[GRASS-user] High resolution dem

[Nick Cahill]

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
>> 
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