Structural analysis of geological phenomena may be enhanced by real or simulated illumination conditions of satellite data. With this concept in mind, an experimental project has been developed to manipulate the Landsat radiances under the control of simulated illumination conditions and viewing geometry. To achieve this, a digital terrain model (DTM) was created by digitizing contour lines of topographic maps on a scale of 1 to 50,000. Using a specially designed algorithm, elevation was later calculated for each grid point at an interval of 50 meters on the ground and overlaid on a geometrically corrected Landsat image. After calculation of (1) the normal to the ground for each "pixel", (2) the direction cosines of the line from each pixel to the simulated satellite position and, (3) the simulated solar illumination, different algorithms were used to derive new satellite radiances.

It is shown that even with a simple algorithm like a multiplication of radiances by cos ψ, where ψ is the angle between the sun illumination and the normal to the terrain, the interpretation of geologic features is dramatically enhanced.

Examples of current geologic maps compared to interpretation made with the present algorithms show clearly the potential of this methodology. The main constraint still remains the long computing time needed to process the data, even though it is still economically advantageous. The foreseen production of a low cost parallel processor will definitely result in the systematic use of the present methodology.

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