[GRASS-user] QGIS GRASS plugin shadedrelief issue

Micha Silver micha at arava.co.il
Sun Sep 19 08:04:01 EDT 2010

On 19/09/2010 13:03, nunosousa84 wrote:
> Hi,
> Im new to the forum and to GIS (both GRASS and QGIS). Im using QGIS GRASS
> Plugin. Im having some issues using GRASS shadedrelief functionality. When i
> manipulate the values of the altitute of the sun above the horizon (in
> degrees) i catch some black spots/dots in the final result. This maybe
> normal since we are working with different altitudes so shadow projections
> vary and some areas might be out of reach? Anyway if you could help me with
> this (try to diminish the problem or try another method) i would apreciate.
> Here are 3 examples:
> http://osgeo-org.1803224.n2.nabble.com/file/n5547443/shaded30270.jpeg
> http://osgeo-org.1803224.n2.nabble.com/file/n5547443/shaded50270.jpeg
> http://osgeo-org.1803224.n2.nabble.com/file/n5547443/shaded80270.jpeg
 From the looks of the images, it seems you have a DEM created from 
contour lines, without any interpolation. You can clearly see the 
"terracing" along the slopes. This might also, indirectly be the cause 
of the black spots. Do you know how the DEM was made? Do you have access 
to the original contour lines? If so, you might get more satisfying 
results with the GRASS v.surf.rst module to interpolate a smoother DEM.

Micha Silver
Arava Development Co.  +972-52-3665918

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