[QGIS-Developer] QGIS WCS is requesting FULL dataset?

Jonathan Moules jonathan-lists at lightpear.com
Sat Oct 20 12:58:33 PDT 2018

Hi Lists,

I'm cross posting this to GeoServer-Users over from QGIS-Dev as the 
current QGIS code seems to be trying to work around a GeoServer quirk 
based on Nyall's code-comment snippet.

Is there something different QGIS can do to work around the stated issue 
in a better way given I guess requesting a 10x10 Coverage via WCS isn't 
going to be doing the host server any favours?



On 2018-10-15 00:54, Nyall Dawson wrote:
> I can't explain the background here, but it seems deliberate. There's
> a comment before this request is made:
> "
>    // It may happen (GeoServer) that extent reported in
> spatialDomain.Envelope is larger
>    // than the coverage. Then if that larger BBOX is requested, the
> server returns
>    // request BBOX intersected with coverage box scaled to requested
>    // GDAL WCS client does not suffer from this probably because it
> probably takes
>    // extent from lonLatEnvelope (it probably does not calculate it from
>    // spatialDomain.RectifiedGrid because calculated value is slightly
> different).
>    // To get the true extent (it can also be smaller than real if
> reported Envelope is
>    // than real smaller, but smaller is safer because data cannot be
> shifted) we make
>    // request of the whole extent cut the extent from spatialDomain.Envelope if
>    // necessary"
>   getCache( 1, mCoverageExtent, 10, 10 );
> Radim may know more and may be able to explain if this workaround is
> still necessary
> Nyall
>> Hi,
>> using our national DEM WCS:
>> https://geodata.nationaalgeoregister.nl/ahn3/wcs?
>> Loading the layer ahn3_5m_dtm for an area of a couple of km in
>> epsg:28992 is very slow.
>> I'm not so familiar with the WCS specs, but I see QGIS asking:
>> https://geodata.nationaalgeoregister.nl/ahn3/wcs?SERVICE=WCS&VERSION=1.0.0&REQUEST=GetCoverage&FORMAT=GEOTIFF_FLOAT32&COVERAGE=ahn3_5m_dtm&BBOX=10000,250000,356250,618750&CRS=EPSG:28992&RESPONSE_CRS=EPSG:28992&WIDTH=10&HEIGHT=10
>> which is the whole of the Netherlands (in a 10x10 pixel output??) and
>> after that a more proper bbox.
>> Is this supposed to go like this?
>> I reckon that (if the WCS data does not have overviews), loading the
>> whole dataset all the time is pretty intensive.
>> Anybody more familiar with this can tell something about this?
>> Regards,
>> Richard Duivenvoorde
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