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AERIAL PHOTOGRAPHY. 
control points. These are the triangulation stations that have been 
accurately determined and marked. If the controls in any one area are 
not properly co-ordinated with those of another area near it, the result 
would be that when different topographical surveys and maps are joined 
there would be overlaps, gaps, and off-sets, which would cause no end 
of trouble and confusion to the map maker. 
Correctine Disrorren Picrures—Auromaric ‘CAMERAS. 
As the methods that have been tried for maintaining the lens vertical 
have not given sufficiently accurate results for some purposes, other 
means have been investigated. One has been to refer each picture, when 
taken, to some stable reference point, so that the tilt of the negative and 
the direction of the tilt may be ascertained. Such data will be sufh- 
cient with the aid of certain apparatus and methods to rectify the 
negative to the plane of the horizon. The gyroscope has also given 
great promise in this direction. 
TRANSFORMING CAMERA, WITH REVOLVING DISKS CARRYING 
NEGATIVE AND PLATEHOLDER. 
So far the most reliable results have been obtained by taking the 
photographs with a fixed camera, and making the necessary. corrections. 
The amount and direction of the tilt are known, and also the height of 
the aeroplane. How can these negatives showing such distortion be 
corrected? Notable results have been obtained by M. L. P. Clerc, who 
has carried out extensive research work for the French Aviation Service 
during the greater part of the war. Clerc investigated, with great 
thoroughness, the mathematical conditions involved in transforming the 
photograph taken by an inclined aerial camera into one corresponding 
with that obtained from the same point of view, but with the lens vertical. 
As a result of his investigations a camera was designed with which the 
distorted photograph is automatically corrected, or, in other words, by 
which an orthographic projection is obtained from a negative taken at 
any angle. A similar result has been achieved in the United States of 
America, and a transforming camera has been designed and constructed 
by F. H. Moffit, of the Geological Survey, by means of which the nega- 
tives taken in any one plane can be transformed into any other plane. 
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