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HYDROGRAPHIC MANUAL 



Page 676 



tion is made, segments of concentric circles are drawn on the sheet from each R.A.R. 

 station as a center, covering the area in which bomb returns were received from the 

 respective stations. These circles are termed distance circles to distinguish them from 

 the short distance arcs, which are drawn only in the immediate vicinity of each R.A.R. 

 position (see 763). 



All distance circles are plotted in units of time intervals from a given station based 

 on an assumed velocity of sound of 1,460 meters per second. The spacing of the circles 

 on the sheet should be according to table 25. 



Table 25. — Distance circle intervals for various scales 



The unit of measurement second is used here to mean a second (time) of sound travel 

 through sea water. Thus, a circle representing 5 seconds of sound travel would be 

 plotted a distanceof 1,460 x 5 = 7,300 meters from the station at the scale of the survey. 



Preparatory to placing the circles on the sheet, and preferably even before the 

 projection is begun, the boat sheet should be studied to determine the area in which 

 bomb returns were received from each R.A.R. station. This will be the area through 

 which the distance circles should be drawn. It is important that this instruction be 

 carefully followed in order to avoid unnecessary confusion where many stations are used 

 on a sheet. A table should also be prepared in advance giving the lengths in meters of 

 the various radii, computed at the plotting velocity of 1,460 meters per second and con- 

 verted to the scale of the meter bar being used. This table is then used for setting the 

 distances on a beam compass. 



To plot the circles the successive radii lengths are pricked lightly on the smooth 

 sheet. This should be done at two or more radial directions from the R.A.R. stations, 

 so that each distance circle when drawn may be checked as it passes through the pricked 

 points. To avoid a repetition of measurements, each distance, as set on the beam 

 compass, should be plotted from every R.A.R. station where it will be needed. 



After the plotting of the radii has been checked, the distance circles are drawn in 

 blue ink through the pricked points, using a fine light line. The distances in seconds 

 may be placed on the circles in pencil for temporary convenience. 



It is essential that the smooth sheet be kept absolutely flat while the radii are being 

 plotted and the circles are being drawn. This can be accomplished, in the first case, 

 by laying a straightedge on the sheet from the station to the pricked points, and in the 

 second case, by having an assistant slide a straightedge along the sheet paralleling the 

 beam compass, immediately in advance of or behind it. A clean stainless steel straight- 

 edge, or one whose underside is covered with a clean piece of paper, must be used in 

 order to prevent soiling the smooth sheet. 



