VALIDATION OF BIDIRECTIONAL AND HEMISPHERICAL REFLECTANCES FROM A GEOMETRIC-OPTICAL MODEL USING ASAS IMAGERY AND PYRANOMETER MEASUREMENTS OF A SPRUCE FOREST

Aircraft imagery and ground measurements acquired for a spruce forest stand in Howland, Maine as part of the 1990 Forest Ecosystem Dynamics Multisensor Aircraft Campaign (FEDMAC) are used to validate the Li-Str ahler geometric-optical forest canopy reflectance model and to demonst rate that both spectral bidirectional reflectance factors and hemisphe rical reflectance can be estimated with some success. With the geometr ic-optical model, a vegetated surface is treated as an assemblage of p artially illuminated tree crowns of ellipsoidal shape, and through geo metric-optics and Boolean set theory, the proportion of sunlit and sha dowed canopy and background is modeled as a function of view angle. Th e model is driven by ground measurements of spectral reflectance and t ree crown shape, size, and spacing. Atmospherically corrected multiang ular radiance measurements of the FEDMAC spruce site from the Advanced Solid State Array Spectroradiometer (ASAS) were found to fit the shap e of the modeled reflectance function quite well along the principal a nd cross-principal planes. Furthermore, integration of the modeled ref lectance functions yielded spectral surface albedos (hemispherical ref lectance), which, when extended to the full solar spectrum, were found to agree closely with pyranometer measurements obtained at the spruce site.