722 



HANDBOOK OF PHOTOGRAPHY 



Line of common 

 scale ^ ^ — ■^/ 



cally the important factor is that the focal plane be horizontal. If the focal plane is 

 truly horizontal, the resulting picture, if taken through a distortion-free lens, is a true 

 conic projection of the ground. When the focal plane departs from the horizontal, a 

 perspective picture is secured in which lines which are parallel on the ground will con- 

 verge on the photograph. If the tilt is slight (less than 

 2°), the resulting errors are fairly negligible from the 

 standpoint of most map uses. If the tilt, however, is 

 greater than this amount, the pictures must be rectified 

 before they may be considered for the basis of map com- 

 pilation (Fig. 9). 



For the purpose of correcting tilt, a rectifying camera 

 is employed in which the negative and easel are inclined 

 into conjugate focal planes to compensate for the tilt. 

 Rectification is too complicated to more than mention in 

 this article. Details are fully available at any good library 

 (Fig. 10). 



A factor which is even more difficult to handle than tilt 

 from the standpoint of map compilation is the variation in 

 scale due to irregularity of the surface of the earth. If, 

 for example, a truly vertical picture is taken of flat but 

 uniformly sloping ground, a picture will be secured with characteristics very similar 

 to a tilted picture. Such a picture may be substantially corrected in the rectifying 

 camera. 



When the surface of the earth is irregular such as in mountainous country, the 

 problem of compilation of a precise map becomes much more difficult. In such a 



Fig. 9A. — Tilt of cam- 

 era results in photo- 

 graphing a trapezoidal 

 instead of a rectangular 

 area. 



Isocenfer or scale poi'nf 

 where normal ana 

 filfed focal planes 

 in fersec / - - ~. ^ ^^w 



Principal poinf 

 where lens axis 

 in fersec is focal 

 plane 



Verh'cai axis — > 



Nadir poinf where 

 verfical ray in fer- 

 sec fs focal plane 



_-Tilfed focal 

 plane 



ji- Nor ma I focal 

 plane 



Tilfed axis 



Fig. 9B. — Diagram illustrating the line of sight and plane of film when camera is level and 



when camera is tilted. 



case there are no large uniform slope areas, and in order to achieve a precise result, 

 elaborate methods or machines must be used. 



Differences in elevation of the ground result in radial displacements on the photo- 

 graph. If, for example, a tall telegraph pole falls exactly in the center of the picture 

 (Pole 1, jTifii, Fig. 11), the top of the pole will appear exactly superimposed over its 

 base (<i6i). If the same telegraph pole is photographed any place else in the picture 



