Hi,

I did not notice that reprojection happens. I had a look at the CRS extension 
standard. First thing that it says is that range set values should not change 
without CRS change.

“The GetCoverage request is extended with two optional parameters indicating 
the CRSs for
subsetting coordinates and for the result coverage. If these parameters are 
omitted then all
access will be performed in the coverage’s Native CRS (as reported in its 
Coverage
Description).
Note This normally ensures that the coverage’s unchanged range set values are 
retrieved, assuming
that no other operations than subsetting is performed in the GetCoverage 
request.”

Then it says that server can do whatever it likes with resampling

“No statement is made in this specification about the interpolation method 
applied by the
server in the course of the CRS transformation. A server is free to employ any 
interpolation
method during CRS transformation, and information about the interpolation 
applied may not
be accessible to the client. While the interpolation method applied may change 
over requests,
and different interpolation methods may be applied to different range 
components of a
particular coverage, a server should not change interpolation across the domain 
of coverage
during execution of a single coverage CRS transformation.”

And this probably explains what should happen with extents and pixel size if 
reader can interpret it (I couldn’t yet).

“The range values of the response to a successful GetCoverage request 
containing an
outputCrs parameter shall be determined according to the following conceptual
evaluation model: Take the bounding box of the request’s subsetting parameter 
(or the full
coverage bounding box if no subsetting was specified in the request); 
re-project this
bounding box to the output CRS provided; generate the result image extent as 
the smallest
bounding box around the reprojected subsetting box; fill this new image extent 
with pixel
values re-projected from the coverage’s Native CRS to the output CRS in a way 
that, for each
axis, the smallest distance between any two reprojected grid points is used as 
offset
(including all interpolation and resampling necessary) unless specified 
otherwise by further
request parameters.

Note Request parameters dictating a different behaviour wrt. grid point offset 
are defined in the
WCS Scaling & Interpolation extension specification”

I have a feeling that without using those parameters from scaling and 
interpolation extensions it is not at all guaranteed that the result is totally 
predictable and identical for all WCS servers.

-Jukka Rahkonen-

Lähettäjä: andrea.a...@gmail.com [mailto:andrea.a...@gmail.com] Puolesta Andrea 
Aime
Lähetetty: 11. elokuuta 2017 12:43
Vastaanottaja: Rahkonen Jukka (MML) <jukka.rahko...@maanmittauslaitos.fi>
Kopio: anna-lena.h...@bkg.bund.de; geoserver-users@lists.sourceforge.net
Aihe: Re: [Geoserver-users] WCS returns wrong GeoTiffs

On Fri, Aug 11, 2017 at 11:32 AM, Rahkonen Jukka (MML) 
<jukka.rahko...@maanmittauslaitos.fi<mailto:jukka.rahko...@maanmittauslaitos.fi>>
 wrote:
Hi,

I would have a try with SCALESIZE and SCALEEXTENT parameters of the scaling 
extension https://portal.opengeospatial.org/files/12-039.  In theory 
GetCoverage should keep the native pixel size but it may be that GeoServer does 
not really know what the native pixel size is.

I don't have an answer for the original question, but mind, there is 
reprojection involved here, http://epsg.io/31467 is not UTM32N.
I don't believe there is in the spec a requirement to maintain the native 
resolution in this case, but I may be wrong.

Cheers
Andrea

--

Regards,

Andrea Aime

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