Validation of the MODIS vegetation indices over a global set of test sites: Preliminary results

Vegetation indices (VI's) are important tools in the seasonal and inter-annual monitoring of the Earth's vegetation. In this study, the vegetation index products from the Moderate Resolution Imaging Spectroradiometer (MODIS) are evaluated over a preliminary set of validation test sites, including a cerrado and rainforest site in Brazil, and two grass/shrub sites in Arizona and New Mexico, U.S.A. Ground and airborne validation experiments were conducted to assess the performance of the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) for vegetation monitoring. Calibrated spectroradiometers were flown for top-of-canopy reflectance retrievals. Vegetation sampling provided the data needed for a biophysical validation of the VI's. Both single-day and 16-day composited MODIS data were processed and corrected for atmosphere at 500m and 1 km resolutions. The MODIS data compared quite well with the validation data with most of the uncertainty associated with the compositing process. Results show the MODIS VI products to offer enhanced sensitivity for land use discrimination and monitoring at both regional and global scales. The EVI was fairly well resistant to residual cloud and aerosol contamination and had a good range of sensitivity over the high biomass, forested areas.