On 25/09/12 12:16, Glynn Clements wrote:
Moritz Lennert wrote:
- A location in EPSG 31370 (Belgian Lambert 1972) - Objectif:
project the elevation raster layer from the nc_spm_08 dataset into
that Belgian projection dataset - A first run of r.proj to get the
coordinates of the layer in the new projection system. Here's the
result:
n=2370907.92051779 s=2378797.24912944 w=-6087464.12326059
e=-6068211.64408602 rows=1350 cols=1500
As you can see s> n. g.region bails out on that.
But, when you just switch n and s, then everything works fine and
the elevation dataset gets projected to the correct spot.
Now, I know that this is not the typical reprojection scenario, but
I'm wondering whether this indicates an underlying issue in
r.proj's boundary calculation algorithm.
This has nothing to do with r.proj's boundary calculation algorithm,
which isn't used for -p/-g. Instead, it simply projects the
south-east and north-west corners of the input map, and assumes that
the projected points are also the south-east and north-west corners.
Clearly, this will be inaccurate for anything other than projecting
between cylindrical projections and/or lat-lon.
The region-cropping optimisation which is used for projecting (when
the -n isn't given) projects the entire boundary, sampled at the
source resolution, then uses the bounding box of the projected
boundary. AFAICT, this works for all but pathological cases (e.g. a
projection which is tangential to the region being projected).
Can you try the attached patch?
Thanks for the patch ! Actually, I wasn't precise enough in my mail. I
was using 6.*. While testing with grass7 with and without the patch, I
noticed that the problem does not appear in grass7. The patch here does
not make a difference for the output of -g (or -p) which is:
n=2384892.88888888 s=2364922.88888888 w=-6087606.16666662
e=-6068356.16666662 rows=1350 cols=1500
I don't know what is different in the code between grass6 and grass7
that explains this issue.
Moritz
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