5.6. Grid Mappings and Projections

When the coordinate variables for a horizontal grid are not longitude and latitude, it is required that the true latitude and longitude coordinates be supplied via the coordinates attribute. If in addition it is desired to describe the mapping between the given coordinate variables and the true latitude and longitude coordinates, the attribute grid_mapping may be used to supply this description. This attribute is attached to data variables so that variables with different mappings may be present in a single file. The attribute takes a string value which is the name of another variable in the file that provides the description of the mapping via a collection of attached attributes. This variable is called a grid mapping variable and is of arbitrary type since it contains no data. Its purpose is to act as a container for the attributes that define the mapping. The one attribute that all grid mapping variables must have is grid_mapping_name which takes a string value that contains the mapping's name. The other attributes that define a specific mapping depend on the value of grid_mapping_name. The valid values of grid_mapping_name along with the attributes that provide specific map parameter values are described in Appendix F, Grid Mappings.

In order to make use of a grid mapping to directly calculate latitude and longitude values it is necessary to associate the coordinate variables with the independent variables of the mapping. This is done by assigning a standard_name to the coordinate variable. The appropriate values of the standard_name depend on the grid mapping and are given in Appendix F, Grid Mappings.

Example 5.6. Rotated pole grid

  rlon = 128 ;
  rlat = 64 ;
  lev = 18 ;
  float T(lev,rlat,rlon) ;
    T:long_name = "temperature" ;
    T:units = "K" ;
    T:coordinates = "lon lat" ;
  char rotated_pole
    rotated_pole:grid_mapping_name = "rotated_latitude_longitude" ;
    rotated_pole:grid_north_pole_latitude = 32.5 ;
    rotated_pole:grid_north_pole_longitude = 170. ;
  float rlon(rlon) ;
    rlon:long_name = "longitude in rotated pole grid" ;
    rlon:units = "degrees" ;
    rlon:standard_name = "grid_longitude";
  float rlat(rlat) ;
    rlat:long_name = "latitude in rotated pole grid" ;
    rlat:units = "degrees" ;
    rlon:standard_name = "grid_latitude";
  float lev(lev) ;
    lev:long_name = "pressure level" ;
    lev:units = "hPa" ;
  float lon(rlat,rlon) ;
    lon:long_name = "longitude" ;
    lon:units = "degrees_east" ;
  float lat(rlat,rlon) ;
    lat:long_name = "latitude" ;
    lat:units = "degrees_north" ;

A CF compliant application can determine that rlon and rlat are longitude and latitude values in the rotated grid by recognizing the standard names grid_longitude and grid_latitude. Note that the units of the rotated longitude and latitude axes are given as degrees. This should prevent a COARDS compliant application from mistaking the variables rlon and rlat to be actual longitude and latitude coordinates. The entries for these names in the standard name table indicate the appropriate sign conventions for the units of degrees.

Example 5.7. Lambert conformal projection

  y = 228;
  x = 306;
  time = 41;

  int Lambert_Conformal;
    Lambert_Conformal:grid_mapping_name = "lambert_conformal_conic";
    Lambert_Conformal:standard_parallel = 25.0;
    Lambert_Conformal:longitude_of_central_meridian = 265.0;
    Lambert_Conformal:latitude_of_projection_origin = 25.0;
  double y(y);
    y:units = "km";
    y:long_name = "y coordinate of projection";
    y:standard_name = "projection_y_coordinate";
  double x(x);
    x:units = "km";
    x:long_name = "x coordinate of projection";
    x:standard_name = "projection_x_coordinate";
  double lat(y, x);
    lat:units = "degrees_north";
    lat:long_name = "latitude coordinate";
    lat:standard_name = "latitude";
  double lon(y, x);
    lon:units = "degrees_east";
    lon:long_name = "longitude coordinate";
    lon:standard_name = "longitude";
  int time(time);
    time:long_name = "forecast time";
    time:units = "hours since 2004-06-23T22:00:00Z";
  float Temperature(time, y, x);
    Temperature:units = "K";
    Temperature:long_name = "Temperature @ surface";
    Temperature:missing_value = 9999.0;
    Temperature:coordinates = "lat lon";
    Temperature:grid_mapping = "Lambert_Conformal";

An application can determine that x and y are the projection coordinates by recognizing the standard names projection_x_coordinate and projection_y_coordinate. The grid mapping variable Lambert_Conformal contains the mapping parameters as attributes, and is associated with the Temperature variable via its grid_mapping attribute.