[GRASS-dev] Unexpected EVI range from "i.vi"
Nikos Alexandris
nik at nikosalexandris.net
Thu Jun 20 09:24:28 PDT 2013
Markus Metz wrote:
> I think I figured it out:
> The EVI formula in i.vi is for MODIS.
That's precise, EVI is MODIS specific. We should clearly describe this in the
manual (I will try to alter the respective text).
> The generic formula is
>
> G * ( nir - red) / (nir + C1 * red - C2 * blue + L)
> where G is a gain factor, C1, C2 are coefficients to correct for
> aerosol influences in the red band using the blue band and L is the
> canopy background adjustment that addresses non-linear, differential
> NIR and red radiant transfer through a canopy.
Right.
> The formula is scale-dependent, i.e. dependent on the range of the
> input data. Scaling the input test values to a [0, 10000] range (the
> range of the MODIS input bands), I get EVI values in the range [-1,
> 1].
Cool! Sorry that I misused the index :-/
> Furthermore, different coefficients might be needed for different
> sensors.
>
> Some more points:
>
> The reference in i.vi for EVI is wrong, it must be
>
> A.Huete, K.Didan, T.Miura, E.P.Rodriguez, X.Gao, L.G. Ferreira. 2002.
> Overview of the radiometric and biophysical performance of the MODIS
> vegetation indices. Remote Sensing of Environment 83 195-213.
O-K, we need to update this as well -- noted!
> The formula we used in R is wrong:
>
> evi <- ( 2.5 * ( nir - red) ) / (nir + 6 * red - 7 * blue + 1.0)
I guess I chewed-up the .5 at some point :D, otherwise I have used it
correctly in the beginning of my tests.
> it must be
>
> evi <- ( 2.5 * ( nir - red) ) / (nir + 6 * red - 7.5 * blue + 1.0)
>
> then I get again 6952.805
>
> g.region rast= res= (-a) does not make sense, unless you want to
> resample a raster map.
O-K.
Great, thanks!
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