Vegetation change monitoring with spectral indices: The importance of view and sun angle standardized data

Remotely sensed reflectance data are often acquired at variable view and solar geometric configurations. Vegetation change monitoring with the NDVI (Normalized Difference Vegetation Index) is sensitive to the effects of solar and view angle geometry. However, by using a BRDF (Bidirectional Reflectance Distribution Function) model, the view and sun angle variability in the NDVI can be standardized. If multi-sun angle data are not available, a second method allows us to extrapolate (nadir) satellite observations to a standard sun angle by using predetermined linear regression relationships between sun angle and ground-based nadir NDVI values for a range of vegetation types. Both methods were applied to one month of daily, atmospherically corrected, multi-angle SeaWiFS (Sea viewing Wide Field-of-view Spectroradiometer) land reflectance data, with promising results. The difference in NDVI due to a sun angle change from 20° to 70° can be up to 50%. The NDVI values for very dense vegetated and bare soil surface areas are less affected by changes in solar zenith angles. This research shows that the sun and view angle effects on the widely used spectral indices could be standardized to improve the accuracy of regional and global vegetation and crop monitoring efforts.