Modis Vegetation Index (mod 13) Algorithm Theoretical Basis Document

One of the primary interests of the Earth Observing System (EOS) program is to study the role of terrestrial vegetation in large-scale global processes with the goal of understanding how the Earth functions as a system. This requires an understanding of the global distribution of vegetation types as well as their biophysical and structural properties and spatial/temporal variations. Vegetation Indices (VI) are robust, empirical measures of vegetation activity at the land surface. They are designed to enhance the vegetation signal from measured spectral responses by combining two (or more) different wavebands, often in the red (0.6-0.7 µm) and NIR wavelengths (0.7-1.1 µm). The MODIS vegetation index (VI) products will provide consistent, spatial and temporal comparisons of global vegetation conditions which will be used to monitor the Earth's terrestrial photosynthetic vegetation activity in support of phenologic, change detection, and biophysical interpretations. Gridded vegetation index maps depicting spatial and temporal variations in vegetation activity are derived at 16-day and monthly intervals for precise seasonal and interannual monitoring of the Earth's vegetation. The MODIS VI products are made globally robust and improves upon currently available indices with enhanced vegetation sensitivity and minimal variations associated with external influences (atmosphere, view and sun angles, clouds) and inherent, non-vegetation influences (canopy background, litter), in order to more effectively serve as a 'precise' measure of spatial and temporal vegetation 'change'. Two vegetation index (VI) algorithms are to be produced globally for land, at launch. One is the standard normalized difference vegetation index (NDVI), which is referred to as the " continuity index " to the existing NOAA-AVHRR derived NDVI. At the time of launch, there will be nearly a 20-year NDVI global data set (1981-1999) from the NOAA-AVHRR series, which could be extended by MODIS data to provide a long term data record for use in operational monitoring studies. The other is an 'enhanced' vegetation index (EVI) with improved sensitivity into high biomass regions and improved vegetation monitoring through a de-coupling of the canopy background signal and a reduction in atmosphere influences. The two VIs compliment each other in global vegetation studies and improve upon the extraction of canopy biophysical parameters. A new compositing scheme that reduces angular, sun-target-sensor variations is also utilized. The gridded vegetation index maps use MODIS surface reflectances, corrected for molecular scattering, ozone absorption, and aerosols, and adjusted to nadir with use of a BRDF model, as input to the VI equations. The …