[GRASS-user] working with orthophotos

[Nikos Alexandris]

On Wed, 2008-10-15 at 12:01 +0000, christian Brandt wrote:
> Dear list,
> 
> I imported  a georeferenced orthophoto (tif with tfw) with all bands (RGB) using r.in.gdal. 
> 
> The result (3 raster layers red, green and blue) looks like very , what means that there is not much to recognize any more on the orthophoto.
> 
> Any suggestions how to enhance the  of the orthophoto?
> 
> Regards
> 
> Christian

Hi Christian!

Splitting the colour orthophoto in three channes means that you are now
able to manipulate the 3 levels of this multi-dimensional "stack" of
information the way you want. You can for example use all 3 channels to
perform a classification or you can selectively use only 2 or 1! It
always depends upon what you want to do.

You can of course view the "normal" RGB image (with d.rgb where r=red
channel, g=green channel, b=clue channel). Now if viewing/visual
interpretation is your aim, you can create a composite (r.composite) and
use this as a single image for viewing (not for classifying).

About the colors you can either stretch all channels before creating the
composite or you might want to look in i.landsat.rgb (it's a very nice
auto-color-balancing tool).

FWIW, I think that GRASS is not well doing when it comes to pan/zoom an
image or visually cross-compare it with another. I prefer to open the
images with OpenEV (opening the raster maps from the /cellhd directory
or exporting them as tiffs).

Hope this helps, Nikos