The technique used in photo-interpretation is known as "pattern analysis". This analysis is based on the premise that the landforms, which constitute the basic units of landscape, have distinct patterns in aerial photographs. This means that all those landforms developed by same geologic processes and in the same environmental setting will have similar patterns, while those developed by different processes will have different patterns.
What are these patterns? Patterns are composed of several major elements that are systematically evaluated in the process of photo-interpretation: a) topographic expression, b) drainage, erosion, c) soil tones, d) vegetation, and e) culture. These are briefly described. For detailed discussions of these one should consult textbooks on photo-interpretation, e.g. Miller and Miller.
a) Topographic expression is generally described as topography, shape and relative relief. These are easily determined by a stereoscopic examination of photographs. Topography is the 3-dimensional aspect of the landscape, such as hill, basin, ridge or mountain. Fault scarps are a common feature.
Shape is a qualitative statement of the general form configuration or outline of an object. It is easier to determine the shape of an object if it is viewed stereoscopically. For example, a drainage channel may appear as deeply incised "V" shaped valley. Shape, therefore, refers to a unique character of the landscape, such as conical hill, sinuous ridge, or A-shaped mountain, that aids in describing or separating different forms of topography.
Relative relief refers to the comparative position of the landform in relation to other landforms in the vicinity, e.g., the position (or relative relief) of the sloping fan-shaped plain lying at the foot of A-shaped hills and sloping downward to a level basin area. In this example, three different landforms are indicated based on their relative relief: mountainous area, fan area and basin. This aspect may also be termed as Place , which is a statement of an objects position in relation to others in its vicinity and this usually aids in its identification. For example, it may not be possible to clearly see streams on some photos but their likely location along a line of trees give a clue to its courses.