\ 
* 
4 
Fig. 2 - Installation of the Wild RC-9 Aerial Camera, 
Wild Horizon Camera, and Zeiss Statoscope 
in a DC-3 Aircraft 
Fig. 2a - Outside View of the RC-9 and Horizon Camera 
Installation in a DC-3 Aircraft 
The horizon photographs are synchronized with the vertical 
photographs in two ways: 
(a) by zeroing exposure counters on both the RC-9 and the horizon camera before 
each flight. Both counters are operated by the same electrical impulse, 
and the same exposure number appears on both negatives (9x9 inches and 
35 mm. ) 
(b) by recording the time on both negatives at the moment of exposure. The 
clocks on the vertical camera and on the horizon camera are synchronized 
before each flight. 
2.2 Technique of Obtaining Horizon Photographs 
To obtain acceptable horizon photographs under variable atmospheric 
conditions, much attention must be paid to the choice of proper emulsions and 
filters. The horizon that appears on the negatives is not a true horizon but a 
vapour horizon, or a very distant cloud layer. Pictures of the true horizon 
can be obtained only over water or under certain atmospheric conditions, when 
flying at low altitudes. However, it is not important whether the true horizon 
or a vapour horizon be photographed since we are determining the values of 
relative tip and tilt. It is important that the horizon appear on the 
negatives as a sharp, well-defined line that can be measured under magnification.