Correcting satellite imagery for the variance of reflectance and illumination with topography

Steephill and mountain slopes severely affect remote sensing of vegetation. The irradiation on a slope varies strongly with slope azimuth relative to the sun, and the reflectance of the slope varies with the angles of incidence and exitance relative to the slope normal. Topographic correction involves standardizing imagery for these two effects. We use an atmospheric model with a Digital Elevation Model (DEM) to calculate direct and diffuse illumination, and a simple function of incidence and exitance angles to calculate vegetation-canopy reflectance on terrain slope. The reflectance correction has been derived from the physics of visible direct radiation on a vegetation canopy, but has proved applicable to infrared wavelengths and only requires solar position, slope and aspect. We applied the reflectance and illumination correction to a SPOT 4 image of New Zealand to remove topographic variation. In all spectral bands, the algorithm markedly reduced the coefficients of variation of vegetation groups on rugged terrain. This produced clean spectral signatures, improving the capacity for automated classification. If illumination correction is performed alone, the coefficients of variation can be increased, and so should not be applied without a reflectance correction. The algorithm output is reflectance on a level surface, enabling the monitoring of vegetation in hilly and mountainous areas.