Greenness of the Conterminous U.S.
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What is NDVI?
The Normalized Difference Vegetation Index (NDVI) has been in use for many years to measure and monitor plant growth (vigor), vegetation cover, and biomass production from multispectral satellite data. The NDVI image maps shown here are prepared from 1-km AVHRR spectral data in the visible (Channel 1; 0.58-0.68 micrometers) and near infrared (Channel 2; 0.725-1.10 micrometers) regions of the electromagnetic spectrum. NDVI is calculated as follows:
NDVI = (Channel 2 - Channel 1) / (Channel 2 + Channel 1)
The principle behind NDVI is that Channel 1 is in the red-light region of the electromagnetic spectrum where chlorophyll causes considerable absorption of incoming sunlight, whereas Channel 2 is in the near-infrared region of the spectrum where a plant's spongy mesophyll leaf structure creates considerable reflectance (Tucker 1979, Jackson et al.1983, Tucker et al. 1991). As a result, vigorously growing healthy vegetation has low red-light reflectance and high near-infrared reflectance, and hence, high NDVI values. This relatively simply algorithm produces output values in the range of -1.0 to 1.0. Increasing positive NDVI values, shown in increasing shades of green on the images, indicate increasing amounts of green vegetation. NDVI values near zero and decreasing negative values indicate non-vegetated features such as barren surfaces (rock and soil) and water, snow, ice, and clouds.
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