I have been reading again and again this:

“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”

I do believe now that “the smallest distance between any two reprojected grid 
points” in your case with 5 m pixels in UTM really converts into 5.0020008 in 
EPSG:31467. If you do not agree I would like to see your interpretation about 
that sentence.  Remember also the case of re-projection between projected and 
geographic systems.


Lähettäjä: Rahkonen Jukka (MML) [mailto:jukka.rahko...@maanmittauslaitos.fi]
Lähetetty: 11. elokuuta 2017 16:25
Vastaanottaja: anna-lena.h...@bkg.bund.de; andrea.a...@geo-solutions.it
Kopio: geoserver-users@lists.sourceforge.net
Aihe: Re: [Geoserver-users] WCS returns wrong GeoTiffs


The native pixel size in UTM32N is obviously 5.00 m. UTM projection is using a 
scale factor of 0.9996 and if you convert a polygon of size 5x5 m (presenting 
one pixel) into EPSG:31467 it will grow into 5.0020008x5.0020008 meters.  That 
process is converting each pixel as they are without any resampling. If exact 
pixel size of 5 meters in EPSG:31467 is to be achieved the data must be 
resampled. I would say that Geoserver is doing the right thing when it does 
minimum amount of resampling and allows pixel size to change. BTW. for example 
gdalwarp is doing the same thing and there are a bunch of questions about that 
in gis.stackexchange.

Now if we consider that it is OK to have 5.0020008 pixel size in the output it 
is impossible to keep extent as  3400000, 5660000 to 3420000, 5680000 because 
you can’t fill such an area with 5.0020008x5.0020008 sized blocks.

So if we want to keep what I consider as the most native pixel size 5.0020008 
we can’t get exactly 3400000, 5660000 to 3420000, 5680000 as extent. If we want 
the extent to be exact we must generate 5 m sized pixels which means resampling 
because of the difference in the projections (the 0.9996 scale factor).

I suppose it would be more clear is you re-projected data from UTM into 
EPSG:4326 because then it would be self-evident that the pixels size of the 
output is not 0.5 meters anymore because the unit is degrees.

By using scaling extension you can force resampling into 5 m output pixels and 
with those you can fill your desired area 3400000, 5660000 to 3420000, 5680000.


Lähettäjä: anna-lena.h...@bkg.bund.de<mailto:anna-lena.h...@bkg.bund.de> 
Lähetetty: 11. elokuuta 2017 15:40
Vastaanottaja: andrea.a...@geo-solutions.it<mailto:andrea.a...@geo-solutions.it>
Kopio: Rahkonen Jukka (MML) 
Aihe: AW: [Geoserver-users] WCS returns wrong GeoTiffs

Hello Andrea,

yes, I did. But I send it again:

I requested in GK3:
AND set the crs to be the same:

Should be 3400000, 5660000 to 3420000, 5680000

But GeoServer returned:
3399999.6508074491284788,5659999.2086437549442053 : 

I just don’t understand why the extent is wrong AND the pixel size and 
dimension. I understand that there are different distortions depending on the 
But how could I get the bounding box I requested?

@Jukka: Thanks for the hint with the Scaleextent – I give it a try… I just 
wanted to avoid to clip the image again to the requested extend.

Von: andrea.a...@gmail.com<mailto:andrea.a...@gmail.com> 
[mailto:andrea.a...@gmail.com] Im Auftrag von Andrea Aime
Gesendet: Freitag, 11. August 2017 14:04
An: Hock, Anna-Lena 
Cc: Rahkonen Jukka (MML) 
 GeoServer Mailing List List 
Betreff: Re: [Geoserver-users] WCS returns wrong GeoTiffs

On Fri, Aug 11, 2017 at 1:45 PM, 
<anna-lena.h...@bkg.bund.de<mailto:anna-lena.h...@bkg.bund.de>> wrote:
As far as I can interpret this, it means that in case we have OUTPUTCRS equals 
SUBSETTINGCRS the extent of the image should be exactly the one requested in 
SUBSET. The request coordinates aren’t transformed – the bounding box is 
“fixed” and the image is filled in this extent. If you vary the dimension and 
the size, then it should be possible to keep the requested extent. Am I missing 
something here?

Hum... you did not provide the output bounding box, in your first mail you were 
concerned about the resolution and pixel width/height only no?
What bounding box did GeoServer return?

During reprojection with no target resolution normally one can choose to 
respect either the native resolution (it seems GeoServer did,
to the letter, taking into account the different scaling factor of the two 
projections) or the target bounding box, but it's impossible in
general to do both.
Looking again at the specification extracts from Jukka I don't see a indication 
that one should be preferred over the other, looks
like an implementation decision.




Andrea Aime

GeoServer Professional Services from the experts! Visit http://goo.gl/it488V 
for more information.

Ing. Andrea Aime
Technical Lead

GeoSolutions S.A.S.
Via di Montramito 3/A
55054  Massarosa (LU)
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