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Abstract
Images from the Compact High Resolution Imaging Spectrometer (CHRIS), on board the space platform Project for On Board Autonomy (PROBA), were acquired in the Brazilian Amazon region in 62 bands (410–1050 nm) at different view angles. They were evaluated for angular variations in Bidirectional Reflectance Factor (BRF), selected vegetation and anisotropic indices, and their relationship with the Above Ground Biomass (AGB) of some forest successional stages using an empirical approach. Results showed that correlations between AGB and reflectance were influenced by the vegetation anisotropy, which was stronger in the visible than in the near-infrared. The anisotropy increased from the forward (−36°) to the backward (+55°) scattering direction, was greater in the blue and green bands and decreased towards the near-infrared. As a result of this behaviour, several narrow and traditional vegetation indices showed correlation coefficients (R) that varied with view angle. The backscattering–forward scattering contrast, represented by anisotropic indices such as the Normalized Difference Anisotropic Index (NADX) and the Hot spot–Dark spot Normalized Difference Vegetation Index (NDVITD), presented only a small improvement of the relationships with AGB when compared with the performance of the other traditional vegetation indices at nadir viewing.
Acknowledgements
The authors are grateful to European Space Agency (ESA) (Category 1 PROBA Proposal 3774), and to Luiz Aragão, Fernando Espírito-Santo and João Roberto dos Santos for providing field inventory data. Thanks are also due to the anonymous reviewers for very useful suggestions.