5. HYDROGRAPHY 



153 



ard speed, except when the vessel is heading 

 into heavy seas thus creating turbulence 

 which blocks the echo. This is the most effi- 

 cient speed, since more sounding lines can 

 be run in a specified time, and better steer- 

 ing control is attained than at slow speed. 



The speed of advance must be reduced (a) 

 when there is risk of grounding; (b) when 

 the existence of submerged dangers is sus- 

 pected; (c) when conditions of the sea re- 

 quire it; (d) when necessary to comply with 

 specifications for minimum distance between 

 successive positions ; and (e) when sounding 

 with leadline or pole in order that true ver- 

 tical measurements can be obtained (see 

 3-69). 



The speed should be as uniform as pos- 

 sible between positions. Each change of 

 speed shall be recorded at the time of 

 change. A position should be obtained at 

 the time of change in speed, or as soon 

 thereafter as practicable. 



5-62 Plotting a sounding line. — After a 

 position has been plotted, a pencil line is 

 drawn to it from the last position. If a 

 change of course has been made between 

 the positions, the line should be drawn to 

 show the course change and shall represent, 

 as nearly as possible, the actual track of the 

 vessel. The curved lines following distance 

 arcs may be drawn by use of a French curve, 

 or by use of small pieces of clear plastic 

 trimmed to fit arcs of various radii. 



When a line is run parallel with the beach, 

 it is often impossible to obtain positions 

 with sufficient frequency to plot all changes 

 of course and the hydrographer should draw 

 a line which closely approximates the path 

 of the launch between successive positions. 

 A note should be made in the sounding rec- 

 ord that the boat sheet plotting shall be used 

 as a guide in smooth plotting the line. A 

 similar situation often exists when sounding 

 in narrow winding channels in streams and 

 mud flats and should be treated in the same 

 manner. 



When hydrography is done by a ship it is 

 usually possible to ink the position numbers 

 as soon as they are plotted, and, in some 

 cases, the soundings may also be inked from 



carbon copies of the sounding record pages. 

 In launch hydrography this is seldom pos- 

 sible. The positions are numbered in pencil 

 as they are plotted, and inked in assigned 

 color at the end of the day. In so far as it is 

 possible to do so, the position numbers shall 

 be placed below and to the right of the posi- 

 tion. The numbers should be much smaller 

 than the soundings. The day letter (see 5- 

 35) shall be shown at the beginning and 

 ending of each line and at every position 

 which is a multiple of 5. The day letter 

 shall also be shown at every detached 

 position. 



5-63 Inking soundings on boat sheet. — 



Soundings shall be plotted on the boat sheet 

 in black India ink each day as hydrography 

 progresses. The soundings shall be reduced 

 to the tidal datum adopted for the area from 

 predicted or observed tides. Significant cor- 

 rections for phase differences or variation of 

 the initial should be applied; small correc- 

 tions can be ignored for boat sheet plotting. 



The soundings between positions shall be 

 properly located by the use of a spacing di- 

 vider, and odd interval soundings correctly 

 shown. If the film positive method of smooth 

 plotting the survey is to be used, the sound- 

 ing on the position should be omitted pro- 

 vided that it is not critical. In any case, 

 the sounding must not obscure the position 

 dot or the position number (see 6-47). 



Only one unit, fathoms or feet, shall be 

 used on the boat sheet (see 1-38). Expert 

 penmanship is not required, but the sound- 

 ings should be uniform in size and clearly 

 legible. In congested areas a selection of 

 soundings should be made but all soundings 

 should be plotted which are necessary to 

 show bottom configuration. The least depth 

 on shoals and dangers should be shown in 

 somewhat bolder figures than the surround- 

 ing soundings. A note should be placed in a 

 marginal area nearby giving the least depth 

 and a reference to the sounding record posi- 

 tion number and day letter. 



Where the depth unit is fathoms, and the 

 bottom is generally flat or has a gentle slope, 

 the soundings shall be plotted in fathoms 

 and decimals in depths less than 11 fathoms. 



