Re: [idvusers] Plotting netCDF data in Mercator projection

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:


  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:

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 Murray                               UCAR Unidata Program
dmurray@xxxxxxxxxxxxxxxx                        P.O. Box 3000
(303) 497-8628                              Boulder, CO 80307