From Azimuth Tables: With Lat. 38° N., Dec. 5° S., & L. A. T. 8h 20m, sun bears NOTE.-[By using Weir's Azimuth Diagram the true bearing of the sun may be obtained without using the Azimuth Tables.] Corr. for Dec. 4°.7 S=.3X46 Corr. for L. A. T. 8h 18m.8=.12X113 Corr. for Lat. 38°05'-.05X23 Total correction True course 296°.8 With distance, 14′.6 (alt. diff.), on course=reverse bearing of sun. Dist. 14.6 N. 6'.6 W. 13.0 D. Lo. 16/.5 W. Corr. in Long. 4'.29 W.=4′ 17′′ W. At 8 a. m. for D. R. Lat. 38 03 11 N. Long. 72 50 26 W. λ 4h 51m 21.7 At a. m. observation the G. A. T. was At a. m. observation the Long. by Obs. was 4 51 21.7 At a. m. observation the L. A. T. was At a. m. observation the W. T. was Long. at 8 a. m. by Obs. Long. at 8 a. m. by D. R. Diff. in Long. due to current Drift per hr. in Long. October 5, 1916.-Work from 11 a. m. to noon. On course 66° (true), at speed 11.7 knots: Northing per hour=4.76. Easting per hour=10.69. D. Lo. per hour=13′.6. Change in Long. per hour due to speed of ship Change in Long. per hour due to current 2m 26.5 17 20.4 13'. 6 E. 1.39 W. Change in Long. from 8 to 11 a. m.=12′.21X3b=36′.63 Watch slow on L. A. T. at 8 a. m. 12. 21 E. Total amount watch will be set ahead 20 19.7 Time of run from 8 a. m. to noon will be 20m 19.7 less than 4b-3h 39m 40°.3=3.66. Ship changes Long. per hour time to noon={1-(19m 46.9)}=40m 13′.1=0.67. Change in Long. from 11 to 12=.67×12′.21=8′.2=0m 32.8. Change in Lat. from 8 a. m. to noon=3.66X4′.76=17′.4. N. Change in Long. 3.66X12.21=44′.7. E. At 8 a. m.: Lat. by D. R. Run to noon D. L. 38° 03′ 11′′ N.; 17 24 N. At noon: Lat. by D. R. 38 20 35 N. Long. by D. R. 71 44 57 W. Long. by Obs. 72° 50′ 26′′ W. D. Lo. 44 42. E. 8 a. m. Long.) 72 05 44 W. run to λ=4h 48m 23 (Used in Noon. Lat. Obs.) In setting watch, left an error making watch 18 fast on L. A. T. L=90°-d-h, or approx. h=90°—(d+L). Corr. (Table 46) Approx. h=46° 52′ }+ 4° 46'. 5 S. 6 H. D. 4 47 06 S. Corr. 9' 02/ Corr. + 10 52 I. C. + 1 50 32.8 d+L 43° 08′ d+Corr. 4 57 58 Total Corr. + 10 52′′ Approx. h=46° 52′ L=L+AL; L/-90°-d-obs. alt.-at+Corr. to alt. L-90°-d-at2+Corr. to alt. ±AL-obs. alt.=K-obs. alt. or K-90°-d-at2+Corr, to alt. +AL. Since ship is making northing, A L in this case is +. If watch is 18s fast, the observations should be taken at watch times: For Lat. 38°.3 N., and Dec. 4°.88 S., we find from Table 26 that a-2.27, and from Table 27, at 2 AL +1 By Obs. Lat. 38 By D. R. Lat. 38 42.2 N. 20. 6 N. Long. 71° 51. 6 W. Long. 71 44. 9 W. .D. L. 21.6 N. D. Lo. Dep. 6.7 W. 5. 3 W. Current, 22.2 miles. Set=346°. Drift=22. 2 miles=1. 2 knts. per hour. From chart: Course for next 24 hrs.=72° (true). Var. 10° W., Dev. 4° W. Compass course=86° 13 15-13.2 on reverse bearing of sun. From azimuth tables: With Lat. 38° Ñ., Dec. 5° S. L. A. T.4h 00 00 sun bears N. 113° 31′ W. Corr. for Lat. 38°.94 = Corr. for Dec. 4.84 .94 X 19 +0° 18' Total Corr. -0° 27' 0° 27' With distance, 13. 2 (alt. diff.), on true course=N. 66° 56′ E. True bearing N. 113° 04′ W. True course. 66°. 9 Dist. N. E. 408. DEFINITIONS.-Surveying is the art of making such field observations and measurements as are necessary to determine positions, areas, elevations, and movements on the surface of the earth, giving its characteristic features, such as, on land, the position of prominent objects, heights, and depressions, and on water, the depth, nature of bottom, position of shoals, and velocity of currents. Topographic Surveying relates to the land, and Hydrographic Surveying to the water; and both are underlaid by Trigonometrical Surveying which, when it is carried on with high precision over such large areas as to contribute to form a basis for determining the size and shape of the earth, becomes a department of Geodetic Surveying. It is not deemed appropriate to include in this work a complete treatise on marine surveying. The scope of this chapter will be to set forth such general information regarding the principles of surveying and the instruments therein employed as will give the navigator an intelligent understanding of the subject sufficient to enable him to comprehend the methods by which marine charts are made, and, if occasion should arise, to conduct a survey with such accuracy as the instruments ordinarily at hand on shipboard permit. For a more detailed discussion of marine surveying, the student is referred to the various publications which treat the subject exhaustively. INSTRUMENTS EMPLOYED IN MARINE SURVEYING. 409. THE THEODOLITE AND TRANSIT.-The Theodolite (fig. 62) is an instrument for the accurate measurement of horizontal and vertical angles. While these instruments vary in detail as to methods of construction, the essential principles are always identical. A telescope carrying crosshairs in the common focus of the object glass and eyepiece is so mounted as to have motion about two axes at right angles to one another; graduated circles and verniers are provided by which angular motion in azimuth and (usually) in altitude may be measured; and the instrument is capable of such adjustment by levels that the planes of motion about the respective axes will correspond exactly with the horizontal and the vertical. The telescope is carried in appropriate supports upon a horizontal plate which has, immovably attached to it, one or more verniers, and which revolves just over a graduated circle that is marked upon the periphery of a second horizontal plate, a means of measuring the motion of the upper plate relative to the lower one being thus provided. Thumb screws are fitted by which the upper plate may be clamped to the lower, and (excepting in some simpler forms of the instrument) others by which the lower plate may be made immovable in azimuth, or allowed free motion, at will; all clamping arrangements include slow-motion tangent screws for finer control. A vertical graduated circle, or arc, with a vernier, clamps, and tangent screws, is fitted to most theodolites, for the measurement of the angular motion of the telescope in altitude. The theodolite usually carries a magnetic needle, with a graduated circle and vernier for compass bearings. The instrument is mounted upon a tripod, and levels and leveling screws afford a means of bringing the instrument to a truly horizontal position. |