An integrated photogrammetric and photoclinometric approach for illumination-invariant pixel-resolution 3D mapping of the lunar surface

High-resolution 3D information on lunar and planetary surfaces is crucial for planetary exploration missions and science. Photogrammetry is a state-of-the-art technology for generating 3D topographic models of surfaces such as digital elevation models (DEMs). The performance of the photogrammetry and the resulting DEMs is affected by image matching. However, most matching algorithms fail when the images have large differences in illumination and subtle textures. This problem can be addressed by integrating photoclinometry into the photogrammetric process. This paper presents an integrated photogrammetric and photoclinometric approach that is able to generate pixel-resolution DEMs of the lunar surface and is illumination invariant. The incorporation of photoclinometry into photogrammetry involves two main steps. First, a photoclinometry assisted image matching (PAM) approach is developed by integrating photometric stereo analysis in the image matching to create pixel-wise matches, even for images with large illumination differences. Second, the DEM derived from photogrammetry using the matching results is further refined to pixel-wise resolution using photoclinometry with a shadow constraint. The proposed approach has been used for high-resolution topographic mapping at the Chang'E-4 and Chang'E-5 landing sites using Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images acquired under different illumination conditions. The results indicate that the proposed approach is robust to severe inconsistencies in illumination and subtle textures in cases where the conventional approaches fail. The approach is able to achieve geometric accuracies comparable to photogrammetry but more small-scale topographic details. The proposed approach can also be used for high-resolution topographic mapping of other planetary bodies such as Mercury or asteroids, and provides a useful reference for similar topographic mapping on Earth.