Equidistant Cylindrical projection

[fedecutraro]

Question

Hello,
At the National Meteorological Service of Argentina, we save radar variables in NetCDF files, which we are now trying to make CF-compliant. My question relates to the grid mapping attribute: the data is in Equidistant Cylindrical projection, but this isn't listed in Appendix F.
What is the correct way to indicate that the data is in this projection?

Kind regards
Federico

[SilverLucFox]

Hi Federico,

You're right that "Equidistant Cylindrical" isn't listed by name in Appendix F. The good news is that the CF conventions do cover your projection you just need to use the name equirectangular instead. Think of it this way: an Equidistant Cylindrical projection is basically the equirectangular projection when the standard parallel is at the equator. So that's the keyword you'll use.

Now, a quick heads-up about where to put this information. A lot of people try to throw a grid_mapping attribute into the global attributes section, but that's not actually CF-compliant. Instead, you need to:

1. Create a separate variable (you can call it something simple like crs or projection) this variable doesn't hold any data, it just acts as a container for your projection info.
2. Set grid_mapping_name = "equirectangular" on that variable.
3. Add the relevant parameters as attributes of that variable. Usually you'll need:
   • longitude_of_central_meridian (your reference meridian, like -60.0 if you're working with Argentina)
   • standard_parallel (for your case, set this to 0.0)
   • false_easting and false_northing if you use them
4. Point to it from your data variable by adding a grid_mapping attribute that references your crs variable.

Here's a quick example to make it clearer:

dimensions:
  x = 100 ;
  y = 100 ;

variables:
  float radar_data(y, x) ;
    radar_data:grid_mapping = "crs" ;   # this links your data to the projection

  double x(x) ;
    x:standard_name = "projection_x_coordinate" ;
    x:units = "m" ;

  double y(y) ;
    y:standard_name = "projection_y_coordinate" ;
    y:units = "m" ;

  char crs ;
    crs:grid_mapping_name = "equirectangular" ;
    crs:longitude_of_central_meridian = -60.0 ;
    crs:standard_parallel = 0.0 ;

Once you set it up like this, your files should pass the CF Checker just fine, and any software that reads CF-compliant NetCDF files will know exactly what to do with your data.

Hope that helps! Best of luck with the work at the National Meteorological Service sounds like a really cool project. Feel free to reach out if you run into any other hiccups along the way.

Kind regards