High Resolution Digital Terrain Models of Mercury

Supplementary material of the article:
Tenthoff, M.; Wohlfarth, K.; Wöhler, C. High Resolution Digital Terrain Models of Mercury. Remote Sens. 2020, 12, 3989.

Abstract:
We refined our Shape from Shading (SfS) algorithm, which has previously been used to
create digital terrain models (DTMs) of the Lunar and the Martian surface, to generate high-resolution
DTMs of Mercury from MESSENGER imagery. To adapt the reconstruction procedure to the specific
conditions of Mercury and the available imagery, we introduced two methodic innovations. First, we
extended the SfS algorithm to enable the 3D-reconstruction from image mosaics. Because most mosaic
tiles were acquired at different times and under various illumination conditions, the brightness of
adjacent tiles may vary. Brightness variations that are not fully captured by the reflectance model may
yield discontinuities at tile borders. We found that the relaxation of the constraint for a continuous
albedo map improves the topographic results of an extensive region removing discontinuities at
tile borders. The second innovation enables the generation of accurate DTMs from images with
substantial albedo variations, such as hollows. We employed an iterative procedure that initializes the
SfS algorithm with the albedo map that was obtained by the previous iteration step. This approach
converges and yields a reasonable albedo map and topography. With these approaches, we generated
DTMs of several science targets such as the Rachmaninoff basin, Praxiteles crater, fault lines, and
several hollows. To evaluate the results, we compared our DTMs with stereo DTMs and laser altimeter
data. In contrast to coarse laser altimetry tracks and stereo algorithms, which tend to be affected by
interpolation artifacts, SfS can generate DTMs almost at image resolution. The root mean squared
errors (RMSE) at our target sites are below the size of the lateral image resolution. For some targets,
we could achieve an effective resolution of less than 10 m/pixel, which is the best resolution of
Mercury to date. We critically discuss the limitations of the evaluation methodology.