3313 HYDROGRAPHIC MANUAL PaGE 214 



mark or control station, as range la-Sm, range la-16m, etc., in which 8m and 16m 

 represent the distances in meters along the sounding line from the control station. 



3313. Frequency of Positions 



The regular interval between consecutive fixed positions along a sounding line 

 should be such that the proposed lines can be followed closely enough so that splits will 

 rarely be necessary and there will be little uncertainty about the placement of the 

 soundings between fixed positions. Thus the frequency of position will depend on 

 the depths, the spacing of the lines, the scale of the survey, the bottom relief, the speed of 

 the vessel, the length of the sounding lines, and the constancy of the current. It is appar- 

 ent that some of the above are complementary ; for example, where the depths are greater 

 the lines are spaced farther apart, and the survey is usually made on a smaller scale. 



A uniform frequency of position should be chosen, which can be generally main- 

 tained throughout most of the survey. This will be of advantage in plotting and spacing 

 the soundings and will aid in detecting errors in plotting. 



For handlead sounding on large scales at the proper speed of vessel, the interval 

 should seldom exceed 3 or 4 minutes, but may be increased somewhat for offshore work 

 on smaller scales. 



A position should generally be taken at each vertical wire sounding, but when the 

 soundings are close together and there is little likelihood of the vessel's being deflected 

 from her course by currents or other causes, a position may be taken on every second 

 or third sounding. 



In an echo-sounding survey of an area of fairly even bottom, the maximum dis- 

 tance between adjacent positions on a line should be about 1 % to 1 ji inches on the survey 

 sheet, regardless of scale. 



Where the bottom is irregular, the slope is steep, or there is difficulty in keeping the 

 sounding vessel on line due to current, or for any other reason, positions should, of 

 course, be taken more frequently. 



When echo soundings are taken from a vessel proceeding at standard speed the 

 distance traveled in any interval of time will be longer than at a lesser speed, but the 

 proposed lines can be followed more closely because of this increased speed. 



In most hydrographic surveys controlled by three-pomt fixes, the signals are 

 readily visible and positions can be taken almost as frequently as they can be recorded 

 and plotted. For a vessel with a speed of 8 knots, taking echo soundings controlled by 

 fixed positions on a 1:10,000 scale, the maximum time between positions should ordi- 

 narily be about 1% to 1% minutes. For smaller scales, the times between positions may 

 be increased slightly less than proportionately. For a 1:20,000 scale and other condi- 

 tions as before, the maximum interval should probably be between 2% and 2% minutes. 



For surveys controlled by R.A.R., there is a practicable minimum time between 

 successive positions because of the time needed to receive the returns, take the time 

 intervals from the chronograph tape, and plot them on the boat sheet. In 1941 this 

 required about 4 minutes, although developments in methods and equipment may 

 reduce this somewhat. Only rarely are surveys controlled by R.A.R. plotted on 

 scales larger than 1 :80,000, and for this and smaller scales an interval less than 4 

 minutes should seldom be required. (See 6812.) 



On dead-reckoning lines controlled by astronomic sights, positions with log readings 

 should be taken at regular intervals of about 10 minutes each. These shoidd be 

 supplemented by positions at times when control data are observed, as for instance at 



