Answer:
Focal length is the distance measured from camera lens
to film. Knowledge of the focal length used, along
with the altitude of the photographic aircraft, makes
it possible to determine the scale of the aerial
photograph (assuming land uniformity). For example,
with a camera focal length of six inches, a flight
altitude of 6,500 feet above mean sea level, and an
average ground elevation of 1,500 feet, the
representative fraction would be computed as follows:
0.5 ft. divided by (6,500 ft. - 1,500 ft.) = 1:10,000
scale
