nost impor-
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igueur. The
in most in-
raphs more
ons may be
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| prime fac-
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have made
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endents and
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niles here in
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roleum dis-
the respon-
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ailed photo
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to the ver-
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cope of the
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at various
literature is
rent, Smith,
| air photo-
, Of course,
] resolution
s a general
(653)
slowness, involved procedure, and usually the problem of making rather precise
measurements of sometimes very small distances. Whether these measurements
are made by a micrometer attachment of a parallax bar or on acetat: film, time
has still remained one of the most important economic factors. The additional
fact that an apparent dip goes through an appreciable change depending on its
distance normal to the line of flight helps not at all in good consistent determi-
nations to fairly close tolerances.
The specifications for photography suitable for a photogeologic evaluation
are not rigid, but are usually prescribed by a combination of factors which will
yield maximum economic coverage with a maximum amount of detail. The op-
timum condition seems to be reached at a scale of approximately 1/20,000.
Special areas may be photographed at a larger or smaller scale depending on the
surface expression of the rocks or the photogeologic detail required. Ideally,
photographs used for geologic interpretation should be free of all tilt or drift.
With adequate ground control some difficulties may be overcome by rectifying
prints into a horizontal plane. Unfortunately, much of the geologic work is
done in areas of little or no ground control and scale differences, and tilt in
individual flights or prints remains a problem. A solution to the tilt problem
may lie in the simple expedient of a simultaneous exposure by prismatic arran-
gement of an instrument which would indicate the amount and direction of tilt
at the instant of exposure. In such case, the print could be easily rectified to
fairly close tolerances without reference to pre-established ground control.
To sum up then, the present status of photo interpretation in petroleum
geology is briefly this: That in the last few years the use of photo interpretation
has known a tremendous increase in area mapped and in the confidence as to
the accuracy of the mapping. That an increasing number of oil companies are
turning to photogeology for exploration. That an ever increasing number of
geologists are engaged in mapping areas hereto denied by conventional methods
and they are doing it quicker and at less cost than by any land-bound method.
That more than a half million square miles has already been mapped and the
possibilities for more appear very good indeed. Finally, that the increase in
technical knowledge through research and experience will continue to benefit
all phases of exploration. The important consideration is that photogeology has
so quickly become an aid in petroleum geology for rapid, low unit area cost
reconnaissance, that a full review of its status must await additional data for a
complete analysis and evaluation.
AIR-PHOTO-INTERPRETATION IN MINING AND ENGINEERING
; GEOLOGY IN CANADA
H. L. Cameron, Nova Scotia Research Foundation Halifax, N.S. *)
Because of its large size and relatively small population, Canada presents
special problems in the surveying and engineering fields. Until the advent of
aerial photography and aerial photogrammetry progress of regional mapping
was slow, and the mapping of engineering sites was confined to relatively small
areas of special importance. The problem of contouring in the mountainous
1) Published with the permission of the Nova Scotia Research Foundation.
