[GRASS-user] Re: QGIS GRASS plugin shadedrelief issue

Hamish hamish_b at yahoo.com
Tue Sep 21 03:01:49 EDT 2010

Nuno wrote:
> For all this steps i worked directly through
> GRASS 6.4 0RC6 and not QGIS. I have opened the grass raster
> layer with the contours (lines) and applyed the commands
> suggested:
> http://grass.osgeo.org/wiki/Contour_lines_to_DEM
> The result was this:
> http://osgeo-org.1803224.n2.nabble.com/file/n5549840/shadedtest1.jpg

that looks much better!

> Well it seems the terrace effect desappeared but there are
> some glintches, do you think its normal?

which glitches? the slight thatched-roof look? perhaps a quick
3x3 r.neighbors smoothing filer could fix that? maybe there is
an off-by-one bug somewhere that's causing that effect at the
contour lines; something for us to look at after floating point
and NULL support I guess.

> Concerning the flat mesa tops they look really ugly, any
> suggestion how to remake/remodel them?
> I suppose i have to go to the original geotiff topography chart
> and remake some vector lines to overcome that problem? What
> should i do?

As for the mesa effect, that is because there is no input data
above it, and unlike a spine interpolation it has no memory about
the slope, so above the highest elevation contour given it does
not know to go any higher. As suggested by the r.surf.contour help
page, it really helps to digitize some trig station / mountain
peak elevation data and if possible spot elevations along ridge
lines to help with that. hopefully your scanned topo map has
some of that information.

individual raster cells are ok, they don't have to be lines.
('v.to.rast type=points' + r.patch to join the points and vector
lines rasters)

> About the blackspots and the color table im not sure but
> changing it doesnt overcome the problem, the spots remain the
> same.

I'm not seeing them in the new version.. ?

> About the Region Settings they were set in QGIS. Since the
> region im working is fairly small i used this parameters:
> N-S Res = 1.00014 and E-W Res = 1.00024 (I think this is the
> minimum resolution supported).

grass doesn't really have a minimum or maximum resolution, just
that a huge number of cells ( > 40,000 x 40,000 ) can be very
slow and take lots of RAM and disk space; and at the other end
the minimum you can have is 1x1 cell.

In grass you can try 'g.region -a res=1' to round outward to
whole numbers.

> One problem i noticed and it might be a QGIS bug is that
> everytime i open QGIS or add a layer it resets the
> projection/coordination values (For example im using
> Datum ED50 UTM zone 29 international ellipsoid but when i
> open a layer or QGIS it always resets this values to others).

no idea, you'd have to ask a qgis expert about that.

grass Locations are locked to all maps using a single map
projection and you need to use r.proj, v.proj to move them
between different ones.



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