[MapServer-users] WMS with a GPKG-based layer : different responses, dependending on version (1.1.1 vs 1.3.0)

[Philippe Ghesquiere]

Hi Jukka,

Thanks for your clear, in depth explanation.
Your "OVERSAMPLE_RATIO=1.0" workaround works just fine. And it makes sense
to me.

Philippe

On Fri, Feb 27, 2026 at 3:27 PM Rahkonen Jukka <
jukka.rahkonen at maanmittauslaitos.fi> wrote:

> Hi,
>
> Thank you for the test package. I think I know what happens and I have a
> fix or a workaround for you.
>
> I added some more debugging first, not sure if all these were really
> needed.
>
> CONFIG MS_ERRORFILE "c:\ms4w_data\test2\errorfile.txt"
> CONFIG "CPL_DEBUG" "ON"
> CONFIG "PROJ_DEBUG" "ON"
>
> The MS_ERRORFILE shows what is the difference.
>
>
> WMS 1.1.0 is using method "msDrawRasterLayerGDAL()"
>
> msDrawRasterLayerGDAL(): Entering transform.
> msDrawRasterLayerGDAL(): src=0,0,4096,2048, dst=0,10,994,498
>
> WMS 1.3.0 probably believes that is must do warping and it selects to use
> method "msDrawGDAL(Image_Layer)"
>
> msResampleGDALToMap in effect: cellsize = 0.181087
> msDrawGDAL(Image_Layer): using RAW_WINDOW=0 0 4096 2048, dst=0,0,1988,994
>
> I do not understand the details, but in WMS 1.3.0 case the values in "dst"
> (destination) are about doubled.
>
> The Apache error log shows that depending on the WMS version, GDAL selects
> different overview levels from the GeoPackage, either "overview 1024 x 512"
> or "overview 2048 x 1024"
>
> Maybe the best fix for this case would be to make Mapserver consider that
> EPSG:4326 and CRS:84 are the same and no warping is needed between those
> two. And therefore there is no need to select the other code path.
>
> As a workaround, see https://mapserver.org/input/raster.html and add this
> line into your LAYER:
> PROCESSING "OVERSAMPLE_RATIO=1.0"
> Oversampling means that higher resolution data than what is needed for the
> output is read in for warping. That improves the quality of the warped
> result. That is fine for orthophotos and scanned maps, but in your use case
> the better resolution overview contains also different data from another
> zoom level.
>
> I tried also "PROCESSING ALLOW_OVERSAMPLE=NO" but for some reason it did
> not work for me. I was testing with a bit old MapServer version 8.3.0-dev
> (MS4W 5.0.0).
>
> -Jukka Rahkonen-
>
>
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