Hello all,
many thanks for your comments. Just to clarify what I want to do: I have a
contour map with 5m contours. I want to create a DEM from it using
v.surf.rst, since r.surf.contour fails to do the job correctly.
Since I can't do the whole map at once with v.surf.rst due to too high
memory usage my plan was to create DEMs from subsets of the region and patch
them together afterwards. But due to the nature of the algo used by
v.surf.rst the border cells from neighboring (sub)DEMs do not match well.
So I am searching for a solution how I can eliminate these differences at
the border. My plan was to use some overlapping and r.mapcalc to create some
kind of average calculation for the overlapping parts to get a smooth
transition for the DEMs.
John, your approach looks promising. The only thing that makes me wonder is
the usage of r.patch, since it takes always the values from the first raster
in the patch list and therefor it cannot average out any differences, at
least not from my understanding ... but I will investigate further.
Hopefully I find some time on monday to illustrate my method and problem a
little bit better with some images.
Many thanks so far for all the answers
Frank
--------------------------------------------------
From: "Nick Cahill" <[email protected]>
Sent: Thursday, February 25, 2010 3:40 PM
To: "GRASS user list" <[email protected]>
Subject: Re: [GRASS-user] High resolution dem
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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