152 



HYDROGRAPHIC MANUAL 



thereon, the hydrographer is ready to begin 

 the survey. If it is proposed to run parallel 

 straight sounding lines, a series of proposed 

 lines are lightly penciled on the sheet. The 

 line spacing should be slightly less than the 

 maximum spacing authorized for the depth 

 or area. The survey should generally begin 

 at a junction with a prior survey and prog- 

 ress in a direction specified for advance in 

 the project instructions. However, it may be 

 desirable to survey an anchorage first, or the 

 selection of the working area may be gov- 

 erned by conditions of weather and sea. 



To start a sounding line the ship or 

 launch is navigated to the beginning of the 

 proposed line, trial positions being plotted 

 as this point is approached. When this point 

 is reached, the vessel is turned to the ap- 

 proximate course. The helmsman is told 

 what course to steer, the recorder is told 

 when to take the first position and sound- 

 ing, and anglemen are told what objects are 

 to be used in the fix. The first position is 

 taken and plotted and the course is altered 

 if necessary to follow the proposed line. Po- 

 sitions are taken and plotted at short inter- 

 vals until a course is established which 

 closely follows the proposed line. The time 

 between positions can then be lengthened to 

 the maximum permissible for the scale of 

 the survey and speed of the vessel (see 5-36) . 



On approach to the inshore end of a line 

 it may be necessary to reduce the speed of 

 the launch. A fix should be taken at the 

 time the speed is changed. Also when a 

 launch begins a line near the beach from a 

 stand-still, the fact shall be noted in the 

 record and the next fix should be taken as 

 soon as possible after the launch attains 

 sounding speed. 



5-59 Following proposed lines. — In hy- 

 drography, an effort should always be made 

 to follow the proposed lines as indicated on 

 the boat sheet. If Shoran or Raydist is being 

 used, this is most easily accomplished by 

 running along distance arcs from one of 

 the stations. Any desired line spacing can 

 be used by maintaining a constant distance 

 from a station. This method is especially 



efficient when strong or erratic currents 

 make it difficult to run straight lines. 



When sextant angle control is used, the 

 coxswain should be trained to select and run 

 ranges as an aid to keeping on the proposed 

 line. Otherwise, the vessel is kept as close 

 to the line as possible by steering compass 

 courses, making small changes as required. 

 Considerable experience is required to gain 

 the knack of making course changes of suffi- 

 cient amounts and at the correct time. 

 Changes in the direction or strength of the 

 current are often indicated by "current 

 streaks." These should be observed and re- 

 ported by the coxswain or anglemen. Changes 

 in the strength of the current may be ex- 

 pected in the vicinity of shoals and banks in 

 offshore areas. 



The line spacing is directly related to the 

 depth of water, becoming wider as depth 

 increases. The project instructions usually 

 specify maximum spacing for a certain 

 depth range, as 200 meters from 20 to 30 

 fathoms, and 400 meters from 30 to 50 fath- 

 oms. When the maximum spacing is exceed- 

 ed, a split should be run to fill the void, 

 except that a wide spacing at the outer limit 

 of the depth range may be accepted on even 

 slopes. The hydrographer must use good 

 judgment in running splits and should base 

 his decision on the character of the bottom 

 as well as the distance between lines. 



5-60 Turns and changes in course. — The 



soundings should be plotted as nearly as 

 possible on the exact path of the vessel (see 

 5-62). When a change of course greater 

 than 10° is made, a fix should be taken as 

 soon as the new course is reached. When a 

 change of approximately 90° is made the 

 soundings on the turn may be plotted or 

 omitted at the discretion of the hydrog- 

 rapher, but it is best to omit them. A fix 

 must be taken as soon as the vessel is on the 

 new course. Soundings shall not be recorded 

 on the turns when a vessel turns about to 

 begin another line. 



5-61 Sounding speed. — When echo sound- 

 ers are used, soundings can be obtained 

 while the survey vessel is operated at stand- 



