Sun and view angle effects on NDVI determination of land cover types in the Brazilian Amazon region with hyperspectral data

Abstract

Solar zenith and view angle effects on the normalized difference vegetation index (NDVI) of land cover types in the Brazilian Amazon region were analysed. Airborne Hyperspectral Mapper (HYMAP) data were collected in 126 narrow bands (450–2500 nm) with a field of view (FOV) of ±30° from nadir. Data collection was performed initially in two flight lines with solar zenith angles of 29° and 53°. In a third flight line, view angles were up to +60° from nadir by airplane banking. Surface reflectance spectra representative of major land cover types were selected, and principal component analysis was applied to indicate their spectral similarity relationships in response to solar zenith angle variations. Reflectance and NDVI differences between pairs of land cover types were plotted for a variable band positioning. Atmospheric and coupled directional effects were analysed for variations in apparent and surface reflectance values, in the depth of the major water vapour absorption bands, and in the NDVI values, as a function of view angle. The results showed that major Amazon land cover types had increased NDVI values with increase in solar zenith angle, especially exposed soil or other vegetated surface components under soil background influence. Small solar zenith angles improved the NDVI contrast between green vegetation and soil and also the land cover discrimination in all bands, except for those placed in the 750–1100 nm range. At large solar zenith angles, this spectral interval was useful to separate primary forest/old woody vegetation regrowth from most of the other surface components due to enhancement in the shade fraction associated with the arboreal covers. Small differences in NDVI values of tropical rainforest were observed between ±25° from nadir. These differences became stronger with the increase in view angle from +25° to +60° in response to the additive and subtractive contributions of atmospheric scattering and absorption, respectively, and of coupled directional effects.

Acknowledgments

The authors are grateful to the Coordenação General de Engenharia e Tecnologia Espacial (ETE/INPE), especially to Carlos Santana, Jânio Kono, Mário Marcos Quintino da Silva and Himilcon Carvalho, for the entire HYMAP mission and for providing essential information related to the planned SSR 1. Thanks are also due to João Roberto dos Santos, Clotilde Pinheiro Ferri, Sílvia Pardi LaCruz and Mauro Prado for participation in the field activities, and to the anonymous reviewers for useful suggestions. L. S. Galvão was also supported by CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) during this investigation.