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: Thu, 11 Dec 2008 16:48:06 -0700
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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