[GRASS-user] Correction of terrain effects for MODIS data?

[Nikos Alexandris]

(...I have to stop using nabble for posting -- it doesn't seem to work
very well -- Sorry for reposting)

Hello Jose! 

Thank you for your clarifications.

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). 

Thank you for your time. 
        
Nikos.

On Sat, 2007-12-01 at 21:54 +0000, José Gómez-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 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
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-- 
Nikos Alexandris
.
Department of Remote Sensing & Landscape Information Systems
Faculty of Forestry & Environmental Sciences, Albert-Ludwigs-University Freiburg
.
Tel.  +49 (0) 761 203 3697 / Fax.  +49 (0) 761 203 3701 / Skype: Nikos.Alexandris
.
Address: Tennenbacher str. 4, D-79106 Freiburg i. Br., Germany
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