ESTIMATING RELATIVE MICRO-TOPOGRAPHIC SURFACE ROUGHNESS USING REMOTELY-SENSED STEREO SPECTRAL DATA

Surface roughness is a scale-dependant property that has been previously identified as a key factor in many geological, hydrological and planetary studies, as well as an essential variable in remote-sensing applications and climate-prediction models. In this study we present a new approach for estimating relative micro-topographic surface roughness (i.e. the roughness of the surface below the scale of available digital elevation models) using stereo imaging airborne and spaceborne sensors.

The fundamental assumptions behind this approach are that reflected solar radiation from sunlit components dominates land-leaving radiation from terrestrial surfaces at 0.4-2.5 microns, and that the aerial fraction of shadowed areas in a pixel is inversely proportionate to measured radiance from the surface. As the amount of shadows actually ‘seen’ by a sensor depends on the view-angle (e.g. down-sun viewing will reveal mostly sunlit surface-facets whereas up-sun viewing will reveal mostly shadowed surface-facets) the variance in measured radiance from the surface at two different viewing angles, under the same solar illumination conditions is mainly a function of the resulting change in the aerial fraction of shadows actually seen on the surface. The linkage between the change in amount of shadows and the surface roughness (e.g. no change for perfectly smooth surfaces with no shadows, and a large change for rough surfaces with many shadows) enables the use of the difference in the radiance from the surface at two viewing angles as a proxy for relative micro-topographic roughness of the pixel surface.

Numerical simulations of simplified surfaces together with laboratory and field measurements of geological surfaces demonstrate that the ratio between at-sensor radiance values from two viewing angles is independent of surface albedo and is well correlated with its roughness. This approach was applied using stereo data acquired by the Advanced Spaceborne Thermal and Reflection radiometer (ASTER) for an alluvial fan sequence in southern Israel and for a salt playa in Northern Chile. In both cases, estimates of relative micro-topographic roughness were in good agreement with independent ground-based roughness measurements.