Syntax: avi(nir, red, green [, mwnir = VAL] [, mwred = VAL] [, mwgreen = VAL])
The optional mwnir, mwred, and mwgreen parameters refer to the mid-band wavelengths of the component images. If not specified, they default to 825.0, 660.0, and 565.0 (the peak wavelengths of the corresponding LandSat 7 bands) respectively.
This function is part of the larger class of vegetation-index methods used in satellite image-processing. See ndi or this discussion of related functions for more detailed information.
AVI is of the subtype which will return both positive and negative values. Positive values generally indicate the presence of vegetation (with greater values indicating healthier vegetation). Negative values generally indicate a lack of vegetation (water, rock, soil, etc.).
The formula used is:
AVI = arctan(((mwNIR - mwRed)/mwRed) * (1 / NIR - Red)) + arctan(((mwRed - mwGreen)/mwRed) * (1 / Green - Red))
Because the mid-band wavelength values are all used in the formula as ratios, the units used to represent them are irrelevant as long as the same unit is used for all three.
This formula was obtained from "Bilko Mini-Lesson 8: Vegetation Indices".