Markus Metz wrote: > > 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)
Hmm, no, it seems that the coefficients for both EVI and EVI2 are meant for scaled MODIS input bands (top of atmosphere reflectance). The coefficients for EVI are a mystery. The MODIS algorithm theoretical basis document refers to Huete and Liu (1996), which does not exist in the references. Huete et al (2002) specifies L = 1, C1 = 6, C2 = 7.5, and G = 2.5, referring to Huete, Justice, & Liu (1994) and Huete, Liu, Batchily, & van Leeuwen (1997), but none of these references explain how these numbers were determined. Interestingly, the MODIS algorithm theoretical basis document sets G = 2, not 2.5, without any explanation. In order to get an EVI in the range [-1, 1], using EVI = G * (nir - red) / (nir + C1 * red - C2 * blue + L) (nir + C1 * red - C2 * blue + L) must be >= G * (nir - red) which could easily be added to i.vi. In any case, L needs to be adjusted to the actual input scale. In summary, rather use NDVI. Markus M _______________________________________________ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev
