Spring Vegetation in North America

Vegetation indices are important tools in the monitoring, mapping, and resource management of the Earths terrestrial vegetation. They are radiometric measures of the amount, structure, and condition of vegetation which serve as useful indicators of seasonal and inter-annual variations in vegetation. 16 days of Moderate Resolution Imaging Spectroradiometer (MODIS) images from the period from March 5 through March 20, 2000, were atmospherically corrected and then composited. The resulting composited reflectances in the first 4 bands of MODIS (blue, green, red, and near infrared) were then used as input into the computation of the NDVI and EVI. The MODIS sensor offers many improvements for vegetation index products, including improved sensitivity to chlorophyll and less contamination by atmospheric water vapor through narrower bandwidths in the red and near infrared, respectively. In addition, the finer pixel size (250 meter red and near infrared bands) provides improved vegetation index monitoring and change detction capability. The MODIS vegetation index products are produced at spatial resolutions of 250m, 500m, 1km and 0.25 degrees, and at temporal intervals of 16 days and 1 month. They include two vegetation index formulations, the normalized difference vegetation index, NDVI, which will serve as a continuity index to extend the Advanced Very High Resolutiuon Radiometer NDVI data record. An improved, or enhanced vegetation index (EVI) is also produced with the goal of providing complementary information on the spatial and temporal variations of vegetation, while minimizing much of the contamination problems present in the NDVI, such as those associated with canopy background and residual aerosol influences. Whereas the NDVI is chlorophyll sensitive and responds mostly to red band variations, the EVI is more NIR sensitive and responsive to canopy structural variations, including leaf area index, canopy type, and canopy architecture.
Source: Department of Soil, Water, and Environmental Sciences at the University of Arizona

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