[GRASS-user] High resolution dem

[John Tate]

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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