Hello Jose! Thank you for your clarifications.
Jose Gomez-Dans wrote: > > 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 just mentioned the BRDF to emphasize that there is no link to any > terrain parameters or to a topographic correction within the description > of the algorithm (ATBD). And it is understandable since it is mainly an > atmospheric correction algorithm." > > 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). > > "Here it is interesting, indeed!" > > 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). > > So I will look for it... MOD43 (?). What is $\rho\$ exactly? > > Anyway, don't bother to reply to this question if it is explained on the > web (for MOD43). > > > Cheers, > José > > Thank you for your time. > > Nikos. > _______________________________________________ > grass-user mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/grass-user > > -- View this message in context: http://www.nabble.com/Correction-of-terrain-effects-for-MODIS-data--tf4924201.html#a14112242 Sent from the Grass - Users mailing list archive at Nabble.com. _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
