Hi Steve:
CF has actually started down the path of specifying the projection
parameters (see Appendix F), which we are taking from FGDC. So I was
really asking for a double check on a "best practice" of how all the
pieces should fit together, and an example added to section 5.6. We are
currently writing code that creates CF netcdf datasets out of GRIB, so
it seemed like a good time to double check.
BTW, as a philosophical matter, I would consider the specification of
the projection as fundamental, and the specification of the explicit
coordinate in lat, lon as redundant, because you can figure out the
values from the projection, but not the other way around. This is not to
say that I disagree with the decision to require the lat,lon values but
not require the projection, however. But i would (and do) recommend
adding the projection info. If the software knows how to use it, it
makes visualization of the grid easier, since model grids are regular in
the projection coordinates, and so they constitutes a kind of "natural"
coordinate system. So I would advocate that CF recommends adding
projection info.
Steve Hankin wrote:
> Hi John,
>
> As you know there are two basic "schools" of thought on how to
> represent projected grids:
>
> 1. by naming the projection and giving its parameters (as is done
> in GIS)
> 2. by giving is coordinates explicitly (as is done in CF)
>
> What you are proposing is a mechanism to harmonize the two approaches
> in CF files. This is an excellent idea. Since the typical purpose of
> including this redundant (but useful) projection information is to
> make it easier for GIS applications to access CF datasets, it makes
> most sense to borrow the GIS encoding standards as closely as
> possible. I presume that this is what you have done in your example
> (??). So the question that I bounce back to you is:
>
> * Can you propose a particular GIS document (standard) for
> encoding projection information and a general mapping from that
> document into CF attributes?
>
> I believe this is the question that we want to address, rather than
> the particulars of the Lambert Conformal projection.
>
> - steve
>
> ========================================================
>
> John Caron wrote:
>
>> I didnt easily find an example of using a projection coordinate systm in
>> the CF docs, so i generated one.
>> It would be great to get it included so i can just point people to that
>> doc.
>> Let me know if i did anything wrong.
>>
>> thanks!
>>
>> dimensions:
>> y = 228;
>> x = 306;
>> time = 41;
>>
>> variables:
>> int Lambert_Conformal;
>> Lambert_Conformal:grid_mapping_name = "lambert_conformal_conic";
>> Lambert_Conformal:standard_parallel = 25.0;
>> Lambert_Conformal:longitude_of_central_meridian = 265.0;
>> Lambert_Conformal:latitude_of_projection_origin = 25.0;
>> double y(y);
>> y:units = "km";
>> y:long_name = "y coordinate of projection";
>> y:standard_name = "projection_y_coordinate";
>> double x(x);
>> x:units = "km";
>> x:long_name = "x coordinate of projection";
>> x:standard_name = "projection_x_coordinate";
>> double lat(y, x);
>> lat:units = "degrees_north";
>> lat:long_name = "latitude coordinate";
>> lat:standard_name = "latitude";
>> double lon(y, x);
>> lon:units = "degrees_east";
>> lon:long_name = "longitude coordinate";
>> lon:standard_name = "longitude";
>> int time(time);
>> time:long_name = "forecast time";
>> time:units = "hours since 2004-06-23T22:00:00Z";
>> float Temperature(time, y, x);
>> Temperature:units = "K";
>> Temperature:long_name = "Temperature _at_ surface";
>> Temperature:missing_value = 9999.0;
>> Temperature:coordinates = "lat lon";
>> Temperature:grid_mapping = "Lambert_Conformal";
>>
>> _______________________________________________
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>> CF-metadata at cgd.ucar.edu
>> http://www.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>
> --
>
> Steve Hankin, NOAA/PMEL -- Steven.C.Hankin at noaa.gov
> 7600 Sand Point Way NE, Seattle, WA 98115-0070
> ph. (206) 526-6080, FAX (206) 526-6744
>
>
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>
Received on Thu Oct 14 2004 - 12:40:55 BST
