466 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
and textures are difficult to define in words because they are evaluated by 
individuals with frequently varying sensory perception. 
With reference to materials on the surface of ground devoid of vegetation, 
the following general conditions prevail as to tone reproduction. 
a. The finer the grain size, the lighter the tone due to the sum of reflectances 
of all surfaces. 
b. The higher the surface moisture content, the darker the tone, as water 
absorbs light energy. 
c. The darker the natural color of the material, the darker the tone. 
The reader will readily recognize that these ideal conditions seldom, if ever, 
are obtained. Thus, the interpreter should never use the principles alone; they 
must be leavened with knowledge of other factors, i.e. climate, time, vege 
tation, etc. 
In any case, each photograph and the photographs of an area will exhibit 
tones and textures that collectively and individually form elements for identi 
fication and correlation. Varying land use types produce distinct tonal patterns. 
Broad classes of vegetation are imaged on photographs in tonal and textural 
ranges. Rocks produce different tones relative to drift materials, and a sand 
texture will produce different tones than a clay texture or an organic condition 
in the same parent material area. 
The total airphoto pattern then is made up of individual elements of topo 
graphy, drainage plan, erosional shape, tones and texture of land use, tones 
and textures of vegetation, and tones of soil and parent material conditions. 
Airphoto interpretation for engineering purposes has evolved around this 
pattern concept. It has evolved by the process of identifying the pattern ele 
ments in both a quantitative and qualitative sense for particular land forms, 
parent materials, and engineer soil groups. 
An analogy with photogrammetry is appropriate. In an underdeveloped coun 
try or area where no ground control surveys exist, photogrammetric mapping 
cannot take place until basic and supplemental ground control surveys are 
performed. In a partially developed area where basic ground control exists, only 
supplemental control need be surveyed on the ground. In highly developed 
areas, using photogrammetric bridging, a small scale map can be compiled 
without ground control except for geographic place names. Large scale maps 
require finite control for each stereo model before photogrammetric compilation. 
The analogy is that throughout most of the world basic control for photo 
interpretation purposes exists in geologic literature and geographic literature. 
Certain areas have supplemental control in the literature of soil science. Air 
photo interpretation then is a technique of bridging in areas where small scale 
engineering soil maps are required. Where large scale maps are required, individ 
ual engineering soil profiles are required for control purposes; but these can then 
be extrapolated to the larger area portrayed in a repetitious airphoto pattern. 
The classifications of land forms, parent materials and engineering soil groups 
should be uniform throughout all areas and all stages of mapping. The material 
that follows is the classification that has been of greatest value in teaching and