staff should be read hourly through as long a period as practicable during the continuance of the work. 424. Simultaneous observations. — In order to obtain the plane of ref- erence for subordinate stations, by comparison with a principal sta- tion. Form 248 should be used. The mean sea level or half-tide level of this form may be obtained by taking the half sum of mean high and mean low waters. In cases where it was impossible to obtain a series of one or more months at the principal station, or when the observations at a subordinate station could not be made simultaneous with those at the principal station, the predicted tides of our tide tables may be used as the standard for comparison in Form 248, but predictions should never be used except in cases of necessity. 425. The plane of reference. — Upon the Atlantic and Gulf coasts of the United States, including Porto Rico, all soundings are reduced to mean low water. Upon the Pacific coast of the United States, or Alaska, Hawaii, and the Philippines, all soundings are reduced to the plane of mean lower low water., with the following exceptions : For Puget Sound the plane is 'i feet helow mean lower low water., and for Wrangell Strait it is 3 feet heloxo mean lower low water. 426. To compute mean low water.— When the tide is semidiurnal, mean low water is the average of all the low waters occurring both day and night during one or more synodic months of about twenty- nine days. When the tide is mixed diurnal and semidiurnal, as is the Gulf of Mexico, mean low water is the average from the sum of all the single low waters combined with the sum of the lower one of each day when two low waters occur, during one or more tropical months GENERAL INSTBUCTIONS FOR EIELD WORK. 99 of about twenty-seven da3's. If the two low waters on anj' tidal day happen to be of equal height, use only one of them in finding the mean. At stations having mixed diurnal and semidiurnal tides, when the later occur the range is usually greatly diminished, and only the lower one of the two tides is taken so as to avoid giving dovible weight to these small tides. In this case the mean low water is sometimes called " Mean diurnal low water," to distinguish it from semidiurnal mean low water, and it is also identical with the plane called " Mean lower low water " for similar tides. 427. To compute mean lower low water. — When the tide is strictly semidiurnal, mean lower low water is the average of the lower one of the two low waters of each tidal day during one or more tropical months of about twenty-seven days each. When the tide is mixed diurnal and semidiurnal, mean lower low water is the average from the sum of all the single low waters combined with the sum of the lower lows when two low waters occur within a tidal day, during one or more tropical months of about twenty-seven days' period. If the two low waters of any tidal day happen to be of equal height, use only one of them in finding the average. In this case the mean lower low water is identical with what is sometimes called " mean low water " or " mean diurnal low water " for similar tides. 428. Interpolation of missing tides. — Interpolated tides should be written in red ink or else inclosed in parentheses to distinguish them from observations. If only a few tides are lost, plot time and height upon profile paper for a number of days before and after the break, leaving space for the missing tides ; connect the alternate tides with curved line, which must be extended across the gap on the plotting as nearly as rnay be in accordance with apparent law of change in time and height. There will usually be eight distinct curves — four for height and four for time, including both high and low waters. In order to prevent the curves from becoming confused by intersecting too much, they may be plotted upon scales which are dropped one below the other. 429. If there is a simultaneous series in the vicinity, the missing tides may be supplied bj' comparison with it. 430. If the series is long enough, take the means of the times and heights occurring twenty-nine days before and twenty-nine days after each missing tide, smoothing out the results by plotting. 431. In some cases a direct linear interpolation will suffice, espe- cially when only a few tides are missing. In this mode of interpola- tion divide the difference between the times of alternate tides and also that of the corresponding heights by one more than the num- ber of missing tides and add the quotients to the preceding time and heights. If it happens that one of the tides occurs near noon or mid- night, it may chance that only one tide of that phase occurs in the 100 COAST AND GEODETIC SXJEVEY. civil day, and care must be taken to allow for this in counting up the number of missing tides. 432. All short breaks should be interpolated before the plane of reference is obtained. 433. Annual inequality in mean sea level. — At most stations there is a variation in the reading of mean sea level upon the staff at different times of the year, depending in general upon the seasonal changes in the direction and strength of the wind, and in river stations also upon the rainfall or melting snow. There is a rough periodicity in these variations of mean sea level, but they can not accurately be foretold. In Table 5 of our tide tables is given the best determination we have been able to make of this annual inequality in mean sea level, and when a subordinate station is located so as to be influenced about the same as any one of the stations given in the table, the plane of refer- ence should be corrected by the mean of the table for the period of observations used. In applying this correction it should be noted that a plus sign indicates that the sea is above its mean value, and Avhen applied to observations must have its sign changed, to reduce them to their mean value. If the predictions are used for comparison with observations, the predicted heights should be corrected by applying the tabular values without change of sign, as the purpose here is to reduce mean values to seasonal ones. 434. To estimate the diiference in time of tide. — As the number of gauges which can be established during a hydrographic survey is nec- essarily restricted, it often happens that in reducing soundings it is desirable to know the difference in time of tide on the sounding ground and at the nearest tide gauge. . When the depths are known the approximate difference in time of tide may be obtained by the fol- lowing general rule : 435. When the length of the wave is not less than a thousand times the depth of the water the velocity of the wave is the same as that which a free body would acquire by falling from rest, under the action of gravity, through a height equal to half the depth of the water. 436. In order to make this rule easier to apply, the following for- mulas are given: F=the velocity of a body after falling through the space s. A=the depth of the water, in feet, at mean sea level, for the average cross section between stations. 7=0.5921X5.(172 /) =3.356 k nautical miles per hour. The time required for the tide wave is , n.ft(;54 . , ,• 1 -1 z;= 1 minutes per nautical mile. Log. ^='1.2520127)— -log. h minute per nautical mile. GENEEAL INSTRUCTIONS FOR FIELD WORK. 101 437. This rule does not take account of variations in range of tide, which would probably affect the plane of reference, if it occurs. The tide stations should, therefore, be selected with a view tohaving about the same range of tide at the staif and the working grounds. For instance, the range within lagoons or bays connected with the ocean by a shallow opening is usually much less than that on the outer bar; and any partial obstruction, like a natural contraction of the channel to comparatively narrow limits, or even bridge piers, alter the relative height of the tide above and below at intermediate stages between high and low water. 438. Bench marks. — No matter how temporary the occupation of a tide station is, if any plane of reference is computed or assumed, the tide staif should be referred to at least three bench marks. This is of greatest importance to make the results of the tidal observations available for future use. These marks should be sufficiently scat- tered, so that they are not likely to be all destroyed by a common cause. The bench marks should be carefully connected with one another and with the tide staff by forward and backward lines of spirit levels. 439. Bench marks may be made in considerable variety, the follow- ing list being merely suggestive. As the future value of the planes of reference depends on the permanency of bench marks, consider- able care should be used in their selection. In new regions it is usually not advisabJe to use copper or other material that might excite the cupidity of natives. The bench marks should be care- fully described, in addition to marking their location on the sketch. 440. (a) A copper or brass bolt inserted in a stone, or a flat tile set in cement underground, belovi^ the frost line, on which rests a 3 or 4 inch pipe, the lower end of which is split and turned out to prevent its being pulled up. This pipe projects above the surface of the ground a few inches, and is covered by a finp secured in place by bolts, as screw threads are liable to be damaged. This constitutes an upper and lower mark, both of which must be referred to the tide staff. This Hurvey furnishes a 3-inch cap marked " U. S. Coast and Geodetic Survey B. M., $250 fine or imprisonment for disturbing this mark." (h) A horizontal mark cut in the end of a copper or brass bolt leaded horizontally into a stone or brick wall or the nearly vertical side of a large rock, with or without lettering. (c) This Survey furnishes a brass plate about 3-| inches in diam- eter, with a stem for insertion in a vertical wall. It is marked " U. S. Coast and Geodetic Survey B. M., $250 fine or imprisonment for disturbing this mark," and has a horizontal line about seven- eighths inch long in the center. 102 COAST AND GEODETIC StJEVEY. (d) The top of a copper bolt leaded vertically into stone or brick, with or without lettering. (e) This Survey furnishes a brass casting known as a " station mark," which will do for a bench mark in a horizontal ledge or top of a wall or pier. It has a cuplike depression on the face v^hich is about 3 inches in outside diameter, with the letters " U. S. C. & G. S." around a raised center, which is turned off smooth for the level rod to rest xipon. The stem of this casting is split, so that a wedge will prevent its being taken out when once set. (/) The bottom of a square hole cut in stone, with or without letters. (g) A cross cut in a vertical or horizontal stone, with or without a circle or other marks. (A) A hole drilled in a horizontal stone, with a circle cut around it, the surface within being smoothed for the level rod. {i) A hole drilled in a vertical stone, with a horizontal line passing through its center, and with or without other marks. (j) A cross -(- on the top of a section of railroad iron driven ver- tically into the ground. (k) The top of an iron pipe driven deeply into the ground. (l) A cross -f- on a door or window sill of a substantial building at a point not subject to wear. . (m) The water table of a substantial building at some marked or designated spot. (n) Whenever any other mark or datum exists in the vicinity, such as a city datum, a triangulation station, or bench mark of some othei organization, this should be connected by spirit levels with the tide staff. 441. Before beginning hydrographic work in any given locality the chief of party should obtain from the office a description of such bench marks as may have previously been made in that vicinity. 442. Eecords. — Tide staff readings are to be recorded in the books provided for this purpose (Form 277) ; the high and low waters are to be tabulated on Form 137 from this record, and no other duplicate need be made. 443. For the automatic tide-gauge stations there should be pre- jiared a description of the tide station, bench marks, reference to bench marks, sketch of location, time used, and plane of reference adopted. This should be in duplicate, the original forwarded with the first tide roll and the copy with the readings of the hourly ordi- nates. Xo duplication is to be made of the tide roll or of the readings from it. GENERAL INSTRUCTIONS FOR FIELD WORK. 103 CURRENT OBSERVATIONS. 444. Each vessel or party engaged on hydrographic work should make such observations and notes regarding currents as may be prac- ticable in connection with the other work. It should be possible to make float observations when at anchor or when drifting and to obtain useful information by determining the set of the vessel when running long sounding lines and on voyages to and from the field of work. The making of current observations is not, however, to be permitted to interfere with the progress of the other survey work unless specific instructions be given. 445. The current information most useful to navigation is the rela- tion of current to stage of tide and the velocity of current in impor- tant passages and straits, the constant and tidal currents along the coasts and in the passages between the islands, particularly in the ordinary tracks of vessels, and the currents in harbor entrances and channels. 446. Currents are dependent on the tides, the seasonal winds, the general oceanic movements, discharges of rivers, and on local weather conditions. It is obvious, therefore, that the most accurate observa- tions covering only a brief period may give a very imperfect or erro- neous idea as to the currents throughout the year. It is desirable, therefore, to collect current data from all available sources, so that by the systematic compilation of reports for various seasons of the year, as well as different stages of tide, general results may be deduced. 447. Only such current observations as are likely to be practicable in connection with general survey work are referred to here. For more extended work special instructions will be furnished. 448. Current observations at anchor. — In connection with general survey operations, current observations are to be made from the ves- sel at anchor as opportunities offer to obtain useful data, but these are not to be permitted to interfere with other survey work, unless specially directed. When weather and depths are favorable an- chorages may at times be selected in channels where current data will be valuable. 449. Only very simple observations with log line and current pole are required, lasting, if practicable, over one flood and one ebb and the time of the three slacks. The observations are to be made each quarter of an hour, on the even quarters, standard time. 450. With log and line. — To observe, place current pole overboard astern, and pay out stray line as pole takes it, as the white tag passes over the taffrail note time, and at the end of thirty seconds check the line and read in knots and tenths per hour. Eecord in Record of Current Observations (Form 270, now furnished in small pam- phlet form) the velocity, time (kind of time), direction of pole by 104 COAST AND GEODETIC SURVEY. compass or sextant, heading of ship, compass error, wind (force and direction) , tide, and eddies or tide rips. Compass directions must be given only in degrees ; sextant angles must show whether the float is to the right or left of the object of reference (mark the angles E, or L, as the case maj^ be). 451. The record must show the position of the ship as located by sextant angles, bearings, or astronomical observations, and there should be attached to the record a piece of chart or tracing showing positions of current stations. Soundings should also be recorded to assist in identifying the position. 452. The time of the current's turning (middle of slack) should be carefully observed and recorded whenever slack water occurs. 453. The current log usually consists of a pole, preferably of hard pine, 2 to 3 inches in diameter, 13 feet long, with a cone-shaped end 1 foot long; the pole is weighted, so as to float upright with a draft of 12 feet, with lead poured into a hole bored up from the bottom or strips of sheet lead wound about its lower end. At the conical end which projects above the surface a couple of screw eyes are inserted to receive a staif or small flag, and a staple is driven about 1 foot below the top, to which end of the stray line is fastened. 454. A small-sized line is desirable, but it must not be so small as to stretch and change its length when hauling in the log. 455. The log line should be so divided that running off from the reel during a period of thirty seconds the marks will measure the drift in nautical miles and tenths per hour. The larger divisions — jniles — should be marked by leather strips, in which holes are per- forated to represent the number, thus: For 1 mile, one hole; for 2 miles, two holes, etc. These strips should be placed 50.67 feet apart when the line is thoroughly wet. Between each pair of leather strips nine knotted cords should be placed at intervals of 5.07 feet to mark tenths of a mile, the number of knots on each cord indicating the number of tenths. 456. Between the pole and the initial division, or zero, there should be at least 60 feet of stray line, in order that the float may pass beyond the vessel before the count begins. 457. Various forms of current meters have been used in connection with hydrographic work, but as at present they are all likely to be supplied to surveying parties only with special instructions they are not further described here. 458. Eeduction of observations. — All observations in a series of moderate length should be plotted upon cross-section paper, taking the times as abscissa>. The velocities constitute the ordinates of one curve and the directions (azimuths) the ordinates of a second curve; but the direction curve need not be drawn where the flood takes only one direction and the ebb takes' the constant opposite direction. If GENERAL INSTRUCTIONS FOR FIELD WORK. 105 this curve is not to be drawn, the directions or azimuths should be written at the base of the velocity ordinates. The times of high and low water at a near-by tidal station should be indicated by marks near the upper margin of the sheets, upon which the currents are plotted. The heights of the tide should be written near these marks. 459. From the plotting it is easy to ascertain the velocities and directions of the current at the times of high and low water and at times one, two, and three hours before and after; or the times and velocities of strength of current and the times of slack water are easily obtained. In either case the velocities of tidal currents may be approximately reduced to mean velocity by multiplying their ob- served values by the mean range of tide, divided by the observed range. If no satisfactory tides, observed or predicted, be available, the time of the moon's transit may be used as an argument for tabu- lation. 460. As a rule the times of a maximum velocity of the flood and ebb streams are more regular than are the times of slack waters, for the permanent flow does not generally alter the former, but it seriously disturbs the latter. 461. Currents from soundings lines. — A ready method of obtaining useful information as to currents will be to compare the course and distance run (by log) on a sounding line with a true course and dis- tance run as determined by sextant positions, the difference being the set due to current and wind ; the direction and force of wind should be noted and allowance made. A separate report should be for- warded for each season's work of currents so observed, giving the date, time (standard), position, total set in distance and direction, wind direction and force, estimated current direction and velocity per hour, after allowing for effect of wind and stage of tide. 462. Currents from voyages. — When a surveying vessel is proceeding to or from the field of work, to or from coaling station, or on a voy- age for any purpose, the log should be so kept as to show the set. This may be done by comparing the position by dead reckoning (from log and course, both corrected) with the true course and dis- tance run as determined by positions obtained from sextant angles or bearings on objects ashore or by astronomical position. In order to develop the effect of the tides the intervals should not exceed three hours; in straits and channels the positions should be obtained at such intervals as to show the current for each important section of the passage. The direction and force of wind should be noted and allowance made. A separate report of currents so observed should be forwarded for each voyage, giving the date, time (standard) of beginning and end of course, position at each end of course, total set in distance and direction, wind direction and force, estimated cur- rent direction and velocity per hour after allowing for effect of 106 COAST AND GEODETIC SURVEY. wind, and stage of tide if observed or known. A convenient report on such work will be made by plotting the information on a portion of a chart, for which purpose extra copies may be obtained. 463. Current observations drifting. — In connection with sounding work currents may be measured by allowing the pole log to drift, and determining its position by sextant angle at measured intervals of time. In this manner a sounding launch or boat can obtain a useful current observation by putting over the pole when stopping for lunch and picking it up again when starting to work, both launch and pole drifting in the meantime, the advantages of referring to the pole be- ing that it will not be ailected hy the wind to the same degree as the launch itself. MAGNETIC OBSEEVATIONS. 464. Land work — General remarks. — Observers engaged exclusively in magnetic work are supplied with a complete magnetic outfit, con- sisting of theodolite-magnetometer, dip circle, half-second, pocket chronometer, small accessories, and nonmagnetic observing tent. Those who are expected to get magnetic results incidental to other work — for example, triangulation or astronomic observations — are supplied with more or less complete outfits according to circumstances. Where only declination results can be secured, under the conditions involved, a compass declinometer is supplied; but to those who can attempt more but to whom a magnetometer can not be supplied, a dip circle with compass attachment is furnished, with which compact outfit, knowing th« azimuth of some mark from triangulation or other source, the declination, dip, and total intensity (Lloyd's method) can be obtained with a very fair degree of accuracy. It has been found by experience that with care and a good instrument the results obtained in this way will not be much inferior to those derived from the more complete outfits, provided that proper standardization obser- vations are made with sufficient frequency. 465. General survey parties working in remote regions, such as Alaska or the Philippines, will in general be furnished with a com- pass declinometer or other instrument for measuring the magnetic declination. In connection with triangulation where the true azi- muths are known, the magnetic declination can readily be obtained, and this should be done at intervals of about 20 miles along the prog- ress of the triangulation, or at shorter intervals where there is indi- cation of local disturbance. In the regions mentioned this should be considered a regular part of the work of general survey parties. The making of complete magnetic observations, including dip and intensity, will be required only Avhen there are special instructions. 466. The declinometer may be set up directly at the triangulation station, or if this is impracticable because of the presence of iron GENERAL INSTKUCTIONS FOR FIELD WORK. 107 height of tripod, or other cause, a magnetic station may be estab- lished by alignment between the triangulation station and the mark. 467. Selection of stations. — The conditions to be satisfied in choos- ing a magnetic station are freedom from present and probable future local disturbance, combined with convenience of access. Proximity of electric railways, masses of iron or steel, gas or water pipes, build- ings of stone or brick, should be avoided. A quarter of a mile from the first, 500 feet from the second, 200 feet from the third and fourth may be considered safe distances. The station should be at least 50 feet from any kind of building. If any doubt arises in the selection of a station on account of the possible existence of local disturbances, two intervisible points a hundred yards or more apart should be se- lected and the magnetic bearing of the line joining them, observed at both. A lack of agreement between the two results is evidence of local disturbance. A station on suitably situated public property, or property belonging to an educational institution, is to be preferred, as it is less likely to be disturbed. 468. Description of station. — Each point occupied should be de- scribed with sufficient detail to render possible its recovery. The de- scription should begin with the general location, enough to indicate the park or field in which the station is situated ; then should follow the measured distances to fences or other fixed objects, and finally the manner in which the station is marked. If a meridian line is es- tablished the distance to and location of the second stone should be given, the magnetic station being selected so as to form one end of the meridian. It is also desirable to give a rough sketch showing the re- lation of the station to surrounding objects, indicating on it the direc- tion of north (which should always be toward the top of the sketch) and the direction of the azimuth marks. The marks should be well defined objects nearly in the horizon and likely to be available for future use. The description should be given as nearly as possible in the form in which it is to be published. Additional details intended for the information of the office, such as character of soil, possibility of or tests for local disturbance, etc., should be given at the close of the description proper. 469. Adjustment of instruments. — Care should be taken to keep the instrument in good adjustment and free from dust. The magnets should be touched with the hands as little as possible and should always be wiped with clean chamois or soft tissue paper at the close of observations. They should not be allowed to touch each other nor come in contact with iron or steel objects and should in the North- ern Hemisphere be kept in the box with north end down. The dip- ping needles should be wiped with tissue paper both before and after observations and the pivots and agate edges cleaned with pith. In reversing polarity, the bar magnets should be drawn smoothly from 108 COAST AND GEODETIC SURVEY. center to ends of needle, as nearly parallel to the axis of the needle as possible. The bar magnets should be wiped after using to prevent rusting and should not be allowed to touch except at ends of opposite polarity. 470. Order of observations. — Unless otherwise instructed the obser- vations at a station comprise morning and afternoon azimuth (Form No. 266), latitude at noon (Form 267), one set of dip (Form No. 42) with each of two needles, two sets of declination (Form No. 37), de- flection (Form No. 39) and oscillations (Form No. 41), and angles between prominent objects. The latitude observations, of course, need be made only when a reliable latitude is not available. It is desirable that the azimuth observations should be made at nearly equal times, preferably not less than two hours before and after the apparent noon, giving nearly the same altitude of the sun for morn- ing and afternoon observations. Latitude observations should begin about ten minutes before maximum altitude of the sun (apparent noon) and continue until about ten minutes after. The second set of deflections and oscillations should be made with the magnets in- verted. As often as possible the chronometer correction on standard time should be obtained by means of Western Union or other time signal, or, in the absence of these, local time observations should be made at reasonable intervals. The longitude must be obtained by observation when a reliable longitude is not available from other sources. A convenient observing programme where work at a sta- tion is begun in the morning is the following : . 471. Azimuth, declination, oscillations, deflections, oscillations, declination, latitude, leaving only dip and azimuth for the afternoon. The horizontal circle should be shifted in azimuth by means of the lower clamp before the second set of declination so as to get readings on a diffei'ent jaart of the circle. The same thermometer should be used throughout the intensity observations if possible. It should be jDlaced in the hole in the magnet house during oscillations and in the end of the bar during deflections. It should be changed from one bar to the other with the magnet and read at the end of the half set, so that it may have time to take up the temperature of the bar. If the temperature is changing rapidly, read at the beginning as well as at the end. Care should be taken to stop up the hole in the magnet house when the thermometer is not in it. 472. Discrepancy limits. — Before leaving the station the computa- tion should be carried far enough to show that there is nothing radically wrong with the observations. Thus in good work the two consecutive sets of azimuth should agree within one minute and the morning and afternoon sets within two minutes. A greater difference is usually due to lack of adjustment or level of the theodo- GENEEAL INSTRUCTIONS FOB FIELD WORK. 109 lite, or to a mistake in pointing on a wrong limb of the sun. The effect of change.s in level of theodolite should be eliminated by the method of observing described under " Elevations by vertical angles " (p. 31). In case the difference between morning and afternoon azimuth amounts to more than five minutes, the observations should be repeated. The two sets of declination should not differ more than two or three minutes when allowance is made for diurnal varia- tion. The average time of 70, or whatever number of oscillation is used, should not differ more than a half a second in the two sets, and in the deflections the two values of log sin u should not differ more than 0.00100 for either distance, when allowance is made for the difference of temperature of the two sets. When the dip results for the tAVO needles differ by more than five minutes in excess of the noi-mcd difference the observation should be repeated. Thus, if com- parison with the standard circle shows that needle No. 1 gives on the average a dip of three minutes greater than needle No. 2, the observations should be repeated when No. 1 gives a result more than eight minutes greater or two minutes less than No. 2. 473. The record should be kept with a hard pencil (or stylographic pen) and entered at once on the proper form (not kept on blank paper and afterwards copied on to the form). All computations should be made in ink. The different sheets should be punched and fastened together in the covers provided (Form 367), arranged in the following order: (1) Description of station, including angles connecting marks with other prominent objects and chronometer correction on standard time; (2) latitude, (3) azimuth observations, (■1) azimuth computation, (5) declination, (6) dip, (7) oscillations, (8) deflections. 474. Abstract of records. — Before the record is sent to the office the computation should be completed and a copy should be made in an octavo volume of the results and also such quantities as would be needed to replace the computation in case the record is lost. This includes brief description of station; chonometer correction on standard time, sun's maximum altitude from latitude observations; mean chronometer, horizontal and vertical circle readings for each set of azimuth, mean reading of mark and magnet, mean scale read- ing erect. and inverted for each declination set; time of whole num- ber of oscillations and effect of 90° of torsion; mean value of 2 m for each deflection distance, the temperature and time of observation ; the mean dip with each needle for each polarity. No other duplicate of the original records is to be made. Records should be sent to the office by registered mail about twice a month, accompanied by a trans- mitting letter. 110 COAST AND GEODETIC SUEVEY. 475. Computations. — Fi\'e place logarithms to be used throughout. In azimuth observations mean of circle readings to be carried to whole seconds, times to tenths of seconds; in computations, carry angles to tenths of minutes (Form No. 269). In time computations (Form No. 269) take out " t " (in arc) to whole seconds. In declina- tion observations carry mean scale readings to hundredths of divi- sion, balance of computation (on observation form) to tenths of minute. In oscillations compute time of one oscillation (on observa- tion form) to four decimal places, mean temperature to hundredths of a degree. Deflection angles to be computed (on observation form) to whole seconds. Dip computations (on observation form) to be carried to hundredths of a minute. 476. Reocciipation of old stations. — In case of old stations special effort should be made to reoccupy the precise point if the local con- ditions warrant it; should it in no wise be desirable to reoccupy the old station, then a new station may be established. There may be cases, however, in which it will be best to occupy the old station and also establish a new one — e. g., the old station, while not satisfying the requirements of future availability, may still suffice to determine the secular variation. In such case sufficient observations should be made to effect a good transfer from the old to the new site. (See also Apjoendix 8 for 1881 and Declination Tables for 1902, pages 90 to 97, for further directions and examples of observations.) 477. Observations with compass declinometer. — The upper portion of the instrument which carries the alidade can be reversed with refer- ence to the bowl, and the needle may be inverted by taking out the agate cup and inserting it from the opposite side of the needle. A full set of observations (Form 38) comprises two separate pointings on both the north and south ends of the needle in each of the two positions " Circle direct," '' Needle direct," and " Circle reversed " and " Needle inverted," with corresponding pointings on the mark at the beginning and end of the set. At places far removed from a magnetic observatory it is desirable to make sets of observations at about the times of easterly and westerly extremes of declination, say about 8 a. m. and 1 p. m., as the mean of the extreme does not differ much from the mean of the day. GENERAL INSTRUCTIONS FOB FIELD WORK. Example of observations and computation. Ill Station : Date: Instrument : Observer f Compass-Declinometer, C. & G. S. No. ■ Cliron. time a. m. Mark. Circle direct, needle direct. Circle reversed, needle inverted. Mark. North end. South end. South end. North end. h. m. 8 46 300° 60' 206° 29' 26° 36' 206° 40' 26° 30' 120° 53' 120 51 26 30 206 39 26 35 206 31 300 53 300 61 206 30 26 40 206 35 26 86 120 61 9 00 120 62 26 31 206 35 26 35 206 38 300 51 Means 300 61.0 206 30.0 26 37.2 206 36.2 26 83.8 120 52.0 Chron., 8.0 m. slow on L. M. T. Local mean time. H. M. 9 01 H. M. 1 40 Remarks. tt. m. p. m. Mark reading 300° 51'. 5 120° 51', 1 Needle reading 206 34.3 / 26 26 . Magnetic azimuth of line 94 17.2 94 26.1 True azimuth of line« 89 51.1 89 51.1 Magnetic declination b +4 26.1 +4 34.0 Index correction + 05.0 + 06.0 Diurnal var. correction c + 03.0 + 03.8 Resulting declination Mean « +4 34.1 +4 35.2 +4° 34'. 6 " Counted from south around by west. ^ When west, + ; when east, — . "^ To be supplied by the office. 478. Dip circle compass attachment. — The so-called " Lloyd-Creak "' dip circles, designed for observations on board ship, but suitable also for land observations, and some of the regular Kew model dip cir- cles are provided with a compass attachment, by means of which the declination may be determined when the true azimuth of the mark is known. This compass attachment consists of a compass needle mounted in a rectangular box, with graduated arc of about 25° at either end. A complete set of observations comprises readings of the horizontal circle for two settings of each end of the needle, first at zero, second 5° or 10° to the right of zero, third 5° or 10° to the left of zero, and fourth at zero again, with pointings on the mark at the begining and end of the set (Form 38). Care must be taken to re- move the compass attachment when dip observations are to be made, and on the other hand to remove dip needles when compass observa- tions are to be made. 112 COAST AND GEODETIC SURVEY. 479. Total intensity. — The total intensity maj^ be determined with a dip circle by Lloyd's method when suitable standardization observa- tions have been made at a station where the dip and intensity are known. This method involves first the determination of the angle of dip with a loaded needle, and second, a determination of the angle through which another needle is deflected by the loaded needle when the latter is placed at right angles to it in the place provided between the reading microscopes and protected by the brass shield. As the determination of total intensity' by this method is relative, it is nec- essary to guard, as far as possible, against any change in the mag- netism of the two needles and to use the same weight in the field as during the standardization observations. Their polarities must never he reversed, therefore, and they must not be allowed in close jjroximity to the bar magnets when these are being used to reverse the polarity of the regular dijD needles. (For further explanation, see pp. 24 and 125, Appendix 8 for 1881 ; p. 195, Appendix 3 for 1904, and Form 389.) 480. Standardization observations should be made at the beginning and end of the season's work to determine the intensity constant and the index correction of the compass needle. 481. Observations on board ship. — Declination on shipboard is deter- mined with the standard compass, dip and intensity with a Lloyd- Creak dip circle mounted on a suitable gimbal stand. The successful determination of declination, dip, and intensitj' at sea requires, first, that observations should be made with the Lloyd-Creak dip circle at a base station on shore at the beginning and end of the cruise to de- termine the intensity constant for the particular weight used at sea and the correction to the dip as derived from the deflection observa- tions ; and second, that the ship be swung at the beginning and end of the cruise (and if possible in the highest and lowest latitude reached) at a place near shore where the declination, dip, and in- tensity are known from shore observations in order to determine the deviations of the standard compass and the deviations of dip and intensity at the dip circle position. 482. The accuracy of the results depends principally upon the suc- cessful determination and elimination of the efl'ect of the ship's magnetism. For this reason observations are usually made on 8 or 16 (preferably 16) equidistant headings, steaming in a circle for- ward and back (with port and starboard helms), holding the ship long enough on each heading to secure good results, and taking usually not over two hours for both swings. Since a complete deter- mination of dip and total intensity on each of 16 headings of the forward and back swings would consume too much time, the practice has been adopted of observing deflections alone while swinging ship in one direction and loaded dip alone while swinging in the opposite direction. Besides the total intensity derived from the combination GENERAL INSTRUCTIONS EOR FIELD WORK. 113 of those observations, a value of dip on eacli heading results from the deflection observations, since the suspended needle is deflected by ap- proximately equal amounts in opposite directions from its normal position. On each heading observations with dip circle are made in only one position of circle and needle, as follows : N. to EXE., Circle East, Needle Face East ; E. to SSE., Circle West, Needle Face AA^est; S. to WSW., Circle AYest, Needle Face East; W. to NNW., Circle East, Needle Face West. In this way the observa- tions with dip circle can be made in about the same time as required for the compass observations, which are being carried on simul- taneously. 483. When instructed to make magnetic observations at sea the ship should be swung at least once a day if possible. When circumstances would not permit a complete swing, results have sometimes been obtained from observations on and near the course, e. g., on course one or two points to starboard, one or two points to port, and back on course. They require a knowledge of the deviations on those par- ticular headings, which may be derived from the complete swings preceding and foUoAving. (See Appendix 3 for 1904, pp. 192 to 197, and Forms 354, 355, 356, compass, and 390, 391, 392, dip circle.) DESCRIPTIVE EEPORTS. 484. Descripti-^-e reports must be submitted to cover all hydro- graphic and topographic surveys. It is preferable to have a separate report for each sheet, but in some cases it may be more convenient to have a single report cover the consecutive sheets of a season's work in one locality where much of the information is common to the different sheets. 485. The descriptive report should not be in the form of a letter, and it should not be a journal of the work, and it need not contain anything about the movements of the party ; it should be entirely dis- tinct from the season's report. 486. It should be headed " Descriptive report to accompany sheet (insert number and title of sheet or sheets)." Writing must not be nearer than 1 inch to left edge of paper. If typewritten, two copies should be furnished. 487. The descriptive report is for the purpose of supplementing original sheets, either hydrographic or topographic, by information not readily shown thereon, and which will be useful in the interpre- tation of the sheets, in the compilation of sailing directions, and in chart construction. Preference should, however, be given to showing- information on original sheets themselves when practicable to do so. 488. The descriptive report should be written concisely, omitting all unimportant detail, and should be arranged in a systematic man- 53695—08 8 114 COAST AND GEODETIC SURVEY. ner with each class of information in separate paragraphs under suit- able miderscored headings. 489. Bearings given in connection with sailing directions and hydrographic information should in general be expressed as from seaward, and in degrees, and it must be clearly stated Avhether the bearings are true or magnetic. 490. Subject heads, — No general rules can be laid down, but the fol- lowing points will be suggestive in prejoaring descriptive reports so far as apiDlicable to any particular region and according to the char- acter of the survey made. The amount of detail to be given requires much judgment; over-minute details tend to obscure the most useful facts. Obviously certain classes of information may be useful as to a new country previously unsur'^'eyed which may not be necessary to give in connection with the resurvey of a well-known coast. (a) General description of the coast, following the geographical sequence of the jDublished Coast Pilots or Sailing Directions, and in- cluding the aspect or appearance of the coast on making the land; describing prominent objects, as, on a bold coast, the headlands, peaks, etc., with their form color and height; or, on a flat coast, the spires, beacons, etc. Especially describe the first landfall, and objects useful as guides to navigation. (&) Oiithjiiig dangers and islands, the limits of tide rips and break- ers, and their relation to wind and tide. {(■) Directions for passing the outlying dangers, if such are at a sufficient distance from the land to conveniently form a second para- graph of directions. ((f) Pilots. — Information as to their station or cruising ground; any special regulations or signals; their charges; the possibility of obtaining tugs, etc. (e) Refuge. — In case of stress of weather the best anchorage or the nearest harbor of refuge to run for ; or in extreme cases of dam- age the best place to beach a ship. (/) Currents., tidal or not tidal. — General conclusions from ob- servation or other information. How long does flood run after high water and ebb after low water? Does current set fair with channel? {g) Weather. — Under this head state briefly only new and impor- tant facts, as prevailing winds and their seasons, directions from which gales come and how they affect anchorage, land and sea breezes, rainy seasons, ice, fogs, and freshets and seasons or conditions when jDrevalent. {h) Description of the shore, with characteristics (as height, color, wooded, cultivated, bold, sandy) of each important headland, jsoint, island, and rock. (?) Landmarl'K. — Descrij^tion of all prominent landmarks likely to be useful to navigation or to future surveying operations. If GENERAL INSTRUCTIONS FOR FIELD WORK. 115 mountains, state whether siimniits are often clouded. (Hve measured or estimated lieights of mountains, liills, cliffs, islets, or rocks referred to. Describe ranj^es in use by pilots and means of identifying them. Suggestions should bo made as to other ranges that would be useful or as to artificial marks that it would be desirable to erect. {)) Inshore (/angers. — Extent and nature, least depth over them; whether visible; if breaking, at what stage of tide; how much, if any, is bare at low ^vater; marks or ranges for clearing them by day or night. (k) Wrecks, where usually occurring ; tendency of wrecks to break up or remain in position. (/) Appivdclu's. — General remarks, usual course from along shore or from sea, dependence on lead, apiDroaching in thick weather. {m) Burs and cluiiinels. — Least depth, best time or place for cross- ing or entering, permanency of bars and of channels; breakers on. bars and their extent and with what winds or tides they occur. [n) Anrlwi'aycs, with descriptions relative to their capacity, hold- ing ground, amount of protection, and circumstances of weather under which tested. (o) Landihij places, especially on a coast exposed to swell. {p) Rlrers. — AATien of any importance give draft and class of ves- sels which can enter; point to which tide reaches; depth on bar and permanency of channel ; current ; effect of freshets ; distance to head of steam navigation. {q) Watering places for ressels, rivers, streams, or springs. At ports state whether water is piped to wharves or supplied by water boats and charges; convenience for watering ships. (r) Lights, light-houses, liiioys, heaeons. life-sarintj stations. — Any useful details not given in light list, reliability of lights and buoys, and visibility of lights. {s) Sailing directions for approaching, entering, and leaving chan- nels and harbors; such directions should be actually tried under dif- ferent conditions and examined when plotted on the sheets. {t) Ports. — Commercial importance, customary anchorage, facili- ties for coaling and watering, supplies obtainable, facilities for re- pairs, marine railways or drydocks and capacity, wharves and depth of water alongside, chief exports and imports, means of communica- tion, time ball, station for reporting vessels, weather and cautionary storm signals, any special harbor regulations (attach copies if prac- ticable), quarantine regulations and stations, hospitals for mariners, harbor improvements in progress. (.m) Change of coast line or depths. — Mention any reliable evidence Eis to recession or growth of shore line or change of depths. If a re- survey, note any important facts regarding changes observed. Give evidence, if any, of subsidence or emergence of shores. 116 COAST AND GEODETIC SUBVBY. (v) Dangers reported or shown on previous charts or surveys; if not found, or if more water found, give in each case detailed state- ment of effort made to find former slioal water, and any important evidence as to tlie reliability of the previous report. (;/') Siirrci/ tnetliods. — Explain any unusual features of survey methods used; mention if any part of the work is incomplete or re- quires further examination, and the reason ; also if any portion is less reliable ; state the system of control of the Avork ; mention any discrep- ancies and adjustments made. PROGRESS SKETCHES. 491. A progress sketch faithfully representing the extent of the entire season's work should be preiDarecl and forwarded at the end of each season. Partial progress sketches need not be furnished at other times unless sioecially required. 492. Progress sketches should be made on tracing vellum, using black ink only. They must not be of excessive dimensions, usually not over 18 by 21 inches. Scales of TooVoir, ^tttoo o, or j^^jVins- are rec- ommended according to the extent and detail. They should be drawn sufficiently strong to be suitable for blue printing. 493. In the Philippines progress sketches of general coast Avork should, if practicable, be on a scale of yo-o-VTrir (the scale of the Philip- pine coast charts) ; for harbor surveys a larger scale may be used if necessary to clearly show the triangulation. The stamped title form is to be used on such sketches, giA'ing the following information : Class of work, island, locality, scale, dates, chief of party, vessel. 494. The progress sketch should indicate the approximate limits of the topography by parallel ruled lines, not closely spaced, the approxi- mate limits of the hydrography hx widely spaced dots, and the trian- gulation as indicated below, including the various operations of a sin- gle partj^ for one season on one sketch. 495. Principal triangulation schemes should be in heaA-y lines, and base lines should be of double width. A line observed at both ends should be full throughout. A line observed at one end should be full at the observed end and broken at the other. Reconnaissance lines should be dotted. Old stations recovered should appear thus: @ New stations should appear thus : A 496. Stations that have been searched for and not found, or that have been found and marked again, or the description of which is defective, should be reported. 497. All important points determined, including mountain peaks, should l)e shown as far as practicable (except that primary triangula- tion parties in the United States should show only the main scheme). GENERAL INSTRUCTIONS FOB FIELD WORK. 117 The intersection lines slioiikl be drawn lighter than those of the main scheme, and a confusion of lines may often be avoided by merely indi- cating with short radial lines tertiary positions determined, showing the general direction of the stations from which observed. GEOGRAPHIC NAMES. 498. Distinct names of points, islands, shoals, rocks, towns, moun- tains, etc., are necessary to the intelligible use of charts and sailing directions, and the surveyor should ascertain the accepted or native names. 499. As far as available, the decisions of the United States Geo- graphic Board in regard to the names are to be followed, and in the Philippines the decisions of the Philippine Committee of Geographic Names. 500. Names already in use on charts and maps and in the coast pilots should be verified; if well established and appropriate they should be adhered to, even though found to differ from the native or original name, especially if the feature is of more importance to navi- gation than it is to the inhabitants, and if the natives name is an awkward or difficult one. 501. Dual names for the same object lead to confusion and incon- venience, and special care should be taken to avoid giving a new name to an object already named, or changing a name already established. Where two names are in use it should be ascertained which is the more appropriate and the more acceptable to the people of the local- ity, and report should be made giving the authorities. 502. For such objects as require them, and for which acknowl- edged names can not be found, names should be recommended, select- ing as far as practicable designations that convey some idea of the form, character, productions, or traditions of the place, or some characteristic of its inhabitants; convenience of length of word and pronunciation should also be considered. Report should be made of names so recommended. 503. In new applications of the terms shoal, bank, and reef, the following distinctions should be made, but these terms already in use should not be changed : Shoal should be applied only to knolls on which there is a depth of 6 fathoms or less. Bank should be employed for knolls of greater depth. A reef is always rocky, and should not be used where there is more than 6 fathoms at low water. 504. Where the native names ascertained have not an established written form, they should be spelled according to the system of the Geographic Board, as follows: 118 COAST AND GEODETIC SURVEY. 505. The true sound of the word as locally pronounced is taken as the basis of the spelling. 506. An approximation only to the sound is aimed at. An attempt to represent delicate inflections of sound and accent would often result in forms of Avords too complicated for use. 507. The vowels are to be pronounced as in Italian and on the con- tinent of Europe generally, and the consonants as in English. a has the sound of a in father. Example : Java. (' has the sound of e in men. Example : Peru. i has the sound of / in ravine, or the sound of ee in beet. Example : Fiji. has the sound of o in mote. Example: Rome. u has the sound of oo in boot (oo should never be used for this sound). Example: Umnak. ai has the sound of i in ice. Example : Shanghai. au has the sound of oir in how. Example: Fuchau. ao is slightly different from above. Example : Macao. ei has the sound of the two Italian vowels, but is frequently slurred over, when it is scarcely distinguishable from ey in the English they. Example: Beirut. r- is alwaj's soft, and has so nearly the sound of s that it should sel- dom be used ; hard c is given by h. eh is alwa^'S soft, as in church. Example : Chingchin. / as in English ; j}h should not be used for this sound. Thus, not Haiphong, but Haifong. g is always hard (soft g is given by j). Example: Galapagos. h is always pronounced when inserted. j as in English ; dj should never be used- for this sound. Example : Japan. h as in English. It should always he used for the hard e. Thus, not Corea, but Korea. kh has the sound of the oriental guttural. Example : Khan. gh is another guttural, as in the Turkish : Dagh, Ghazi. ng has two slightly different sounds, as in finger, singer. th as in thing. Example : Bethlehem. sli as in shall. Example : Shasta. q should never be employed ; qu is given by Ic or hw. h, d, I, m, n, p, r, s, t, c, vc, x, and a, as in English. y is always a consonant, as in yard, and should not be used for the vowel /. Thus, not Wady, but Wadi. 508. All vowels are shortened in sound by doubling the following consonant : Examples a in fatter, c in better, ■/ in sinner, o in sohhing, u in riibher. 509. Doubling a vowel is only necessary where there is a distinct repetition of the single sound. Example : Nuulua. GENERAL INSTETTCTIONS FOR FIELD WORK. 119 510. Accents should not, generally, be used, but where there is a very decided emphatic syllable or stress which affects the sound of the word it may be marked by an acute accent. Example : Palawan. 511. In the Philippine Islands, in translating from Spanish into English nouns which are combined with geographical names, the foUoAving system should be followed, except in specific instances where a different usage has already been established : River, island, hay, point, and gulf are to follow the proper name. Mount, port, and cape are to precede the proper name. Rio grande is to be translated simply river. GENERAL REMARKS. 512. Office work. — During working days when for any reason field work is not done, all survey officers not performing other duty will per- form office work. Systematic office hours should be kept, not less than seven hours per day. 513. Completion of field results. — It should be the aim of a chief of party to turn in field records, computations, and sheets in a com- pleted condition, as far as circumstances may permit. All records and results must be transmitted as early as practicable, and in any event before the commencement of another season's work. 514. Records in general. — All records should be kept in a systematic manner in the standard forms so far as provided. They must be sufficiently distinct and clear to avoid all chance of misunderstanding ; particularly numbers must be written plainly. Explanation must be given wherever necessary so that the record may be intelligible to one not familiar with the field work. 515. Original records should not be made on loose sheets of paper to be afterwards copied into the regular form of record book, but should in all cases be made at once in the book which is to be trans- mitted to the office, and must be consecutive and continuous in the order of time in which the observations are made. 516. Erasures should not be made in original records. Where an error is discovered, draw a line through it and write the corrected figures above or to one side. 517. Original records in pencil must not be inked. Pencils softer than No. 3 should not be used in making records. It is preferable, but not essential, to make original records in ink. 518. The duplication of records is usually to be avoided, except in cases where called for in the general or specific instructions. The requirements are .specified under each head. The function of dupli- cation is the insurance against loss in transmission, and this should be kept in view in deciding special cases. 519. Records or computations sent by mail are to be well wrapped and registered. When there is duplicate information (in whatever 120 COAST AND GEODETIC SURVEY. form) it should not be forwarded by the same mail as the original, and in general should not be kept in the possession of the observer any longer than necessary after the completion of the work. 520. Computations in general. — Computations should be kept up during the field work as far as practicable, and at least far enough to sht)W that the observations are sufficient and the record complete. 521. Computations should be transmitted to the office promptly, as soon as reasonably complete. In no case should computations be held Avith the idea of making them perfect in the field, as the final revision of the computations is the function of the office. 522. All computations must be in a neat and orderly form, and complete, so as to be readily intelligible to others. Every important operation must be shown. 523. Standard forms for computations should be followed wherever jDracticable. 524. E^-ery computation must shoAV by whom made and by whom checked, with dates. 525. Proper titles should be written or pasted on each cahier of computations, giving all essential information, as kind of work, local- ity, date, observer's name, computer's name, etc. Printed labels are available to cover ordinary requirements. 526. No writing should be placed within 1 inch of the binding margin of the sheets. 527. Information affecting navigation, reports of dangers, and changes m aids to navigation. — All persons in the service of the Coast and Geo- detic Survey should communicate to the Superintendent any valuable information obtained affecting the interests of navigation along the coasts. Special reports should promptly be made of any information of the following classes, giving in each ca^e the authority and such recommendations as ma}' seem desirable: Eocks, reefs, shoals, or sunken wrecks (with depth of water over same), either not shown or incorrectly shown, aids to navigation differing in any respect from tlie data given on the charts or in the light or buoy lists, important errors or omissions on charts or in coast pilots or sailing directions, changes in depths or directions of channels, changes in coast line, currents, etc. 528. Determination of aids to navigation. — When of importance, such investigation should be made as may be possible without inter- fering with other work. Even outside of the limits of proposed work, when practicable, lights and buoys established by proper authority should be determined in position and described when they are not shown on the charts or have not previously been determined by this Survey. 529. Accidents to aids to navigation. — All reports of accidents to aids to navigation, or information as to difference from the data on charts GENEEAL INSTEUCTIONS FOE PIELD WOEK. 121 or in light or buoj"- lists will be sent (but without recommendation) directly to the light-house inspector in whose district the aid in question belongs, and copies of all such reports will be forwarded at the same time to the Superintendent. Suggestions or recommenda- tions as to the positions of lights, light vessels, or buoys should be forwarded only to the Superintendent. 530. Vessels en route from one port to another, when weather and other circumstances will permit, should verify the positions of light- ships and seacoast buoys. The positions of all buoys and light-ships on the field of work should be accurately determined. 531. Verification of coast pilots. — The published coast pilots and sail- ing directions should be examined in connection with the results of each season's work, or with information gathered elsewhere, and report of corrections or desirable changes made to the office. 532. Local officials, pilots, and fishermen, or others having valuable knowledge of the vicinity, should be freel}^ consulted. 533. Suggestions and recommendations of a definite character are in- vited as to survey methods or instruments, need of surveys or charts in any particular locality, economies in work, improvement or correc- tion of charts or other publications, and concerning aids to navi- gation. 534. Maps, charts, and sketches (or copies of them) containing in- formation as to geography, topography, or hydrography likely to be of value to the Survey should be obtained when practicable and for- warded to the Office. 535. Photographs. — Photographs illustrative of the geographical features of new regions visited are desirable — more especially views from seaward of important features of the coast, harbor entrances, and prominent landmarks. Views illustrative of surveying opera- tions, or of the people of the region, may also be of value when un- usual. The following information should accompany every photo- graph: Subject, locality, position from which taken (an exact loca- tion for views of important coast featvires is desirable), date, and by whom taken. 536. All negatives worth preservation taken with supplies and out- fits furnished by the Survey are to be transmitted to the Office. 537. In the Tropics, owing to climatic conditions, plates and films should be especially cared for, used as fresh as practicable, and de- veloped soon after exposure. If necessary, they should be forwarded for development. 538. Special effort should be made to protect plates and films from beino- fogged or light struck. Orthochromatic plates are recom- mended. 122 COAST AND GEODETIC SURVEY. 539. Coast views. — Hand sketches of important features of the coast are desirable when photographs are not available. It is important that the correct relation of objects be retained. Sketches as well as photographs should be taken from the position that will be of most advantage. for the use intended. Recommendations should be made as to views useful in coasting or entering harbors and desirable for illus- trating sailing directions or charts. 540. Care of instruments. — Proper care of instruments is important in all classes of surveying work. The officer using the instrument should personally see that it is kept in good order and not leave this to anyone else. Instruments in good condition and adjustment are essential to good work. 541. The arc of a sextant may be cleaned by wiping lightly with chamois skin or a soft rag dipped in weak ammonia. Never polish the arc with paper or cloth, as this is liable to deface the graduation. 542. Sounding wire, even when galvanized, is subject to rust if not well cared for. The reel should be wrapped around with oiled cloths and well covered from rain. When the sounding machine is idle for a short period the wire should be dried by running through cloths, and oiled, and this should be repeated once a month when the machine is not in use. 543. All surveying instruments should be cleaned from time to time. Surfaces that are liable to stick together when left in place for a long time should be moistened slightly with oil or tallow after cleansing and before assembling; this applies to the cells holding object glasses. 544. Particular care should be taken of steel tapes, steel parts of drawing instruments, etc., as all steel instruments are subject to rapid deterioration, particularly on board ship or in a tropical cli- mate. Steel tapes should be cleaned and oiled after use, and the chief of party should make sure that they are carefully handled at all times ; special care is required in reeling tapes. 545. A lens may be dusted with a camel's-hair brush, and when necessary may be cleaned by rubbing gently with soft tissue paper, first moistening the glass slightly by breathing on it. A lens should be examined occasionally to see that it is tight in its cell. 546. A three-arm protractor used in plotting hydrography should be examined occasionally to see that it is in good adjustment and has not lost motion in clamps or tangent screws. A protractor may be tested by measuring with it several angles which have been ac- curately constructed geometrically on drawing paper. 547. Care of property. — Reasonable and proper care should at all times be taken of property, boats, and vessels employed in the survey work. II>^DEX. Aids to navigation: p^u-agraph. Accidents to 520 Determination of -,28 Angles : Horizontal, observation of 21 Vertical, observation of 77 Position 216 Astronomical positions 1 Azimuth : General remarks HO Observation and compntatiou 111 ^ , , 2 sin" i t Table 111 = — : — —-; — _ ^ 114 Base: Accuracy, standard of _^ 15 Catenary correction 20 Computation of length __ . IS Measurement ' 15 Sites and base nets 14 Bases, frequency __ l:; Bench marks :;r>i, ;;!:(i, 438 Blank areas on charts 337 Bottom characteristics 26G Channels, development of 195 Comparisons of lead lines :!21 Compass declinometer 477 Comjiletion of field results 513 Computations 520 ComiDutation, field, of triangulation 4s Contours and contour Intervals 122, 136 Control of topography 116 Current observations 444 Curves, depth --- -- 104 Dangers, navigation;,] : Precautions in case of indications 1!J2 Eeported, to be searched for _- -- — .'135 Declinometer compass 477 Depth curves 2Sx Depth units 264, 303 Description of stations : Triangulation 3S Magnetic 468 Descriptive reports 484 Direction method observations 23 123 124 INDEX. Para;;i'aph. Dividers, spacing 320 Dragging for rocliS, ledges 207 Duplication : Trlangulatlon records 55 Hydrographlc records 263 Eccentric reduction, example 53 Elevation computatious : From nonreciprocal cjliservations 97 Prom reciprocal observations 91 Indefiuite objects 104 Elevations : By vertical angles 77 In plane table work 135 Observations 77,79,81 Record 82 Field results, completion of 513 Freezing of gauge, to prevent 382 General remarks 512 Geographic names 498 Horizontal angles, form of record 24,27,29 Hydrograpbic signals 166, 175, 177 Hydrography 162 Inclination correction tables (base) 76 Indefinite and temporary objects , 30 Information affecting navigation 527 Initial position 2 Instruments, care of 540 Intersection stations 28 Laud-survey marks ^ 45 Latitude, approximate determination 1,2 Lead lines 323 Lead lines, marking 329 Lengths of trlangulatlon lines 12 Lens, care of 545 Lettering 142 Locations, preliminary 140 Log and line for current observations 450 Longitude, approximate determination 2 Magnetics: Adjustment of instruments 469 Computations _ 475 Description of station 468 Dip circle compass attachment 478 Observations im land 470 Observations on board ship 481 Record - 473 Selection of stations 467 Maps, local 141, 534 Marking stations 33 Names, geographic 498 Office work 512 Pencil records 517 Photographs 159, 535 INDEX. 125 Paragraph. Pilots, infoi-mation from 1 336 Plane of reference : For elevations 78^ 134 For soundings 279,425 Plane table, use of _ __ 115 Plotting hydrographic sheets 283 Polaris, observations for latitude 2 Position angles 21(j Positions : Initial 2 Plane table, list of 160 Hydrograpbic list of 176 Pressure tubes 24S Progress slvetcbes 491 Property, care of 547 Protractor, tbree-arm 318 Radius of curvature, table of logarithm 108 Ranges : For compass deviation 338 For running sounding lines 196 Records : Base measurement 18 Elevations ^ 81 Horizontal angles 24, 27, 29 In general 514 Magnetics 473 Soundings 250 Reduction of soundings 276 Reefs : Development of 200 Locating in heavy weather 204 Reference marks 33, 38 Repetition method of observations 25 RocliS, dragging for 207 Scale equivalents, table 124, 317 Scale: For hydrograpbic sheets 165,299,317 For topographic sheets 121 Seeing, poor 47 Sextant : Care of 332 Use in topography 138 Sheets : Hydrograpbic — Completed, details on 286 Comparing with previous surveys 289 Plotting 283 Scale 165 Topographic — Dimensions 124 Inldng 150 Laying out 123 126 INDEX. Paragraph. Signals, hydrograpbic ^ 166,175,177 Sounding lines : Running by compass 107 Systems of 185 Sounding : Interval, distance 189 Interval, time ^ i- 190 Machine 249 Records -,~ 250 Reductions 276 Speed 191 Wire, case of 540 Soundings : Out of sight of signals 239 Selection of ^98 With vessel underway 243 Spelling of new native names 504 Spherical excess 54 Star charts 2 Statistics for hydrographic sheets 287 Strength of triangulatiou figures 11 Examples 65 Table 60 Suggestions and recommendatiuiis 533 Symbols, topographic 142 Tapes, care of 540 Theodolites for triangulatiou 22 Tide observations ; Plane of reference 425 Purposes - 339 Simultaneous 424 Tide gauge: Aero-mercurial 366 Automatic 368 Care of 393 Clocks 415 Comisarison with staff 356 Freezing, to prevent 382 Location of 340 JManometer : 362 Tide rips 302 Tide staffs 345, 347, 349, 352 Titles : For hydrographic sheets 286 For progress sketches 491 For toiJOgraphic sheets 157 Topography : Control 116 Features to be included 125 Inking sheets 150 Scale 121 With sextant 138 INDEX. 127 Paragraph, Towns, plans 141, 531 Triangulation : Accuracy 21 Field computation 48 F'gures ^ 9, 11 Records ^^ 24, 27, 29 Selection of instrument 22 Tertiary 1 6 Views 539 Vertical angles for elevation 77 o