[gdal-dev] wcs extraction

[Frank Warmerdam]

Bruno Binet wrote:
> Hi list,
> 
> I'm currently trying to use the wcs gdal driver to extract data from a
> wcs service (mapserver), using a specfic projection, extent and
> resolution.
> I managed to extract the all dataset in the original projection, and
> format (geotiff).
> But I would like to be able to specify a value for the format, the
> extent, the projection, and the resolution for the data to be
> extracted.
> 
> Given the docs: http://www.gdal.org/frmt_wcs.html we can specify a
> preferred format for extraction with the PreferredFormat element, but
> what about projection, extent, and resolution?
> Do we have to download the entire dataset and use gdal capabilities to
> extract and reproject a subset of the downloaded data?
> It would be great if it could be handled directly by mapserver.

Bruno,

I have skimmed the WCS driver implementation and it does not appear
to be convenient to request data in a projection other than the default.
Basically the driver really wants to establish an image size and it does
this from the GridCRS/BoundingBox (1.1) or GridEnvelope (1.0) and this
information is generally (always?) only available for the default coordinate
system.

It might be possible for you to manually massage all these values in
the CoverageOffering/CoverageDescription in the GDAL WCS control file
to "fake it out" as if another coordinate system was the default.  However,
I've never tried that and it might be fairly hard to get right.

As things are currently setup, it is really intended that GDAL "knows" the
true native coordinate system, grid size and georeferencing and accesses
on that basis.

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