Directional vegetation index interactions in ASAS imagery

The use of vegetation indices (VIs) for monitoring the Earth´s vegetation must not only produce composited, cloud-free imagery but must also consider and adjust for land surface and atmospheric directional effects. In this study, the view angle profiles of various vegetation indices (VI´s), along the principal plane to the Sun, were investigated with ASAS imagery, acquired over different land surface cover types at the Walnut Gulch Experimental Watershed in Arizona. The normalized difference vegetation index (NDVI) and several soil- and atmospheric-correcting versions of the NDVI were analyzed to examine land surface anisotropic effects for both land cover discrimination and for temporal compositing of the VI. Atmospheric optical depth and soil and vegetation biophysical measurements were simultaneously made. View angle profiles were highly dependent on the VI utilized. Furthermore, view angle profiles of some VI´s, such as the NDVI, varied with land cover type, while other VI´s, like the SAVI and modified NDVI (MNDVI), yielded similar profiles for all the land cover types studied. The NDVI view angle profiles were sensitive to atmosphere and topography-induced brightness variations while these same variations were minimized with use of the atmospheric resistant and soil-correcting variants of the NDVI, particularly the MNDVI. Hence, the improved, self-correcting VI´s facilitated multiangular interpretations, including compositing, by minimizing complex atmospheric-vegetation-soil-angular interactions