Abstract
The technique of photoclinometry has frequently been used to determine planetary topography without proper consideration of possible sources of error. Previous studies of error sources have been limited in extent and have overlooked the importance of factors such as atmospheric scattering and the choice of a surface photometric function. This paper adopts a thorough and more direct approach to error analysis, whereby known topography is compared with photoclinometric profiles derived from synthetic quantised reflectance scans.
Instrumental and geometric sources of error are found to exert a minimal influence on profiles in practice, provided that sufficient care is taken in the selection of images and the extraction of scans from those images. Environmental factors — relating to the scattering properties of the surface and, if present, atmosphere — are far more important. It is found that a simple Lommel-Seeliger law is unlikely to be appropriate to the majority of planetary terrains, given its inability to model the effects of multiple scattering or unresolved macroscopic roughness. It is further demonstrated that a Minnaert function or combination of Lommel-Seeliger and Lambert laws may empirically compensate for the first of these phenomena but not the second; in this respect, Hapke's equation is a far superior model of surface optical properties. In the case of an atmosphere, the need to correct for scattering by aerosols or suspended dust becomes more acute as atmospheric opacity increases and as particle scattering becomes more forward-biased. To perform this correction, a model for the combined reflectance of surface and atmosphere must be used when deriving profiles.
Two case studies — of a small impact crater on Triton and a dust-mantled basaltic lava flow on Mars - are presented here. Regarding the latter, the implications that errors in photoclinometric flow thickness measurements have for inferred lava rheology are examined. Conservative estimates of errors in yield strength and apparent viscosity easily exceed 100% when one of the simplest photometric models possible — a Lommel-Seeliger law — is used to derive a profile.
In the light of these findings, strategies are suggested for improving the results obtained from photoclinometry in the future.
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Efford, N.D. Sources of error in the photoclinometric determination of planetary topography: A reappraisal. Earth Moon Planet 54, 19–58 (1991). https://doi.org/10.1007/BF00055046
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DOI: https://doi.org/10.1007/BF00055046