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

  • Subject: Re: [thredds] future TDS WCS support for irregular/curvilinear grids?
  • From: Tom Kunicki <tkunicki@xxxxxxxx>
  • Date: Wed, 15 Jun 2011 23:09:25 -0500
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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