726 



HANDBOOK OF PHOTOGRAPHY 



already established. This control system will have been put in either by the Coast 

 and Geodetic Survey or the Geological Survey; by one of the other federal agencies, 

 such as the Forest Service; or possibly by a state, local, or commercial organization. 

 In any event, this existing control will probably be available to the map compiler in 

 the form of a description of the control point with information as to how to reach it and 

 the coordinates, i.e., the latitude and longitude of the point. 



It will now be necessary to take the contact prints to the ground, follow the 

 published description of how to reach the point, and then identify this point upon the 

 photograph. The control point on the ground will be marked, perhaps, by a concrete 

 monument, by a brass cap cemented in the rock, or in the case of less permanent 

 surveys by a stake driven into the ground, none of which will show on the photograph. 

 Nearby, however, will always be identifiable objects, such as a tree, fence corner, a 

 house, a bend in a stream or some other feature from which a measurement can be 

 made to the control station. Then the position of this station can be pricked on the 

 picture, circled, and annotated for future reference. 



Fig. 13^.— 

 Points marked oji 

 photograph for 

 purposes of align- 

 ing individual 

 prints for making 

 a mosaic. 



Fig. 135.— Slot- 

 ted templet for 

 radial control as- 

 sembly. 



After all the control points have been identified upon the pictures, the assembly 

 of the mosaic may be started. On each photograph at least eight points, disposed as 

 indicated in Fig. 13, should be selected. These same points should be identified in the 

 overlapping section of the adjacent picture. Occasionally a point will fall on as 

 many as six separate pictures. It must be precisely pricked and circled on every 

 picture. When at least eight such points have been selected for every picture in the 

 area, radial lines should be drawn through the points extending perhaps an inch on 

 both sides of the point. 



Next the radial-control assembly may proceed bj' any of three methods: 



1. Graphic method. 



2. Transparent-templet method. 



3. Slotted-templet method. 



In all three methods, the fundamental principle is identical, the main differences 

 between them being in the ease of adjustment, time consumed, and ultimate quality 

 of the result. 



Perhaps the easiest method to understand is the slotted-templet method, which 

 will be described in full. The operator who does not have slotted-templet equipment 

 available will be able to accomplish the same result by more tedious means by either 

 of the other two methods, which will be briefly described hereinafter. 



The picture with the points which are to be used for the radial control, pricked, 

 is placed over a piece of thin cardboard, which should be the same size as the picture, 

 and the points which are pricked on the picture should be pricked through to the 



