|
|
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 reflected signal from measured spectral responses by combining two (or more) wavebands, often in the red (0.6 - 0.7μm) and NIR wavelengths (0.7-1.1μm) regions.
The MODIS VI products (MOD13) provide consistent, spatial and temporal comparisons of global vegetation conditions which can 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 inter-annual monitoring of the Earth's terrestrial vegetation.
Two VI products are made globally for land regions. The First product is the standard Normalized Difference Vegetation Index (NDVI), which is referred to as the continuity index to the existing NOAA-AVHRR derived NDVI. There is a +27-year NDVI global data set (1981 - 2009) 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 second VI product is the Enhanced Vegetation Index (EVI), with improved sensitivity over high biomass regions and improved vegetation monitoring capability through a de-coupling of the canopy background signal and a reduction in atmosphere influences. The two VIs complement 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 VI maps use MODIS surface reflectances corrected for molecular scattering, ozone absorption, and aerosols, as input to the VI equations. The gridded vegetation indices include quality assurance (QA) flags with statistical data that indicate the quality of the VI product and input data.
The MODIS VI products are currently produced at 250 m, 500 m, 1 km and 0.05 deg spatial resolutions. For production purposes, MODIS VIs are produced in tile units that are approximately 1200-by-1200 km, and mapped in the Sinusoidal (SIN) grid projection. Only tiles containing land features are processed, with the aim to reduce processing and disk space requirements. When mosaicked, all tiles cover the terrestrial Earth and the global MODIS-VI can thus be generated each 16 days and each calendar month.
With over 12 years of Earth Observing and consistent data used in research, application, and modeling, the MODIS era is coming to an end. To ready the product suite for that eventual date the team is currently working on the following topics to help with long term archives and address earlier issues:
These changes and more are being implemented at the PI SCF level and the new products will be available when complete at https://vip.arizona.edu/viplab_data_explorer.php
