Syntax: evi(nir, red, blue [, l = VAL] [, c1 = VAL] [, c2 = VAL] [, gain = VAL])
The optional l, c1, and c2 parameters refer to coefficients in the original formula, with l representing "the canopy background adjustment that addresses non-linear, differential NIR and red radiant transfer through a canopy", and c1/c2 being "the coefficients of the aerosol resistance term, which uses the blue band to correct for aerosol influences in the red band". If not specified, they default to 1.0, 6.0, and 7.5 (the values from the source paper) respectively. The gain parameter is a simple scaling factor and is included only for complete representation of the original formula. It defaults to 1.0 if not specified.
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.
EVI 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:
EVI = G * ((NIR - Red) / (NIR + C1 * Red - C2 * Blue + L))
This formula was obtained from "Tropical Wet Realms Of Central Africa, Part II" (Paul R. Baumann, 2009).
Note: as the example below indicates, this formula generates results over a much broader range than most other vegetation-index formulas. As a result, even when the results are normalized, they are "grainy".