Hi, On Saturday 01 December 2007 21:11:28 Nikos Alexandris wrote: > > Did you check the ATBD08 (Algorithm Technical Background Document)? > > http://modis.gsfc.nasa.gov/data/atbd/atbd_mod08.pdf > > But indeed - it doesn't seem to mention it. > > I ve read almost every line from the MODIS algorithm. > > There seems to be an attempt to couple BRDF but no DEM is incorporated > in the approach.
That's two different things altogether. Atmospheric correction tries to minimise the effects of the... well, atmosphere :) For that, it does need an idea of the angular reflectance properties of the surface (or assume something). In order to bring this into the algorithm, the BRDF (the variation of the reflectance as a function of illumination and viewing geometry) is used. Apart from the fact that the correction needs to take into account the width of the atmosphere (and hence needs an estimation of height), terrain is not a (major) issue for this correction. I think your problem comes to correct the variations in reflectance of the terrain in different images. This is the expected behaviour: in areas with high relief, you are looking at an interesting BRDF function, and small changes in illumination/viewing geometry will be greater than in flat terrain (which is more Lambertian-like). The good news is that you can use the BRDF product to ensure a constant illumination/viewing geometry for all your data. Using this does away with the geometry of acquisition variations, and depending on your terrain relief, you can choose the most optimal setup (you just work out $\rho$ from the kernel parameter estimates in MOD43 product). Cheers, José _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
