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HYDROGRAPHY 



3741 



3741. Graduated Perpendicular Method 



a. Preliminary construction 



Before this method can be applied the following preliminary construction is 

 required on the layout sheet (fig. 63) : 



(1) Erect a perpendicular bisector (Line of Centers) to the line joining two control stations 

 selected for one angle. 



(2) With centers on the perpendicular bisector and radii equal to distances to one of the stations 

 (P in the figure) four arcs are drawn within the limits of the large-scale sheet, one near the top, one 

 near the bottom, and two approximately equidistant beween the top and bottom arcs. 



(3) With a protractor set on any point on each of these four arcs, measure the angle a (to the 

 nearest degree) subtended between the two control stations. In figure 63 these angles are 17°, 19°, 



APPROXIMATE SCALE 1:400,000 

 Figure 63.— Layout sheet for graduated perpendicular method. 



22°, and 25°. Also measure on the layout sheet the maximum range of angles that will be required 

 on the large-scale sheet. This will determine the limiting circles to be drawn. These angles should 

 all be laid off on the Line of Centers so that when construction of the circles is begun, it will require 

 merely an inspection of the layout sheet to place the boat sheet on the drafting table so that the Line 

 of Centers will be within the limits of the table top. 



(4) On each of the four arcs select three points. A, B, and C — one near each edge and one near 

 the centerline of the sheet. With a protractor measure the azimuths (to the nearest half degree) 

 between the center for each arc and the three selected points on that arc. (See azimuth line from 

 Line of Centers to point C on 25° arc in figure 63.) These scaled azimuths are used later in the 

 computation of the geographic positions of the selected points (see 37416(5)). 



