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CONSTRUCTION SOURCES OF GRAVEL, MINTZER 141 
therefore, this writer will briefly define the basic principles of photo interpretation, 
and how the essential elements of the airphoto pattern are applied to locate sources 
of gravel. Following this will be three examples of the use of airphotos to locate sources 
of gravel. 
Basic principles of photo interpretation. 
Only the fundamental principles of photo interpretation are discussed here. There 
are more elaborate treatments available [see ref. 1, 2, and 8]. The airphoto is a detailed 
record of the earth's surface within the photo coverage; the images of similar surface 
materials can be grouped into recognizable patterns, and these observed patterns are 
repetitive. In any two physically similar, but geographically different locations, existing 
under similar climate, similar topographically and geologically explained origins, very 
similar ground conditions will also exist. Similar patterns will represent similar gravel 
deposits. Generally, sources of gravel from similar parent materials may be repeated 
over a general physiographic region. A standard surface material, i.e., rock or gravelly 
soil of sufficient depth and areal extent with fairly uniform characteristics may be 
classified as a given type. Each pattern that represents a source of gravel has 
distinct elements that may be delineated, identified and associated with that given 
granular-type pattern. 
The photo interpreter, in applying the areal concept, analyzes the photo pattern in 
terms of its elements. The elements frequently identified as a part of a given pattern 
are: landform, regional drainage, erosional features, photo gray-tones and land use. 
The processes of thinking — deductive and inductive — and the imagination of the 
interpreter bring about the visualization of the sequence of natural events in the 
evolution of a given pattern. Thus the representative ground condition is associated 
with a given photo image. The ground conditions of interest in this discussion are those 
associated with sources of gravel. 
Landform. 
The topographic arrangement of the ground surface is a feature of general relief, 
shape, and slope referred to as “landform.” The landform identifies its location with 
respect to the surrounding area. Relief and often the materials underlying the surface 
reflect the weathering processes that gave the land its shape. Gravel is found within 
landforms having particular characteristics. Beaches, ridges, terraces, and ice-laid plains 
are easily identifiable landforms on the ground and on the airphoto. The landform is a 
most important element because of its close association with the origin of the material. 
The landform pattern of a granular source can be identified by stereoscopic study 
of the photos. It is possible when visualizing the relief in three dimensions to know 
whether a particular landform is rounded or angular, has vertical or sloping faces, is 
flat-topped or rolling. The particular shape of the landform, its origin and geological 
history, enable the engineer interpreter to visualize the physical and textural character- 
istics of the gravel deposits that lie within the landform itself. Each type of gravel deposit 
will have its own landform and set of characteristics. 
Regional drainage. 
The regional drainage pattern is often characteristic of the given landform system. 
Rainfall falling on the earth tends to give identifying features to a terrain surface in a 
fashion peculiar to the particular resistance of the materials underlying the landform. 
There is a clue to dip and type of rock and general depth of surface mantle based upon 
the lines which mark the relief and drainage. These linear features are observable in the 
photo image of the terrain. The soils and rocks of the earth’s surface reflect the way 
water run-off erodes the land surface. Clays and silts also resist penetration of moisture, 
promoting run-off, and definite erosion removes the surface mantle if some protection 
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