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HANDBOOK OF PHOTOGRAPHY 



The result is, of course, that the ratio print made from the comparative measure- 

 ments between the mosaic board points and the points on the contact print will fit 

 the mosaic board and permit the assembly of a precise map. 



The first method listed above was the graphic method of radial control. This 

 method utilizes the radial lines which were first drawn on the print. Instead of 

 buttoning the print templets together as was done in the slotted templet method, the 

 graphic method traces the radial lines from each print on a piece of tracing cloth and 

 causes the successive radial lines to intersect as nearly as possible at a point. This 

 method is tedious because, after the first run of radial prints between the two known 

 control points, it is usually found that the scale selected for the control plot is too large 

 or too small. A correction factor must then be established between the first two 

 prints of the run, and the whole run must be duplicated perhaps two or three times 

 until by trial and error the proper scale is found. 



-A B 



Fig. 14. — Fairchild four-couple transformer jig set over plateholders. A, five micro- 

 scopes so set as to correspond to the five index marks of the four-couple camera; B, adjust- 

 ment screws used to bring plates into proper position under the microscope; C, clamping 

 down nuts to hold the plates securely in position after adjustment. 



The second method of radial control which was enumerated is the transparent- 

 templet method. In this method, a celluloid templet is traced from the contact print, 

 i.e., the center and the radial lines are traced from the contact print. The method 

 differs from method 1 in that the celluloid templets are laid down one over another 

 and the radial lines so adjusted that they intersect at a point. Then when the second 

 control point is reached, if the scale is too large or too small, each templet is moved a 

 little closer or a little farther from its adjacent templets so that the scale is adjusted. 



A simpler method of mosaic assembly, which has proved satisfactory, particularly 

 in regions of very moderate relief, is known as the print-to-print ratio method. This 

 method is based on the geometrical principle that regardless of displacement due to 

 difference in elevation, the distance of an image when measured perpendicularly to the 

 line joining the centers of two adjacent prints is the same on both prints (disregarding 

 lens distortion and tilt). The purpose of this method is to compensate for variations 

 in the airplane altitude. If the airplane altitude changes between one picture and the 

 next, a direct ratio may be derived by making measurements perpendicular to the 

 center line and to common images on the adjacent prints. When the ratio from print 



