WORKING GROUP 9 MILES 473 
stratification processes and by all the weathering processes. Therefore, sub 
divisions must be made. The primary variables in mapping soil, after the 
parent material type has been classified, are topography and horizon [5]. At 
this point in mapping of discrete units, time, parent material, climate, and 
organisms are essentially constant. 
There are two approaches to the delineation of soil types by the technique of 
airphoto interpretation. One approach is to obtain complete descriptions of 
agricultural soil types and associations as related to parent materials and slope 
classes. By field correlation studies, the engineer determines the engineering 
characteristics and construction problems associated with these soil catenas, 
soil series, and soil types. This is an analogy technique. It has a great advan 
tage in that published agricultural soil surveys serve as supplemental 
control for nearby unmapped areas. The aerial photographs are used to 
compare one area with another; and, by analogy, the agricultural soil types 
are mapped. 
This technique has been used to advantage in the State of Indiana. The 
correlation information on this method was published in 1943 [8]. This book 
is now somewhat obsolete as many new agricultural soil series and types have 
been added, and some areas have been remapped using more modern concepts 
\ of soil individuals and their affect upon farm management practices. Of course, 
the technique is restricted to areas of similar soil associations. 
A new cooperative program of correlation was instigated in 1960 whereby 
the agricultural soil surveyor collects samples of individual horizons within 
selected modal soil individuals. These samples are then analyzed for their 
engineering properties using Standard Methods of Test of the American 
Association of State Highway Officials. In a few more years, information will 
be accumulated; and the bulletin on the correlations between agricultural soil 
types, engineering soil porperties and their airphoto patterns will be repub 
lished. 
The other approach to the delineation of soil types is by the use of a quanti- 
titative engineering soil classification system as a qualitative engineering soil 
identification system in airphoto interpretation studies. 
The author has investigated the three major engineering soil classification 
systems previously mentioned, and has concluded that the Unified Soil Classifi 
cation System has great merit as a system for mapping soil groups by the tech 
nique of airphoto interpretation. A system of this type differentiates mineral 
soil groups from organic soil groups. Each of these form distinct airphoto 
patterns. It is based on a major subdivision of soil textures into coarse-grained 
soils and fine-grained soils, which also form distinct airphoto patterns. 
The fine fraction (combined silt and clay) is defined on the basis of their 
plasticity characteristics. The outline of system is shown in table 7. The fifteen 
engineering soil groups are represented by symbols that can be used as plani- 
metric mapping symbols and as soil profile symbols. They should be preceded 
by an ‘T’-meaning “inferred” - when used in pure interpretation without 
ground control.