On 05/08/2010 09:38, Frank Broniewski wrote:
Yesterday I received a note off list which pointed me to an error I
made when calculated the elevation
If you used r.mapcalc, make sure to divide by 100000.0 and not 100000
in order to get DCELL output, otherwise the result will be integer
with terraces, as in the aspect map, particularly on 5m horizontal
resolution.
Now I have a nice aspect map :-)
What would be the best approach to calculate a dtm for a contour map
where large parts miss elevation data? I 've uploaded an example on
our server:
http://www.gis-hosting.lu/static/img/grass/no_contours.png
Since the algorythm from r.surf.contour takes much longer when there
are large gaps or no elevation information is present, there should be
soem kind of solution. My idea is to take elevation data from srtm or
aster data and create contours from that data and patch it into the
finer contour map. Any other ideas?
I would be very surprised if you could get any match in elevations
bewteen your high resolution data, and the SRTM or ASTER data. I've
tried with some local, high quality DTM and was always disappointed.
THere was always a step at the edge where the overlap ends. And it was
not necessarily uniform, so it wasn't possible to just add i.e. x meters
to the ASTER DEM to get them to match.
In any case, let us know how it goes...
Many thanks
Frank
Am 04.08.2010 09:53, schrieb Frank Broniewski:
Hi,
Hamish:Feb 28, 2010; 03:15am Re: High resolution dem
I am curious to know what the problem with r.surf.contour was. Usually
it does a great job as long as you take a moment to work around the
integer and NULL/0 issues.
Sorry that I quitted this discussion so silently, but I had to drop this
and could not persue the issue further. Sometimes I really could use a
48h day :-)
Of course my problems are still present. To take the thread up again, I
want to answer to Hamish's question about my problems with
r.surf.contour first:
DEM interpolation seems to work nice, but when you start deriving maps,
especially the aspect map looks simply not right ;-)
I've uploaded some example images on our server, so that you can see
what I mean:
http://www.gis-hosting.lu/static/img/grass/contours5m.png
http://www.gis-hosting.lu/static/img/grass/dtm5m.png
http://www.gis-hosting.lu/static/img/grass/dtm3d.png
http://www.gis-hosting.lu/static/img/grass/expo5m.png
The data source is a shape file with contour lines with 5 meters
interval. Conversion to raster went well and the pixel values match the
height values for the contours. Before using the raster map, I
multiplied the map with 100000, just like it is mentioned in the wiki.
Interpolation took approx. 5 minutes. The region is 10,000m * 10,000m
wide and has a 5m resolution. Before deriving the aspect map, I divided
the elevation map by 100000 to gain real height values again ...
Now I wonder what could cause the gaps in the aspect map - since the
terrain model itself looks OK to me ...
Many thanks
Frank
--
Micha Silver
http://www.surfaces.co.il/
Arava Development Co. +972-52-3665918
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