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The extraction of geologic information from remotely sensed data, which consist of encoded radiometric signals detected above the terrain, is procedurally governed by the same logical sequence and argument employed by a geologist when the data are typical ground-based observations and measurements. Each respective data set relates physical and compositional conditions of the geologic environment. In either situation, the sequence of problem definition, data collection, data reduction, interpretation, and testing is followed and, commonly, dictates the geologist's success or failure in arriving at an adequate geologic interpretation.
The most useful data format for geologic analysis and interpretation is an image display in which both radiometric and geometric relations in the data can be correlated with conditions on the earth's surface. The objectives of applying a systematic approach in obtaining geologic information from images are: (1) to provide a framework in which geologic interpretations are logically derived from the imaged data; (2) to separate the more objective aspects of image analysis from the subjective considerations imposed on a geologic interpretation; and (3) to facilitate the efficient reduction of imaged data by separating tasks, concentrating attention, and thereby minimizing omissions.
In geologic investigations, imaged data are analyzed and their geologic significance is interpreted; consequently, both spectral and spatial aspects of the data are considered in deriving geologic information. Preceding the interpretation of geologic relations, the data must be grouped according to their spectral characteristics. Subsequently they are reduced into landscape elements based on their spatial distribution and association. These two actions, the spectral classification and spatial reduction, constitute the two phases of image analysis. The interpretation of geologic information from the analyzed data must be made by a person trained in geology. Beyond this, the individual must be able to correlate and interpret the geologic significance of the landform, drainage, and cover patterns that are products of image analysis.
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