Directional sensitivity analysis of vegetation indices from multi-angular CHRIS/PROBA data

View angle effects present in vegetation indices are either being seen as unwanted information or as an additional source of information. However, the magnitude of these angular effects remains for most indices unknown. We use the ESA-mission CHRIS- PROBA (Compact High Resolution Imaging Spectrometer–Project for On-board Autonomy) providing spaceborne imaging spectrometer and multidirectional data to assess the directional sensitivity of broadband and recently developed narrowband indices. Apart from the illumination and viewing geometry as well as the atmospheric composition, the surface reflectance anisotropy is a prime factor determining indices directional response. Two contrasting structural vegetation types, forest and meadow, were selected to study the affect of different land cover types on the indices angular response. This work demonstrates that the tested broadband indices (NDVI, SRI, ARVI) as well as the narrowband indices NDVI705, PRI, ARI1 & ARI2 were significantly sensitive to angular effects, while other indices (mSR705, mNDVI705, SIPI & RGRI) tested invariant to multiple viewing angle observations. The results suggest that caution is required when using some, but not all, indices since angular effects may differently impact the results, finally seriously hampering interpretation.

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