AERIAL PHOTOGRAPHY— B AISLE Y 385 



proximately 25 miles wide. From these photogi^aphs features can be 

 plotted on a map probably more accurately, and certainly much more 

 rapidly, than can be done by ground survey methods. 



The pictures taken for mapping purposes are the kind ordinarily 

 called "still" j^ictures to differentiate them from "moving" pictures. 

 These pictures are taken at intervals during flight, varying from a 

 few seconds to several minutes, depending on the altitude and speed 

 at which the airplane is flying as well as on the area covered by 

 each picture. Most mapping processes require that all parts of the 

 area to be mapped show on at least two photographs, and this re- 

 quirement, together with the conditions outlined above, determines 

 the time interval between exposures. 



At the present time there are in use in this country two types of 

 cameras that are considered to be up to date. One of these, the 

 K3-B (pi. 5, fig, 1), which is a single-lens camera using a lens of 8145 

 12, or 24 inches focal length, is electrically operated and can be set 

 to run automatically, taking pictures at a predetermined interval of 

 anywhere between 3 or 4 seconds and several minutes. It can also be 

 manually operated and is adapted to both vertical and oblique pho- 

 tography. The picture size is 7 by 9 inches. The other, the T3-A 

 (pi. 6, fig. 1), is suitable for mapping purposes only and is manually 

 operated. It has five lenses, taking five pictures simultaneously. A 

 contact print is made from the negative exposed in the central cham- 

 ber, and the other four are printed through a transforming printer. 

 The final five prints are assembled as a single vertical photograph as 

 shown in plate 11, figure 2. The picture taken with this camera 

 measures 30 inches across. 



PICTORIAL PHOTOGRAPHY 



Aerial photography is divided into two broad classes, known as 

 \'erticals and obliques. The former class comprises photographs for 

 which the axis of the camera is held as nearly vertical as possible at 

 the instant of exposure, and the latter class includes all other 

 photographs. 



Oblique aerial photographs were obtained from kites and balloons 

 long before the invention of the airplane, and the earliest airplane 

 photographs also were oblique views. They have the advantage of 

 presenting a view as we are accustomed to seeing it, and accordingly 

 they convey information that would be overlooked in the unnatural 

 view afforded by the vertical photograph. Examples of obliques 

 are the pictures of San Bias Island and of Irazu volcano and other 

 mountains shown in plates 7 and 8. These pictures are useful in 

 mapping work as well as being of scenic beauty. The photographs 

 of the United States Army-National Geographic Society stratosphere 



