code: def _get_cf_grid_mapping_var(self):
          
          gmaps = ['fixedgrid_projection', 'goes_imager_projection',
                   'lambert_projection', 'polar_projection',
                   'mercator_projection']
          if 'grid_mapping' in self.filename_info:
              gmaps = [self.filename_info.get('grid_mapping')] + gmaps
          for grid_mapping in gmaps:
              if grid_mapping in self.nc:
                  return self.nc[grid_mapping]
          raise KeyError("Can't find grid mapping variable in SCMI file")
docs: Figure out which grid mapping should be used