[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.

Nikos A:
> > That's precise, EVI is MODIS specific.  We should clearly describe this in
> > the manual (I will try to alter the respective text).

MM:
> From the literature, I gor the impression that EVI can be calculated
> from other sensors as well, as long as you get the coefficients right.

Yes, but this is not an easy job, is it?  This is (also) why, I think, they 
(MODIS science team) developed the EVI2, which is cross-sensor.


> >> 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.

> Assuming that the input to i.vi should be properly preprocessed bands
> with a theoretically maximum range of [0, 1], you could set L to
> 0.0001 and would get reasonable EVI values, sensor-independent.

This reminds, if I am not wrong (didn't check) the scale factor for MOD09 
surface reflectance products.  Makes sense.

Thank you Markus for the time you invested for this.

Nikos