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

[José Gómez-Dans]

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