View angle effects on canopy reflectance and spectral mixture analysis of coniferous forests using AVIRIS
The dependence of vegetation reflectance on sun and sensor geometry can potentially provide information on canopy properties, but also may be a source of unmodelled systematic error in single-angle remote sensing measurements. In this study, we investigated the angular variability of reflectance measurements from the NASA Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), and the resulting impact on spectral mixture analysis (SMA) using both full-range (400-2500 nm) and shortwave-infrared wavelengths (2080-2280 nm; AutoSWIR). The study was conducted in coniferous forests in Central Oregon using five AVIRIS overpasses to generate multiple view angle measurements. Canopy reflectance was highly anisotropic, with the strength of the angular signal controlled by species type, canopy cover and soil reflectance. Canopy cover estimates from full-range SMA averaged only slight decreases (~6% relative) toward the retro-solar direction for 16 field plots in the study region. AutoSWIR was even less influenced by view angle, exhibiting changes only for large differences in view angle. In addition, AutoSWIR's ability to accommodate endmember variability led to stronger agreement with field cover values than full-range SMA. The results suggest that while view angle can significantly affect reflectance measurements from AVIRIS, the consequent variability in vegetation cover estimates from SMA and AutoSWIR is low.