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

[Nikos Alexandris]

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.
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> 

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