6821 HYDROGRAPHIC MANUAL PaGE 640 



stations. The elapsed times must be used as distances in order to plot them on a 

 hydrographic sheet, and for this purpose the velocity of sound in sea water must be 

 known. The distances from the stations may be plotted graphically by several different 

 methods, the position sought being at the intersection of the distance arcs (see 7631). 



During the survey the positions need to be known as quickly as possible, so that 

 the ship may be maintained on the proposed line. Therefore, for the boat sheet a 

 rapid method of plotting is required— extreme accuracy is not essential so long as com- 

 plete coverage of the area is ensured. 



For the smooth sheet the converse is true, the positions must be plotted as accu- 

 rately as practicable and speed is not especially important. At this time better values 

 of the velocity of sound to be used will have been determined and the positions can be 

 more accurately plotted. 



There are several general methods which may be employed in plotting R.A.R. 

 positions. The elapsed time to each station may be converted into distance from an 

 R.A.R. velocity scale (4824), and the distances plotted on the sheet with a beam com- 

 pass. This method is generally used for plotting on the boat sheet in areas where 

 different velocities of sound must be used. 



Circles representing intervals of distance from the R.A.R. station may be drawn 

 on the sheet and the distance from each station may be plotted by increments added 

 to, or decrements subtracted from, the nearest circle representing the appropriate 

 distance from the station. The circles may be drawn in even multiples of distance in 

 meters or they may be chawn in distances representing even multiples of travel time 

 at a selected velocity (see 7341). Positions can be plotted more rapidly from circles 

 representing seconds of travel time, and this method is used for plotting the positions 

 on the smooth sheet (see 763). 



6821. Plotting Positions on the Boat Sheet 



After the elapsed times have been received from the chronograph station they are 

 used to plot the position of the bomb explosion (see 6811). If distances in meters are 

 used for plotting, the elapsed times are converted into distances on an R.A.R. velocity 

 scale, using a beam compass with the fixtures fitted to a suitable length bar. The 

 distance from each station is plotted as a short arc near the estimated position on the 

 boat sheet. The position is determined from the intersection of the distance arcs 

 from two or more stations. 



The approximate velocity of sound must be known before positions can be plotted 

 on the boat sheet. Apparent horizontal velocities (6351), determined from tests 

 made over distances measured by taut wire or from tests at positions fixed by sextant 

 angles, are generally used for plotting during the survey. "Where the apparent 

 horizontal velocity is not known, or is not applicable, velocity calculated from the phys- 

 ical characteristics of the water, as explained in 6343, may be used. (See also 636.) 



6822. Plotting a Position With One Distance Arc 



Occasionally a return is received from only one R.A.R. station, which, of course, 

 will not fix the position. If this one distance arc intersects the sounding line at an 

 angle greater than 45°, the position may tentatively be assumed to be at its intersection 

 with the course from the preceding position. If the distance arc intersects the sounding 

 line at an angle less than 45°, the position should be assumed to be at its intersection 

 with the corrected log distance plotted as an arc from the preceding position. 



