4 PHOTOGRAMMETRIC ENGINEERING
standards of comparison for geologic features in different geographic environ-
ments. What is needed is an atlas of typical landforms and geologic features in
all parts of the world.
It is generally agreed that some degree of ground checking is essential to
secure maximum results from photo interpretation. Many interpreters prefer
to do the field work after office interpretation, and follow this with such
restudy of the photos as may be needed. Others feel that preliminary field
reconnaissance, with photos in hand, is a desirable prerequisite for office
study, even when the latter is to be followed by more rigorous field checking.
Still others prefer to carry on interpretation and field study more or less con-
currently. The conditions under which the interpreter works, particularly the
accessibility of the area studied, commonly are deciding factors in determining
the relative roles of field and office study. In some instances, circumstances
may even require that field checking be entirely dispensed with, and the
interpreter’s best judgment (or guess) be the sole guide in planning mineral
exploration, land acquisition, or other operations. The type of ground checking
varies with circumstances. In many cases, routine geologic examination may
be sufficient. In other cases, the inadequacy of surface exposures may require
that trenching, drilling, or geophysical testing be carried out.
Another type of checking is by direct observation from the air, preferably
with a light, low-speed plane flying at low altitudes. In this way much significant
detail that is lost on photos of small or intermediate scale is readily observed,
and may be recorded, if necessary, with a small hand camera. For this type of
observation, the helicopter would be ideal, but has not yet come into general
use.
The advantages arising from photo interpretation all lie in the direction
of increased efficiency. The degree of increase varies widely with the type of
terrain investigated. Where surface expression of geologic features is poor,
owing to various types of surface cover, the main advantage may be in providing
for more effective planning of field work. Localities favorable for ground study
may be ascertained in advance, and routes selected, so as to avoid the lost
time occasioned by blind traversing. Interpolation between localities studied
on the ground, furthermore, may be facilitated. Where the terrain is more
favorable, as in many arid regions, it frequently is possible to go much further,
and outline all or nearly all of the essential geologic features directly from photos.
The time required for mapping is accordingly reduced, and costs are lowered,
sometimes to a small fraction of that required for the conventional methods of
former years. Still another advantage is the increased accuracy and complete-
ness of mapping made possible where surface expression of geologic features is
favorable. Features obscure or invisible on the ground may be clearly revealed
on photos, with the result that important structures or other phenomena un-
noticed in ordinary field mapping are found, and problems encountered in
field mapping are solved. It has even been commented by Raasveldt that no
geologic study of an area can be truly complete and thorough unless air photos
are used. Certainly when office interpretation is combined with adequate field
checking, the best possible geologic map is obtained with maximum economy.
As a result of the advantages noted above, virtually all geologic mapping in
North America now utilizes photo interpretation to a greater or lesser extent,
if carried on in areas for which suitable photos are available. This applies alike
to mapping carried on under governmental, academic, and commercial auspices.
In the petroleum industry, photogeologic interpretation has been found par-
ticularly valuable, and, although specific data have not been released, it is
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