REPORT OF WORKING GROUP NO. 3
COMMISSION VII
by Page E. Truesdell,
U. S. Naval Photographic Interpretation Center
A review of the progress made in the fields of photo-
interpretation covered by Working Group No. 3 shows
that much work has been accomplished in the past sever-
al years and indications are that even more projects of
this type will be conducted in the years to come.
Actually each year more and more work performed by
photointerpretation is being accepted as standard
practice by government and commercial firms. Each year
more companies are either adding photointerpreters and
photogrammetrists to their staffs or contracting work .
to an ever increasing number of firms who specialize in
this type of investigations. In projects other than
Strictly mapping where various types of special equip-
ment is used, the work is accomplished largely by rather
simple instruments. The use of aerial photography to
conduct investigations covering the various phases of
earth sciences as reported on by this Working Group is
rather extensive, not only in areal coverage but also
in its application. Each one of the phases is not only
a study in itself but they also overlap as one phase
can hardly be completed without considerable reference
to the others.
INTERPRETATION OF SURFACE CONFIGURATION
This phase of photointerpretation is probably the
one most active of this group as it is generally only
the surface of the earth that can be recorded on
aerlal photography. However it is often separated into
more Specialized phases such as drainage, soils, geology,
geography, etc. In some instances, such as in open pit
mining, eroded areas, etc. subsurface formations or con-
ditions can be interpreted from aerial photography. In
general though much of the interpretation of natural
conditions is accomplished by means of pertinent land-
forms, (revealing surface configurations or patterns),
tonal gradations, including color, erosion, drainage
patterns, vegetation types, and many others. The proper
interpretation of the earth's surface by these physical
records is very critical and cannot be done in a hap-
hazard manner.
The earth's surface configuration is exposed on al-
most every photograph, the major exceptions being those
exposed entirely over water or in some cases over dense-
ly forested areas, although even here many aspects of
the earth's surface condition can be determined. The
various pertinent landforms, drainage patterns, tonal
variations, etc. recorded on a photograph are the basic
"keys" to many of the earth's natural resources when
properly and accurately interpreted by trained personnel.
These resources, whether they be send and gravel, min-
erals, or petroleum are in turn exploited by many groups
who more and more appreciate the value and reliability
of properly applied photointerpretation.
The interpretation of the earth's surface configura -
tion for sub-surface conditions is very important for
many fields of engineering construction, geology, min-
ing, etc. The exact boundary between surface and sub-
surface interpretation and their respective extent is
difficult to establish. However for the purpose of this
paper the interpretation of surface configuration may
include such engineering aspects as sand and gravel de-
posits, rail roads and highways, canals, pipelines and
communication line locations, soil investigations, urban
area planning and development, surface drainage, and
many other similar studies which require a relatively
rapid and economical but dependable description of the
ground conditions.
The required photographic scale and area covered for
use in these fields of interpretation of the earth's
surface vary as to the exact nature of the investiga-
tion. It would be rather presumptuous for such a limit-
ed paper as this, to attempt to outline the scale ranges
used for various surface confi ation studies. Suffice
it to say that as a rule relatively small scales roughly
-l-
in the 1:20,000 - 1:30,000 range are used for overall
planning purposes and some detail studies, while the
larger scales of 1:10,000 or even larger are required
for the more detailed investigations. In the fore-
seeable future, with the better quality of aerial photo-
graphic equipment and processing which produce much
higher resolutión values, sharper images, etc., smaller
Scales with their greater areal coverage will be used.
The fact is that regardless of the end results, the
earth's surface configuration plays an important part
in almost every aerial photograph that is interpretated.
This is confirmed by the many excellent publications
dealing with this factor which are available oh the
market today as well as by the numerous engineering pro-
Jects based essentially on photointerpretation. Many of
these applications are mentioned in this and similar
papers.
INTERPRETATION OF DRAINAGE
The interpretation or use of surface drainage patterns
as a reliable indicator of ground conditions is second
only to that of the earth's surface configuration. Act-
ually the two should be considered together as a single
phase, as with few exceptions one cannot be studied with-
out the other although this may not always be apparent.
Surface drainage patterns are often very revealing as to
the physical characteristics of the underlying materials.
This may &pply to large regional areas or to & small area
or even a specific location. As in the case of surface
configuration the reliable interpretation of any area
based on the drainage pattern requires a specific know-
ledge obtained only by years of training and experience.
Once this knowledge is gained however, valuable indica-
tions of terrain conditions can be interpreted. Usually
drainage or stream patterns are not interpreted for their
value alone, except perhaps in certain water resource
studies, drainage network plans, dam site investigations,
etc. more often they are used as guides or indicators of
engineering or natural resources investigations.
The superimposed surface drainage pattern, caused by
varying periods of erosion and climatic conditions, on
the soll or rock formation indicates the surface and to
some extent the sub-surface terrain condition. Certain
relatively impervious materials such as clay or shale
resist infiltration thereby contributing to surface run-
off and the resulting relatively dense drainage network.
In contrast to this are the more pervious materials such
as sand and gravel, sandstone, and which may be included
here, limestone because of its solubility. These latter
formations because of their more or less high filtration
capability will usually have a much coarser drainage net-
work. From this and other factors a properly trained
interpreter can indicate areas or formations having well
developed drainage patterns as probably being of an im-
pervious nature while a lack of drainage patterns
usually implies a high infiltration or pervious condi-
tion. Surface drainage patterns may also indicate local-
ized changes in material having different characteristics
as to texture, infiltration, and solubility. Another
condition that can often be indicated by drainage pattern
is the change in bedrock or underlying formations by be-
ing influenced by planes of weakness, fractures, faults,
etc. In fact much information concerning the under-
lying or sub-surface condition may be indicated by the
shape of the drainage pattern. Erosion features which
are the results of surface runoff or drainage often re-
veal significant information which will enhance the in-
terpretation of terrain conditions or geological forma-
tions.
As in the interpretation of surface configuration the
scale of the photography will be dependent upon the
nature of the investigation. However the small scales
from 1:40,000 up to 1:60,000 or smaller are suitable
for overall area study giving indications of drainage
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