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[thredds] Curious effect from attributes semi_major/minor_axis on southern hemisphere images created by ncWMS

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  • Subject: [thredds] Curious effect from attributes semi_major/minor_axis on southern hemisphere images created by ncWMS
  • From: Egil Støren <egil.storen@xxxxxx>
  • Date: Fri, 25 Jan 2013 16:28:32 +0100
Dear all,
We have a netCDF file containing ice concentrations around antarctica that is available at http://thredds.met.no/thredds/catalog/egiltest/ES/data/catalog.html?dataset=egiltest/ES/data/iceconc.nc. Using the Godiva2 viewer, it is obvious that the data is displayed inverted along a vertical line centred on the map. This is most easily seen using the south polar stereographic projection. 

The netCDF file has a variable containing projection info:

        int Polar_Stereographic_Grid ;
                Polar_Stereographic_Grid:grid_mapping_name = 
"polar_stereographic" ;
                Polar_Stereographic_Grid:straight_vertical_longitude_from_pole 
= 0.f ;
                Polar_Stereographic_Grid:latitude_of_projection_origin = -90.f ;
                Polar_Stereographic_Grid:standard_parallel = -70.f ;
                Polar_Stereographic_Grid:false_easting = 0.f ;
                Polar_Stereographic_Grid:false_northing = 0.f ;
                Polar_Stereographic_Grid:semi_major_axis = 6378273.f ;
                Polar_Stereographic_Grid:semi_minor_axis = 6356890.f ;
Polar_Stereographic_Grid:proj4_string = "+proj=stere +a=6378273 +b=6356889.44891 +lat_0=-90 +lat_ts=-70 +lon_0=0" ;
When I removed the following attributes from this variable, the data suddenly displayed correctly in Godiva2: 

                Polar_Stereographic_Grid:semi_major_axis = 6378273.f ;
                Polar_Stereographic_Grid:semi_minor_axis = 6356890.f ;
The modified version can be seen at http://thredds.met.no/thredds/catalog/egiltest/ES/data/catalog.html?dataset=egiltest/ES/data/iceconc_1.nc.
Since these attributes only gives a more exact description of the shape of the earth (compared to not having them), I think there must be a bug somewhere. Have anybody experienced this behaviour? Is this a known bug? 

We are using thredds version 4.2.9.

Best regards,

  Egil Støren
  Norwegian Meteorological Institute
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