[GRASS-dev] Unexpected EVI range from "i.vi"

[Nikos Alexandris]

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!