[OSGeo-Discuss] WCS/WMS accuracy tests?

Steven M. Ottens steven at minst.net
Mon Dec 7 10:11:30 PST 2009

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?
>>> 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)
> Steve,
> 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
> speed:
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 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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