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! _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
