GVI - Normalized Difference Vegetation Index
The basic index for measuring the 'greeness' of the earth's surface is the Normalized Difference Vegetation Index (NDVI), which is basically a calculation of the differences between AVHRR channels 1 and 2. A reasonable estimation of the density and coverage of green vegetation can be determined by measuring how green the earth's surface is.
NDVI values range from -.1 to .703 and are unitless. Values greater than .1 generally denote increasing degrees in the greeness and intensity of vegetation. Values between 0 and .1 are commonly characteristic of rocks and bare soil, and values less than 0 sometimes indicate clouds, rain, and snow.
NOTE: Low values of NDVI do not necessarily denote lack of vegetation. For example, during the winter months deciduous forests may appear more orange than green.