Evaluation of spectrodirectional alfalfa canopy data acquired during DAISEX´99

Field goniometer measurements are a tool to generate a priori bidirectional reflectance distribution function (BRDF) knowledge for correction and validation of directional reflectance data acquired by air- and spaceborne sensors. This study analyzes the diurnal hemispherical-directional reflectance factor data of an alfalfa canopy measured during the Digital Airborne Imaging Spectrometer Experiment 1999 (DAISEX´99). We analyze the variation of measured and modeled spectrodirectional vegetation data, revealing that measurement noise is negligible compared to the variation due to the canopy´s anisotropy. The deviations of the spectral albedo (bihemispherical reflectance) and of field spectrometer nadir measurements throughout a day prove to be larger than modeled deviations. Calculated anisotropy factors quantify the spectral-dependent effects of the vegetation reflectance anisotropy. This paper is a contribution toward the generation of a reliable BRDF database by suggesting methods to preprocess and analyze observed directional vegetation reflectance data, with special emphasis on the spectral dimension.