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Structural features which can be readily mapped from radar imagery are those features which are represented by some elements of physical relief. These structural features are usually fractures or physical discontinuities in the rock materials, which can be seen because of the effects of selective or differential erosion. These effects are seen on radar imagery in two distinct manners. First is the linearity of physical features, and second is the discontinuity of tone, shape, texture, and/or pattern. The discontinuity of tone, shape, texture, or pattern is created by differential erosion of the terrain on one side of a fracture in respect to the other side.
The ability of a remote sensing system to enhance minor physical features is the most important aspect in mapping structural features. The available tonal contrast is of secondary importance. The direction of illumination is also important in the mapping of structural features as determined by studies on the Alaskan Peninsula.
Maps showing the major structural features of the study area were compiled from real aperture radar imagery, synthetic aperture radar imagery, and Landsat MSS imagery. Three simple frequency diagrams were produced from maps of the study area showing the major structural features. The frequency diagram of the linear features taken from real aperture radar imagery, shows a strong bimodal distribution. The two major directions indicated are approximately 50°W and 50°E. The frequency diagram of the linear features taken from synthetic aperture radar imagery also shows two major directions of orientation. The two major directions are approximately 70°W and 20°E. The frequency distribution for Landsat MSS imagery is trimodal showing major orientations of approximately 60 #176;W, 10°E, and 60°E. All three frequency diagrams are strikingly different.
What is shown in the frequency diagram is preferential enhancement of linear features by a particular direction of illumination. Field reconnaissance indicated three major directions
of fracturing, 55°W, 55°E, 15°E, presented here in order of decreasing dominancy. These directions show up in relation to directions of illumination. Along a line of illumination there will be two triangular areas, one on either side of the line, in which the maximum enhancement of linear features will occur. These areas are the zones of maximum shadow effects. When radar imagery is acquired with two directions of illumination, the two directions should not be at 180° to one another. The second direction of illumination should be about 120° from the first direction of illumination. In an area in which Landsat data are available, the direction of illumination for the radar imagery should be acquired 120° from the direction of illumination of the Landsat da a.
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