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

[Markus Metz]

On Fri, Jun 21, 2013 at 10:04 PM, Nikos Alexandris
<nik at nikosalexandris.net> wrote:
> 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.

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)

again, L needs to be adjusted to the actual input scale.

AFAIU, both EVI and EVI2 can be applied to different sensors, given
that the required bands are available and that the coefficients are
adjusted if need be.

>
>
>> >> 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.

I suggested L = 0.0001 exactly because this is the MODIS scale factor.

BTW, the satellite data you mentioned are ETM, not MODIS, thus
applying the EVI formula developed for MODIS to ETM bands is a bit
adventurous. In any case, EVI was developed for tropical rain forests
because NDVI can saturate there. The Landsat scene you mentioned has
only ocean and desert, no forest of any kind. NDVI should be just fine
in this area.

I would suggest to test the EVI(2) formulas in i.vi with a MODIS NDVI
product which also includes the required input bands. All bands in the
MODIS NDVI product would need to be scaled according to the
documentation prior to feeding them to i.vi, or r.mapcalc with
adjusted formulas.

Markus M