Having worked with the importing WCS output via GeoTIFFs for use with
NetCDF-Java i'm not sure it's possible to define a curvilinear model
coordinates using the current GeoTIFF tag set.
The pertinent portion of the GeoTIFF specification is here:
http://www.remotesensing.org/geotiff/spec/geotiff2.6.html#2.6.1
There are two transformation types:
1) Define a tie point as x,y[,z] along with pixel x,y,[z] deltas.
*or*
2) Define a transformation matrix such that m(x,y,z,1) = M_t * i(x,y,z,1) with
M_t being a 4x4 transformation matrix and m being model coord and i being image
coord.
I've validated the GeoTIFF IOSP against all the sample GeoTIFFs available at
the remotesensing.org site and have yet to see an implementation of
type 2. I would take this to mean that even if one were to output a
transformation of type 2 it would be unlikely that client software would
support it (since there doesn't seem to exist validation data for it). That
being said I don't think a type 2 definition could be used to define
curvilinear mode coordinates.
I guess the point is that WCS support for curvilinear grids would need an
output format other than GeoTIFF. The only way to specify a curvilinear grid
with GeoTIFF would be to provide model data as a regular grid with a (known)
projection mapping to the curvilinear coordinates. I don't think this provides
the desired flexibility (projections are limited to known ESG
projection-algorithm/parameter-sets or projections via EPSG codes). NetCDF-CDF
output from WCS would be needed. I apologize if i've stated the obvious...
Tom
On Jun 15, 2011, at 10:00 PM, john caron wrote:
> some terms as far as i understand:
>
> regular grids: 1d x, y coordinates evenly spaced. only
> ones WCS 1.0 allows
> irregularly spaced grids: 1d x,y, but dont have to be even, eg gaussian
> latitudes.
> curvilinear grids: 2d x,y but each section is a quadilateral
> and connected to its neighbor (implicit topology)
>
> CF allows both irregular and curvilinear grids. one could extend WCS to
> return netcdf/CF irregular and curvilinear grids, without regridding.
>
> unstructured grids: arbitrary meshes, explicit topology, no CF standard yet.
>
> John, why do you want WCS? To get geotiffs, or ??
>
> On 6/15/2011 7:25 PM, Ben Domenico 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> 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> 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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