AERIAL PHOTOGRAPHY 727 



cardboard. In addition to this, the center point of the picture, which is indicated by 

 most aerial cameras, should also be pricked through to the cardboard. The card- 

 board templet is then given a number to correspond to the picture, and a hole, perhaps 

 y^ in. in diameter, is punched at the center of the templet. Radial slots are pimched 

 centering at each radial point which has been pricked to the cardboard. 



Amongst the eight points which have been pricked on each picture, it is, of course, 

 essential to prick all the control points which have been identified in the field, as these 

 are the key points upon which the entire construction depends. At the time these 

 control points are pricked through from the picture to the cardboard templet, the name 

 or designation of the control point should be written on the cardboard control templet 

 for future use. This designation should be written in such a position on the templet 

 that it will not be cut away when the control point is punched. Posts with a fiat base 

 and a neck of just the diameter of the slot in the templets are now nailed down to the 

 mosaic board over the plotted position of each known control point. To begin with, 

 a group of templets is selected representing a strip of pictures on which two known 

 control points separated by a number of pictures appear. The templet on which the 

 first control point appears is now laid down on the mosaic board with the slot which 

 represents that point placed over the post. Additional posts are now placed in all the 

 other slots on this templet. The next templet in series is now picked up and laid over 

 the first templet which may be accomplished by sliding the posts in the first templet 

 back and forth in their slots until they fit the slots of the second templet. Now in the 

 second templet, posts are placed through all the remaining slots and the third templet 

 is placed over these latter posts in the same manner. This process is continued 

 through the series of prints until the templet is reached in which a slot has been cut 

 representing the next control point. It will probably be found that the assembly 

 of templets is either too long or too short to permit the slot representing this control 

 point to be placed over the post which is nailed to the mosaic board at this control 

 point. However, the entire assembly of templets is flexible and may be lengthened 

 or contracted like an accordian. If the string of templets is lengthened, all the posts 

 riding in the slots are spread out; if the string of templets is shortened, all the posts 

 ^vill move inward, automatically assuming a disposition proportional to the distance 

 between the control points. Thus, by lengthening or shortening the string of tem- 

 plets, it may be adjusted so that the proper slot may now be placed over the post 

 i-epresenting the second control point. We have now established the scale of assembly, 

 and every post is in the correct scale position for the point it represents. The balance 

 of the teinplets in each strip — strip by strip — may be buttoned to their respective 

 posts until the entire area is covered. 



The posts have a vertical hole in them, and the next step is to drive a pin, fastening 

 each post to the mosaic board. Now the templets may be lifted off, one by one and 

 as each templet is removed from its posts, the post which passed through the center 

 hole of that templet is numbered on the board to correspond to the negative and 

 templet number (which are, of course, identical). After the templets are removed, 

 the posts are pulled from the board one by one, and the small hole which is left by the 

 pin which secured the post in position is circled. This hole represents the correct 

 position for each image point which was represented by a slot and a post. 



We now have a pattern laid out on our mosaic board with the center and at least 

 eight outlying points established for each print which is to go into the mosaic. Meas- 

 urements may now be compared between these points and the corresponding image 

 points on the pricked set of contact prints and a ratio factor calculated for the enlarge- 

 ment or reduction of the print or any part thereof. In more elaborate mosaics these 

 measurements are used as the basis for determining the angle of tilt by which the 

 entire print or parts thereof are to be corrected in the rectifying camera. 



