×

You are using an outdated browser that does not fully support the intranda viewer.
As a result, some pages may not be displayed correctly.

We recommend you use one of the following browsers:

Full text

Title
Transactions of the Symposium on Photo Interpretation

WORKING GROUP 2
ALLUM
77
collect all the evidence; but this is incorrect. Rocks covered by residual soil fre
quently indicate their presence and their structures by the vegetation, and the
distribution of that vegetation, which they support. This variation in vegeta
tion, although frequently producing easily recognised and clearly defined
lineaments on the aerial photographs, is of a statistical nature. If the field geol
ogist were able to make a statistical study of the vegetation, he would produce
a pattern similar to that of the lineaments of the photographs; he would not
normally be aware, however, unless he had aerial photographs, that the ano
malous distribution of vegetation, justifying a statistical study existed. Thus the
residual-soil-covered structure would be overlooked and remain unmapped.
It has been suggested above that geological information which, in practice
is unobtainable in any other way, can sometimes be obtained from aerial photo
graphs. The question now arises as to what should be the geologist’s attitude if,
because of paucity of outcrops or some other reason, his field work produces
no evidence either for or against the existence of the structures indicated by the
photographs. In the writer’s opinion, the photogeological data have a status,
value, and reliability in their own spheres fully equal to those obtained from
other geological sources. It follows that, if the geologist, after considering the
data obtained from the aerial photographs, postulates the existence of certain
structures, these structures should be plotted with suitable qualifications on the
geological map, unless positive field evidence is found which proves that the
postulated structures are inaccurate or non-existent. The geologist has no right
to ignore structures “seen” on aerial photographs merely on the grounds that
he has found no positive evidence for them in the field; if he does so, he is
rejecting much of the most valuable assistance which aerial photographs can
offer him. It is in those areas where field observations are sometimes incon
clusive that the geologist should pay increased attention to, and put increased
reliance on, the evidence provided by the aerial photographs.
References
Allum, J. A. E. Photogeological interpretation of areas of regional metamorphism. Trans.
Inst. Min. Metall., London, Vol. 70, 1960-61, Part 9, 521-543. (Bull. Inst. Min. Metall.,
Lond., No. 655, June, 1961).
Hemphill, W. R. Small-scale photographs in photogeologic interpretation. Photogramm.
Engng. 24, No. 4. Sept. 1958, 562-7.
Ray, R. G. Aerial photographs in geologic interpretation and mapping. United States Geol
ogical Survey, Professional Paper 373 (Washington U.S. Govt. Print. Off., 1960).
Discussion
F. Ruellan wished to emphasize the importance of field checks in order to avoid errors. Exam
ple : the pre-cambrian volcanic region of Rio Branco in Brasil, which from photos alone gives
the impression of monoclinal ridges. The author agreed that field work is necessary for the
production of a final geological map. If, however, a geological structure is visible on the aerial
photographs and it is not possible to confirm it by field work, the structure should be plotted
on the final map in photogeological symbols only. There will then be no ambiguity as to the
source of the information and all data available will have been recorded.