Dear all, Just to refer to a previous question - when I have had to make relatively high resolution DEMs (much smaller than yours, only 5000 x 3000 cells), I found it most effective to use Arc/Info to create a TIN from contour lines and points, then rasterize that and import the raster into GRASS, rather than having GRASS interpolate from contours. I was never able to get the parameters right in *.surf.rst, and processing times were very long. Arc/Info does the job very quickly and effectively, and doesn't end up with overshoots and depressions, which were a problem for me. I wish this were an option in GRASS. I would also like to be able to work with other vector-based surface models in GRASS.
Nick Cahill Dept of Art History UW-Madison On Feb 25, 2010, at 5:11 AM, John Tate wrote: > Hi Frank, > > Not sure this will help, as I am slightly unsure what you are attempting, but > you may be able to apply what I did to create a DEM from tiles. > > I interpolated my 70 1km tiles with v.surf.rst by interpolating a 1050x1050 > cell area (1.05kmsq). I cropped with g.region and r.mapcalc to 1020X1020 > (incase of any artefacts - trees are bad for that), and then patched > (r.patch) > them all together. The patching should average out any differences. I then > cropped out each 1km tile (1000x1000). > > This was done so that the 1km tiles could be combined for specific areas by > different people (e.g. only a 4kmsq area for academic 'a' or a 6kmsq area 2km > away for academic 'b'). > > Anyway, from what I understand of your scenario, a hole in a DEM to insert > another DEM, create at least a 20 cell overlap for both datasets and r.patch. > > Hope this helps > > John > > On Thursday 25 February 2010 08:45:37 Frank Broniewski wrote: >> Hello, >> >> I am trying to create a high resolution dem from contour lines Until now >> all my tries where not successful. At first I tried r.surf.contour, but >> since my interpolation region is not rectangular and the contours are not >> evenly distributed (rough terrain), the result was unfortunately not >> usable ( but it took around 7 days to compute, that alone was already >> impressive ;) ) >> >> My contour map is a combinatin of a national contour line map (5m vert. >> resolution) and contours from SRTM with 20m vert. resolution. I created a >> "hole" in the srtm contours for the national contour map and patched both >> together to avoid large gaps with no height values (mostly for >> r.surf.contour) >> >> My region is 17.000 x 13.000 cells wide (5m horiz. resolution). So my >> current approach is to use small regions (2000 x 2000) to calculate small >> subsets of the dem. Because of the algo used by v.surf.rst to create the >> dem the neighboring tiles do have different height values calculated at >> the borders. So it was not possible to just create the tiles and patch >> them together. >> >> My next approach used an overlapping of 20 cells for each tile and a moving >> window average to calculate the mean of the overlapping tiles. The result >> was quite good, but the moving window approach resulted in null values >> where one tile ended and the other started (similar to the slope and >> aspect maps, where there is a 1 cell null border around the map in >> comparison to the input dem). >> Unfortunately I was not able to remove the null values satisfactorily. >> r.fillnulls fails because of the large region, and r.resamp.rst does the >> job not very well. The stripes are still visible, though filled with >> values. When calculating a derivate from the dem, like aspect, the errors >> from filling null values are quite obvious. >> >> So to make my long text short: Is there a technique to combine two or more >> raster dem with (or without overlapping) with good transition/intersection >> (don't know the correct word) between two tiles? >> If necessary I can illustrate my efforts by creating a web page or similar >> ... >> >> Many thanks for tipps >> >> Frank >> >> Frank Broniewski >> >> Metrico s.àr.l. ( http://www.metrico.lu ) >> 36, rue des Romains >> L-5433 Niederdonven >> Luxembourg >> >> Fon: +352 26 74 94 28 >> Fax: +352 26 74 94 99 >> >> _______________________________________________ >> grass-user mailing list >> [email protected] >> http://lists.osgeo.org/mailman/listinfo/grass-user >> > _______________________________________________ > grass-user mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/grass-user _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
