Vegetation Index - NDVI
A common method of monitoring surface vegetation is through the Normalized Difference Vegetation Index:
This has long been used to monitor the vegetation, and changes in vegetation of the entire earth. NDVI for vegetation generally range from 0.3 to 0.8, with the larger values representing 'greener' surfaces. Bare soils range from about 0.2 - 0.3.
Healthy vegetation reflects very well in the near infrared part of the spectrum. Green leaves have a reflectance of 20 percent or less in the 0.5 to 0.7 micron range (green to red) and about 60 percent in the 0.7 to 1.3 micron range (near infra-red). NDVI provides an estimate of vegetation health and a means of monitoring changes in vegetation over time. The the typical range of NDVI is between about -0.1 (NIR less than VIS for a not very green area) to 0.6 (for a very green area).
Here is an example of NDVI over Europe, derived by German Remote Sensing Data Center.
The NDVI is typically derived from observations from the Advanced Very High Resolution Radiometer (AVHRR), which makes observations a five wavelengths (early versions has only four observations). The AVHRR makes observations in the visible, near-infrared, and thermal infrared portions of the electromagnetic spectrum. This sensor is carried on NOAA's Polar Orbiting Environmental Satellites (POES), beginning with TIROS-N in 1978.
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