[OSGeo-Discuss] WCS/WMS accuracy tests?

Frank Warmerdam warmerdam at pobox.com
Mon Dec 7 10:04:20 PST 2009

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?
>> Steven,
>> 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


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 at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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