Modeling crater topography and albedo from monoscopic Viking orbiter images: 1. Methodology

A new photoclinometric technique for extraction of topographic data from single planetary images is presented that overcomes many previous limitations of photoclinometry. The procedure fully compensates for oblique viewing geometry prevalent in spacecraft images. Albedo variations have been one of the most serious obstacles in the application of photoclinometry to planetary surfaces. This problem is overcome in the topographic solution by simultaneously utilizing brightness data from a pair of profiles; both segments are assumed to have the same topographic and albedo variations along their lengths. Profile directions are chosen where the orientation of downslope or upslope is obvious, thus resolving a major ambiguity in photoclinometry. This requirement is particularly easy to satisfy for craters and not very difficult for many irregular features. An additional procedure is presented that eliminates even the requirement of topographic symmetry along the pair of profiles. If two profiles have the same relief but their shapes are very different, another method can be used in an iterative process to derive topographic profiles; however, this procedure does, require that the albedo not vary along the profiles. Test results indicate that both procedures have an accuracy and precision of approximately 2° for slopes of typical bowl-shaped craters, which translates to approximately 5% for depths.