Re: [idvusers] Plotting netCDF data in Mercator projection

  • To: "Kemp, Eric M. (TASCSD)" <ERIC.KEMP@xxxxxxx>
  • Subject: Re: [idvusers] Plotting netCDF data in Mercator projection
  • From: John Caron <caron@xxxxxxxxxxxxxxxx>
  • Date: Fri, 12 Dec 2008 09:15:45 -0700
Mercator and Orthographic Projections have been added to Appendix F of CF:

  http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/cf-conventions.html


John Caron wrote:
> It has been added to CF, but because of an oversight, hasnt been
> published yet. Follow the specs on this page:
> 
>   http://cf-pcmdi.llnl.gov/trac/wiki/GridMapNames
> 
> Don Murray wrote:
>> Hi Eric-
>>
>> Kemp, Eric M. (TASCSD) wrote:
>>> I am writing a post-processor for the WRF model.  My intent is to
>>> generate new netCDF files with optional derived variables on pressure
>>> surfaces.  I have been writing my code to use the CF-1.0 convention
>>> as recommended in the IDV documentation, but this week I discovered
>>> that the CF convention does not include the Mercator map projection.
>>> (Note that the Transverse Mercator projection included in CF
>>> is not the same projection.)
>> We have proposed Mercator as a standard for CF, but it has not been
>> accepted yet.  However, you can define it as:
>>
>> mercator
>>
>>    char Mercator_Projection;
>>       :grid_mapping_name = "mercator";
>>       :longitude_of_projection_origin = 110.0;
>>       :latitude_of_projection_origin = -25.0;
>>       :standard_parallel = 0.02;
>>       :_CoordinateTransformType = "Projection";
>>       :_CoordinateAxisTypes = "GeoX GeoY";
>>
>> See the reference at:
>>
>> https://www.unidata.ucar.edu/software/netcdf-java/reference/StandardCoordinateTransforms.html
>>
>>> I've made two attempts to work around this while remaining in the
>>> CF convention:  (1) label the data as Lambert Conformal with the
>>> standard latitudes equidistant from the equator, which is supposed
>>> to be equivalent to the Mercator projection; and (2) not write out
>>> any grid_mapping metadata, but include the latitudes/longitudes at
>>> each grid point.  In the first case, IDV freezes up and has to be
>>> killed.  In the second case, IDV claims there are no gridded data
>>> in the file, even though the data meets the CF convention (the
>>> grid_mapping metadata is listed as optional in the CF documentation).
>> Can you provide sample files so we can look into this?
>>
>>> So at this point, I'm looking for help on how to encode this for
>>> IDV.  One extreme possibility is to abandon the CF convention and try
>>> using the _Coordinate convention described in the NetCDF-Java
>>> documentation, but I'd appreciate feedback before taking such a
>>> drastic step.
>> Try using the definition above.
>>
>>> (Note that my post-processor should also support other map projections
>>> used by WRF, so I'd like the output convention to be as flexible
>>> as possible.)
>> Don
>> *************************************************************
>> Don Murray                               UCAR Unidata Program
>> dmurray@xxxxxxxxxxxxxxxx                        P.O. Box 3000
>> (303) 497-8628                              Boulder, CO 80307
>> http://www.unidata.ucar.edu/staff/donm
>> *************************************************************
>>
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