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

[Hamish]

Nikos Alexandris wrote:
> > > I don't get the desired results... The output is too bright from what one
> > > would expect it to be.
> > > 
> > > Any recommendations?
Hamish:
> > I've never used it for MODIS, but you could try i.landsat.rgb to see if
> > that helps. (it wants three 0-255 bands, it doesn't care about it being
> > LANDSAT or not)
Nikos:
> Will this approach enable a correction for terrain as well? I don't see
> how I can incorporate elevation data or anything relevant.
> 
> I used the cosine correction approach which is described in GRASS-GIS in
> order to correct a MODIS image. But the result is over-corrected (!) as
> mentioned also in the book.

No, i.landsat.rgb does not touch the data, it only normalizes the color tables
while preserving the relative hues (-p). If your image is simply over-bright it
can lower the visual brightness*, but not fix the contrast.

[*] actually it is (mostly) designed for brightening dark images, so you would
need to edit the script and swap the hard-coded MIN=`r.univar percentile=2` to
$((100-BRIGHTNESS)) and MAX=`r.univar percentile=$BRIGHTNESS` to "2" to make it
dimmer. Or get rid of the "2" and cut from both ends with $BRIGHTNESS.

I guess we could add a new flag to the module to do that if there was demand.

The result is dependent on the image, so not much use for quantitatively
comparing a time series of images with different reflectance angles.


Hamish



      ____________________________________________________________________________________
Be a better sports nut!  Let your teams follow you 
with Yahoo Mobile. Try it now.  http://mobile.yahoo.com/sports;_ylt=At9_qDKvtAbMuh1G1SQtBI7ntAcJ