GVII-82 
PHOTOGRAMMETRIC ENGINEERING 
simplified delineation of sharp subdivision 
lines, recognizing the complexity of the 
landscape associations which affect man. 
The geologist can draw sharp lines with 
impunity, but not the human geographer, 
who must recognize the blending of influ 
ences and draws his lines to represent 
transitions where there is a correspondence 
of many factors. 
The Terrain Type and the 
Air Photo 
The earlier land classification surveys 
were made by ground methods, but as air 
photography became available photo in 
terpretation was gradually adopted as a 
tool. It was an ideal combination of theory 
and practice because the air photo was 
especially well suited to show spacial rela 
tionships and the multitude of details 
which make up the elements of terrain. 
In practice the terrain type is remark 
ably flexible and can be broadened to the 
regional level or narrowed down to show 
great detail, depending upon almost any 
criteria. It can involve any combination 
of features which are associated together 
and which are repeated again and again 
over the landscape. It can include such 
specialized groupings as forest cover types 
and crop classifications, or broad types 
such as lands suitable for settlement and 
those to be reserved as wild lands. Prob 
ably the outstanding advantages of the 
terrain type technique are the ease with 
which most types can be recognized on air 
photos, and the relative environmental ho 
mogeneity of the type, which permits spe 
cific interpretations of ground conditions 
to be made. 
The terrain type method has been 
widely used for the classification of ter 
rain for military purposes. Best known of 
these applications is the world wide 
Strategic Engineering Survey made by 
the U. S. Geological Survey for the U. S. 
Army Engineers. It classified geographical 
regions into broad terrain types which 
could be related to trafficability and 
construction problems. 
Physiographic Subdivision and the 
Air Photo 
All geographers did not adopt the classi 
fication of land by types, however. Fenne- 
man’s system of multiple physiographic 
subdivision has seen wide application, 
especially by physical geographers, who 
used it as a tool for geographic description. 
The broad description of regions seems to 
be the best use to which it can be put and 
it has been widely successful. Physio 
graphic subdivisions have also been used in 
military intelligence surveys, one of which 
was the Joint Army Navy Intelligence 
Study of worldwide coverage during World 
War II. Geographic regions were deline 
ated on topographic lines, then subdivided, 
and a description was supplied for each 
area. However, the technique has rarely 
been used in photo interpretation stud 
ies, principally because it is a tool for 
generalized descriptions of regions, and 
not for the analysis of spot locations. The 
air photo is not a generalization except at 
the smallest scales. At operational scales 
it inescapably narrows the field of interest 
very quickly to specific images. Physio 
graphic subdivisions, as broad generaliza 
tions, have heterogeneous elements and 
many diverse conditions, making it im 
possible to use their descriptions to analyze 
specific sites. For instance, if a subdivision 
contains high mountains, plateaus and 
deep basins it is difficult to make generaliza 
tions which will fit any particular site. 
Such subdivisions usually cannot be 
recognized on air photography because of 
this diversity. 
Examples 
Geographic problems are difficult to 
examine in the abstract and examples 
seem to be essential for understanding. 
Regional photo interpretation keys have 
been used in the following illustrations 
as the vehicle for regional analysis. Al 
though the regional key is still in a devel 
opmental stage, it has already proved itself 
an extremely effective means for analyzing 
the geographic associations of a region. 
The two keys used herein are fragmentary 
and are only intended to show how each 
example keys out one of its end products, 
so that they may be compared. 
The Fenneman approach has been 
adapted to photo interpretation, and it is 
demonstrated by keying out the Navajo 
Section of the Colorado Plateau shown in 
Plates 1 through 4. In addition single 
representative oblique photos 1 are used to 
1 The author is indebted to the Air Chart & 
Information Center of the U. S. Air Force for 
the trimetrogon photography used here, and to 
the Division of Forest Economics, U. S. Forest 
Service, for help in preparing the plates.