7632 HYDROGRAPHIC MANUAL PaGE 704 



the proper distance circle, the center of the device will be a point on the distance arc required. The 

 intersection of the two distance arcs can be plotted in the same manner by making the appropriate 

 concentric circles tangent to the two distance circles involved. The center is then a point on both 

 distance arcs. There is no limit to the number of distance arcs that can be drawn with this device at 

 any one position, although only two distance arcs can be used simultaneously. It is especially well 

 suited for verification work. 



b. With dividers and scale. — In this method a special scale is used, similar to a metric scale, con- 

 structed for the plotting velocity of 1,460 meters per second at the scale of the survey. The incre- 

 ments or decrements are measured with dividers. One point of the dividers is set on the distance 

 circle and, with the aid of a large celluloid triangle alined toward the station, the desired point is 

 pricked along the alined edge. This method is slow because the increments or decrements must be 

 plotted in an exact radial direction from the respective stations. 



c. With R.A.R. plotting scale. — A long rule similar to an engineer scale is used, with scales 

 graduated in seconds for the plotting velocity of 1,460 meters per second at various survey scales 

 (see 4828). The rule is long enough to be alined with the station by eye, the desired point being 

 pricked directly along the edge of the graduated scale. If such a scale is not available a substitute can 

 be made on a long narrow piece of celluloid graduated in seconds, with 1 or 2 seconds at one end divided 

 into tenths of seconds, the hundredths being estimated. 



After the points for the distance arcs have been pricked on the smooth sheet, short 

 pencil arcs are drawn through them with a beam compass set at the approximate plotted 

 positions of the respective stations. These pencil lines should be short, just long enough 

 to ensure their intersection with the sounding line. For drawing distance arcs of 

 various radii, it is convenient to use two sets of beam compasses, one with a long beam 

 and the other with a short beam, to avoid having to shift the fixtures an excessive 

 amount. 



Where two R.A.R. stations comparatively close to one another have been used, dis- 

 tance circles are drawn from only one (see 7341). To plot an arc from the station for 

 which no circles have been drawn, the increment or decrement is measured from the 

 distance circle plotted on the sheet. The total distance from this pricked point to the 

 true center of the distance circles is set on the beam compass, but the distance arc 

 must be drawn from the R.A.R. station from which the bomb return was received. 

 In this case the point of the beam compass must be centered exactly at each station. 



7632. Plotting the Sextant Angles 



After the distance arcs have been plotted the sextant angles are plotted. Sextant 

 positions can generally be plotted with a celluloid protractor since they are usually not 

 far from the buoy stations. Single angles are often obtained at R.A.R. positions and 

 the loci of these angles should be plotted, since the locus of each angle so obtained is 

 in effect another distance arc which must be considered in the determination of the 

 final position (see 7635). 



7633. Plotting the Bearings 



Bearings may be plotted by several methods. Parallel rulers may be used, but 

 their undersurface should be covered with paper so that the metal will not soil the sheet. 

 An inconvenience in their use is that the pieces of celluloid over the stations are fre- 

 quently detached by the rulers. The best method of plotting bearings, which is suf- 

 ficiently accurate, is by using the circular no-arm protractor (4539) . When this instru- 

 ment is used it is centered at the station and oriented by means of a short north-south 

 line drawn through the station, the bearings being plotted directly without shifting the 

 protractor. 



