[GRASS-dev] Unexpected EVI range from "i.vi"
markus.metz.giswork at gmail.com
Mon Jun 24 12:28:32 PDT 2013
Markus Metz wrote:
> I am pretty sure that the EVI2 formula in i.vi is not cross-sensor,
> but also tailored to unscaled MODIS input bands:
> EVI2 = G * ( nir - red) / (nir + C1 * red + L)
Hmm, no, it seems that the coefficients for both EVI and EVI2 are
meant for scaled MODIS input bands (top of atmosphere reflectance).
The coefficients for EVI are a mystery. The MODIS algorithm
theoretical basis document refers to Huete and Liu (1996), which does
not exist in the references. Huete et al (2002) specifies L = 1, C1 =
6, C2 = 7.5, and G = 2.5, referring to Huete, Justice, & Liu (1994)
and Huete, Liu, Batchily, & van Leeuwen (1997), but none of these
references explain how these numbers were determined. Interestingly,
the MODIS algorithm theoretical basis document sets G = 2, not 2.5,
without any explanation.
In order to get an EVI in the range [-1, 1], using
EVI = G * (nir - red) / (nir + C1 * red - C2 * blue + L)
(nir + C1 * red - C2 * blue + L) must be >= G * (nir - red)
which could easily be added to i.vi.
In any case, L needs to be adjusted to the actual input scale.
In summary, rather use NDVI.
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