Slope Measurements in the Vallis Marineris and Kasei Vallis, Mars: A Comparison between Photogrammetry and Photoclinometry

Abstract

Two of the most prominent structures of the Martian surface are the large equatorial canyons of Valles Marineris, and the circum-Chryse system of anastomosing outflow channels. The knowledge of topography is of utmost importance in the understanding of the geological processes responsible for the formation of both the canyons and the valleys. Tectonic information like the dipping of fault planes, as well as information about the amount of erosion and sedimentation, can foten be gained only through three-dimensional analysis of imaging data. In planetary science, two common techniques have been used to derive topography: 1) the photogrammetric approach is based on the identification of common points in two or more images, a subsequent bundle-block adjustment, and the derivation of object point coordinates which can form the basis for a digital elevation model. 2) in photoclinometry, the brightness variation of the surface (i.e. the DN values in an image) is due to varying illumination caused by topography (shape from shading), assuming thatthe albedo is homogeneous along the measured profile. Small-scale features (e.g. small grabens near the main canyons, valley walls) were selected for the measurements where appropriate high-resolution Viking Orbiter imaging was available. The results of the two techniques are compared and the implication for the geologic interpretation are discussed.