The effect of small topographic variations on reflectance

The influence of topography on the bidirectional reflectance factors (BRFs) of vegetation canopies is commonly neglected when retrieving canopy parameters by inversion of reflectance models. However, the nonlinear dependence of vegetation BRFs on slope magnitude and orientation means that topography cannot necessarily be assumed to have an insignificant effect, even when slopes are small. The study presented here shows that, at a sun zenith angle of 60degrees, the reflectance of a set of terrain elements may differ from that of horizontal terrain by more than 10% at forward scattering directions for slopes of 10degrees or less. The errors may also be strongly correlated to observation conditions, which raises questions about using least squares minimization schemes to find the best fit between measured and modeled reflectance data.