[GRASS-user] High resolution dem

[Frank Broniewski]

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