Re: [thredds] future TDS WCS support for irregular/curvilinear grids?

Hi all,

Besides GeoTIFF not supporting curvilinear grids, the WCS specification
does not currently support *requesting* irregular or curvilinear grids.
What it does allow is listing supported Coordinate Reference Systems
(CRS) on which requests must be made. Those CRS can be predefined (e.g.,
EPSG codes) or parameterized projections (though how those are
advertised is still being debated). So, if you can describe your grid as
regularly spaced on some CRS, you are good to go.

The WMS specification is similar to WCS in that it supports listing
available CRSs on which requests can be made. The reason for the
difference in what the TDS WCS can serve and what the TDS WMS can serve
is that the WMS code (ncWMS) uses the GeoToolkit library to support
regridding of the data.

So, two TDS WCS enhancement ideas that are on our list (though currently
not high on our list):

1) Add regridding capabilities to the WCS code (hopefully borrowing as
much as possible from WMS).

2) Any square array based grid can be described on what the OGC WCS
calls an "image" grid. Basically, the index space of the array. The TDS
WCS code does not currently support requests on the index space CRS. But
it could be enhanced to do so. And if that was done, it could serve up
both irregular and curvilinear grids (unstructured/triangular grids are
another story). The problem then is (at least for GeoTIFF response) that
the client won't know how to geolocate the data.


On 6/15/2011 7:47 PM, John Maurer wrote:
> Thanks, Ben! Sounds good! Glad to hear things are progressing along
> these lines. Sounds like it could be quite some time, though, before
> irregular grids are formally adopted by the international standards
> community and trickle down to all of us here in TDS-land. This is why I
> was hoping Unidata could maybe short circuit this process by regridding
> the data onto a regular grid via the TDS backend and then serving it out
> through WCS (1.0) that way. But that is maybe asking (way) too much.
> Again, ncWMS is presumably doing something similar to provide WMS for
> irregular grids (see attached image)? Or is the WMS spec ahead of WCS in
> terms of its adoption of irregular grids? Perhaps Jon Blower could
> elucidate (cc'd)...
> sst_roms_curvilinear.png
> Cheers,
> John Maurer
> On Wed, Jun 15, 2011 at 3:25 PM, Ben Domenico <Ben@xxxxxxxxxxxxxxxx
> <mailto:Ben@xxxxxxxxxxxxxxxx>> wrote:
>     Hi John et al.,
>     I am currently at the OGC Technical Committee meetings so your note
>     is quite timely.  My concept of how this sort of capability might be
>     worked into the standards -- from the netCDF point of view anyway --
>     is via the CF conventions.    We are currently working on a CF
>     extension for the OGC netCDF core standard.  At the same time we are
>     working on a CF-netCDF encoding standard for WCS 2.0.  But my
>     understanding is that irregular/curvilinear grids are not formally
>     adopted yet as part of the CF conventions.  When these grids
>     officially become part of CF, we can then do a revised extension to
>     CF-netCDF and to WCS.
>     Please correct me if I'm wrong about the status of
>     irregular/curvilinear grids in CF.  If you have suggestions for
>     better ways to accomplish this, let me know.   But for now this is
>     the general outline of the plan for getting this into the formal
>     standards world.  
>     For what it's worth.
>     -- Ben
>     On Wed, Jun 15, 2011 at 6:23 PM, John Maurer <jmaurer@xxxxxxxxxx
>     <mailto:jmaurer@xxxxxxxxxx>> wrote:
>         Thanks for the explanation, Ethan. If TDS could handle the
>         regridding itself, then it could work. I assume this is what's
>         happening on the backend via TDS/ncWMS for GetMap requests since
>         the WMS spec doesn't handle irregular/curvilinear grids either
>         (right?). I was hoping a future enhancement could someday do the
>         same for TDS WCS. Something for the suggestion box.
>         Cheers,
>         John
>         On Wed, Jun 15, 2011 at 2:17 PM, Ethan Davis
>         <edavis@xxxxxxxxxxxxxxxx <mailto:edavis@xxxxxxxxxxxxxxxx>> wrote:
>             Hi John,
>             TDS currently only supports WCS 1.0.0. The WCS 1.0.0
>             specification
>             itself does not support requesting or returning
>             irregular/curvilinear
>             grids. I believe the same is true for WCS 1.1.0.
>             I don't belive this is necessarily the case for WCS 2.0.
>             However, the
>             2.0 specification is written in the new core/extensions
>             paradigm and I
>             don't believe the extensions necessary to request or return
>             irregular/curvilinear grids are yet written.
>             The TDS WCS implementation does not currently support
>             regridding data.
>             So, currently one gets the data from the WCS in the same
>             projection in
>             which the data is stored. The combination means the TDS WCS
>             can only
>             serve regularly gridded data.
>             Ethan
>             On 6/15/2011 5:31 PM, John Maurer wrote:
>             > Hi All,
>             > We now use the WCS service through TDS to serve GeoTIFF
>             files to users.
>             > Since it only works for regular grids, I was wondering if
>             TDS could be
>             > enhanced in the future to support irregular/curvilinear
>             grids via WCS?
>             > TDS/ncWMS already handles curvilinear grids via WMS, so I
>             was hoping the
>             > same sort of intervening magic could be applied for TDS'
>             WCS service?
>             > Thoughts?
>             > Thanks for listening!,
>             > John Maurer
>             > Pacific Islands Ocean Observing System (PacIOOS)
>             > University of Hawaii at Manoa
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