An assessment of the MODIS vegetation index compositing algorithm using quality assurance flags and Sun/view angles

The normalized difference vegetation index (NDVI) products, as derived from the NOAA AVHRR sensor series, have been shown useful for the studies of the land biosphere characteristics and dynamics at regional to global scales. Standard pre-processing in generating these AVHRR NDVI products include the compositing process, in which the highest NDVI value from a series of multitemporal georeferenced images is retained for each pixel location in order to minimize cloud and atmosphere contamination. While this maximum value composite (MVC) procedure has been shown to produce NDVI images with a relatively high degree of radiometric consistency, adjacent composite pixels may have been acquired at widely varying viewing geometries due to the bidirectional reflectance distribution effects of the surface targets, of which magnitude varies with land cover types and atmospheric corrections, resulting in large inconsistencies. In response, the compositing algorithm of the Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation index (VI) products emphasizes a global, operational view angle standardization. It utilizes a bidirectional reflectance distribution function (BRDF) model to produce nadir looking equivalent reflectance values if enough cloud free observations are available during a 16-day compositing period. Otherwise, a backup, MVC criterion that includes a view zenith angle constraint (namely, the constraint view angle MVC criterion, CVMVC) is utilized to composite. In this study, the authors assess and characterize this new compositing algorithm using early MODIS data and compared it with the conventional MVC algorithm