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.