Physics for Geologists, Second edition

Optics 57
point onto a 'horizontal' surface. Looking at the geometry of air photographs
in a little more detail we see that the scale of the photographs is the focal
length divided by the height above the ground (always in consistent units).
This means that over hilly ground the scale changes according to elevation.
The other difference is that the camera 'looks' from a single point, so there is
both distortion radially outwards from the principal point and also distortion
due to the angle of view. For example, a small steep hill will appear farther
from the principal point in a single photograph than it really is, and a valley
nearer; and the camera may not see down a steep gorge near the edge of
the photograph. The radial distortion can be corrected by triangulation if
a feature appears on three consecutive photographs (as it should with 60 per
cent overlap).
The line joining the principal points of the photographs of a run is called
the base line. The change of apparent position of an object on account of
a change of viewpoint is called parallax, which is measured parallel to the
base line. In the figure, the parallax of the base of the pyramid is 55.5 mm, of
the top, 54.3 mm (reproduction may have altered these figures slightly) and it
is the difference of parallax that creates the stereoscopic image. If there is no
difference of parallax between points in the two images, the points appear to
be at the same elevation. Photogeology has been replaced by satellite imagery
to a very large extent, but there are still large areas of the earth that were
mapped first from aerial photographs - large areas of Canada, Australia and
New Guinea, for example. Nothing, however, replaces the examination of
the rocks themselves.
For a more detailed, but still concise, description of basic photogeological
techniques, see Allum (1966) and/or Foster and Beaumont (1992).
Copyright 2002 by Richard E. Chapman