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-Strahler geometric-optical forest canopy reflectance model and to demonstrate that both spectral bidirectional reflectance factors and hemispherical reflectance can be estimated with some success. With the geometric-optical model, a vegetated surface is treated as an assemblage of partially illuminated tree crowns of ellipsoidal shape, and through geometric-optics and Boolean set theory, the proportion of sunlit and shadowed canopy and background is modeled as a function of view angle. The model is driven by ground measurements of spectral reflectance and tree crown shape, size, and spacing. Atmospherically corrected multiangular radiance measurements of the FEDMAC spruce site from the Advanced Solid State Array Spectroradiometer (ASAS) were found to fit the shape of the modeled reflectance function quite well along the principal and cross-principal planes. Furthermore, integration of the modeled reflectance functions yielded spectral surface albedos (hemispherical reflectance), which, when extended to the full solar spectrum, were found to agree closely with pyranometer measurements obtained at the spruce site.