[Gfoss] [OSGeo-Discuss] WCS/WMS accuracy tests?

come da titolo : Test di accuratezza su wcs/wms


Inizio messaggio inoltrato:

Da: “Steven M. Ottens” <steven@minst.net>

Data: 09 dicembre 2009 14.03.31 GMT+01.00

A: OSGeo Discussions <discuss@lists.osgeo.org>

Oggetto: Re: [OSGeo-Discuss] WCS/WMS accuracy tests?

Rispondi a: OSGeo Discussions <discuss@lists.osgeo.org>

Hi all,

I’ve finished my tests.
The conclusion:
Geoserver has a bug which offsets all the results by half a pixel, this is a known issue with the definition of the location of a pixel. Added to this there’s the no-data border which appears with non-native, non-multiple requests. I presume that will be gone once the pixel issue is resolved.

Mapserver doesn’t offset the data unless it is physically impossible (non-native, non-multiple resolutions, extents which don’t snap to source data) but produces a multi-band geotiff where the source data is single band.

Deegree has a bug which offsets some of the results, but I don’t know what causes it and although it is resolution-related I don’t see a pattern. It also produces a multi-band geotiff instead of a single band.

The full report can be found at:
http://blog.minst.net/2009/12/09/perceived-wcs-inaccuracy (slow site warning)

The issue for Geoserver can be found at http://jira.codehaus.org/browse/GEOS-3702
The issue for Deegree can be found at http://wald.intevation.org/tracker/index.php?func=detail&aid=1216&group_id=27&atid=212
For mapserver I didn’t file a bug since I’m not entirely sure if the GeoTiff multi-band image is me or mapserver and I didn’t inverstigate it any further.

If there are any questions or remarks let me know


On Dec 8, 2009, at 9:31 AM, Judit Mays wrote:

Hello Steven,

the deegree crowd would also be interested in a description of your test

cases and the results. If you could send an email either here or,

preferably, to the deegree developer list [1], this would be much


I talked to one of the main deegree WMS developers and he told me that

deegree-wms passes all the OGC CITE tests of CITE 1.3.0, and that there

are specific tests which check whether the returned tiff contains the

expected pixels. So it would indeed be interesting to see, what is

different in your tests to cause the offset.

Kind regards,


[1] deegree-devel@lists.sourceforge.net | register at:


Steven M. Ottens schrieb:

On Dec 7, 2009, at 7:04 PM, Frank Warmerdam wrote:

Steven M. Ottens wrote:

On Dec 7, 2009, at 6:48 PM, Frank Warmerdam wrote:

Steven M. Ottens wrote:

Hi all, Working with Geoserver as a WCS we discovered that requesting a

GeoTIFF in the same projection as the original GeoTIFF produces a

shifted dataset. (http://jira.codehaus.org/browse/GEOS-3702) The shift

is small, less than one pixel of the original dataset, but with a coarse

dataset of 100m/pixel it can be 70meters. The Geoserver people are aware

of the problem and at some point in time will fix it I’m sure, but it

prompted me to test other OSS WCS servers (mapserver and deegree). Both

of them showed a shift of the data as well. Deegree has about the same

error as Geoserver, while Mapserver does a better job but is still off. I know there have been speed tests between different WMS services, but

I’m wondering has there been any data-quality/accuracy test been done

between WMS and/or WCS services?


I would appreciate your filing a detailed ticket on this issue against MapServer. Please be specific about the exact request made, provide the data and mapfile, and explain why you think the results are wrong.

Will do once the tests are completed. Currently we overlay the original

GeoTiff with the result of the request in QGIS. Other ways of testing are

welcome. (I was thinking gdal-info output, overlay in uDig and ArcMap to

rule out bias of QGIS)


Are you requesting the data at greater than the natural resolution of the

image? Is it the DescribeCoverage extent details that are wrong? If

you request the imagery supersampled (at higher resolution than the underlying

image) then there is a known issue with MapServer that can be fixed by

setting adding the following line to the LAYER at some cost in processing


I will test both the same resolution and a greater resolution to be sure. Currently we request a greater resolution since that’s what we need.


I discovered that already and included it in my mapfile. The trouble is that the image from Mapserver (and the other services) is shifted to the South East. I only did a quick test before the office closed so the exact shift is still to be determined, but it was noticeable smaller than with Geoserver and Deegree.

For Geoserver we tested it both by defining a resolution of the output image and with a width and height with the same BBOX. For Mapserver I only tried the resolution request (&resx=#&resy=#)

If that is the issue then a ticket might still be appropriate, but I will

take a different approach which is to force use of the more precise resampler

when any raster draw request is made at supersampled resolution.

Best regards,


I set the clouds in motion - turn up | Frank Warmerdam, warmerdam@pobox.com

light and sound - activate the windows | http://pobox.com/~warmerdam

and watch the world go round - Rush | Geospatial Programmer for Rent

Judit Mays

l a t / l o n GmbH

Aennchenstrasse 19 53177 Bonn, Germany

phone ++49 +228 18496-0 fax ++49 +228 18496-29

http://www.lat-lon.de http://www.deegree.org

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