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HYDROGRAPHIC MANUAL 



symbol. If an Edmonston Beam Holder is not 

 available, a Horn center should be placed 

 over the station mark to prevent damage to 

 the sheet as the arcs are drawn. Radii for 

 the circles should be measured along three 

 lines drawn from the station. When boat- 

 smooth sheet procedures are to be used, it is 

 best to compute three points on the circle 

 of largest radius and plot these points on 

 the projection. If the measured distance does 

 not check the plotted points the radius of 

 each arc shall be multiplied by a factor which 

 will distribute the distortion uniformly 

 across the sheet. 



The station may be off the limits of the 

 sheet, in which case it is necessary to com- 

 pute at least three points on three or four 

 circles. When the area to be surveyed is not 

 too far distant from the station, the outline 

 of the boat sheet limits can be plotted on a 

 nautical chart and three or four radial lines 

 drawn from the station across the sheet as 

 shown in Figure 51. The azimuth of the 

 lines may be measured with a protractor and 

 the distances scaled off the chart. Because 

 of the distortion inherent in the Mercator 

 projection, a Lambert conformal projection 



should be used when long distances are in- 

 volved, especially if the scale of the survey 

 is large. World Aeronautical Charts (WAC) 

 at a scale of 1 : 1,000,000 are suitable for this 

 purpose. An alternative method is to scale 

 the coordinates of a point near the center 

 of the sheet and compute an inverse be- 

 tween the point and the station. From the 

 computed azimuth and distance a pattern of 

 points on the circles can be developed and 

 their coordinates computed. The scaled dis- 

 tances must be expressed in terms of the 

 units measured by the system, that is : Micro- 

 seconds for EPI, statute miles for Shoran, 

 and lanes for Raydist. The distances are 

 then converted to meters. Table 7 can be 

 used to convert microseconds to meters and 

 Table 8 to convert statute miles to meters. 

 Since lane width for the Raydist system is a 

 function of the frequency used and is ex- 

 pressed in meters, no conversion table is 

 necessary. 



The geographic coordinates of the points 

 Al, Bl, CI, etc., in Fig. 51, are computed 

 and plotted on the sheet. The radial lines 

 should pass through the computed points 

 along each azimuth, and a circle should 



Station 



Figure 51. — Principle of drawing distance arcs when the station is off the sheet. 



