Variations in reflectance with seasonality and viewing geometry: Implications for classification of Brazilian savanna physiognomies with MISR/Terra data

Bidirectional Reflectance Factor (BRF) data, collected at nine view angles, four bands and six dates by the Multi-angle Imaging SpectroRadiometer (MISR), were used to characterize the seasonality and viewing geometry effects on the discrimination of five selected physiognomies of a specific Brazilian savanna environment. Spectral-angular profiles for each physiognomy (Seasonal, Dry and Pluvial Forests; Arboreous and Park Savanna) were obtained from nadir-normalized BRY data at each MISR band and date of image acquisition. The maximum likelihood classification technique was applied at each camera and date using a common set of pixels as training samples. A reference map was used as ground truth to obtain the classification accuracy for each physiognomy, view angle and date. Results showed that the surface anisotropy signatures of the savanna physiognomies were not unique and varied with Sun-view geometry and seasonality. Directional effects increased from data collected in the orthogonal plane to those acquired close to the solar principal plane, and with increasing Sun zenith angles. Such effects were also affected by seasonality due to differences in the dynamics of the vegetation response to precipitation, as indicated by the Fraction of Photosynthetically Active Radiation (FPAR) and Leaf Area Index (LAI) values. Dry Forest presented a faster rate of green-up in the beginning of the rainy season and more abrupt changes in LAI values earlier in the dry season than the other physiognomies. In relation to the nadir response, the strongest anisotropy was observed in the backward scattering direction and in the red band at large Sun zenith angles. Directional effects were also observed after the Normalized Difference Vegetation Index (NDVI) determination. Classification accuracy of vegetation improved from the rainy to the dry season. The exception was Park Savanna, which was also well discriminated from the other physiognomies in the beginning of the rainy season due to the spectral effects of non-photosynthetic vegetation (dry grass understore) that produced an increase in the red reflectance. In general, classification accuracy of the physiognomies improved also from the forward to the backward scattering direction. The best view angles for classification purposes ranged from 0 degrees (nadir) to -45.6 degrees, and were associated with viewing directions of maximum backscattering at the different dates. In comparison with single view direction results, the use of Anisotropy Index (ANIX) images produced a general decrease in classification accuracy values. Results indicated that off-nadir viewing can improve discrimination and mapping of major physiognomies in the Brazilian savanna environment. (c) 2006 Elsevier Inc. All rights reserved.