Chapter 5.  Coordinate Systems

A variable's spatiotemporal dimensions are used to locate data values in time and space. This is accomplished by associating these dimensions with the relevant set of latitude, longitude, vertical, and time coordinates. This section presents two methods for making that association: the use of coordinate variables, and the use of auxiliary coordinate variables.

All of a variable's dimensions that are latitude, longitude, vertical, or time dimensions (see Section 1.2, “Terminology”) must have corresponding coordinate variables, i.e., one-dimensional variables with the same name as the dimension (see examples in Chapter 4, Coordinate Types ). This is the only method of associating dimensions with coordinates that is supported by [COARDS].

All of a variable's spatiotemporal dimensions that are not latitude, longitude, vertical, or time dimensions are required to be associated with the relevant latitude, longitude, vertical, or time coordinates via the new coordinates attribute of the variable. The value of the coordinates attribute is a blank separated list of the names of auxiliary coordinate variables. There is no restriction on the order in which the auxiliary coordinate variables appear in the coordinates attribute string. The dimensions of an auxiliary coordinate variable must be a subset of the dimensions of the variable with which the coordinate is associated (an exception is label coordinates (Section 6.1, “Labels”) which contain a dimension for maximum string length). We recommend that the name of a multidimensional coordinate variable should not match the name of any of its dimensions because that precludes supplying a coordinate variable for the dimension. This practice also avoids potential bugs in applications that determine coordinate variables by only checking for a name match between a dimension and a variable and not checking that the variable is one dimensional.

The use of coordinate variables is required whenever they are applicable. That is, auxiliary coordinate variables may not be used as the only way to identify latitude and longitude coordinates that could be identified using coordinate variables. This is both to enhance conformance to COARDS and to facilitate the use of generic applications that recognize the NUG convention for coordinate variables. An application that is trying to find the latitude coordinate of a variable should always look first to see if any of the variable's dimensions correspond to a latitude coordinate variable. If the latitude coordinate is not found this way, then the auxiliary coordinate variables listed by the coordinates attribute should be checked. Note that it is permissible, but optional, to list coordinate variables as well as auxiliary coordinate variables in the coordinates attribute. The axis attribute is not allowed for auxiliary coordinate variables. Auxiliary coordinate variables which lie on the horizontal surface can be identified as such by their dimensions being horizontal. Horizontal dimensions are those whose coordinate variables have an axis attribute of X or Y, or a units attribute indicating latitude and longitude (see Chapter 4, Coordinate Types ).

If the coordinate variables for a horizontal grid are not longitude and latitude, it is recommended that they be supplied in addition to the required coordinates. For example, the Cartesian coordinates of a map projection should be supplied as coordinate variables in addition to the required two-dimensional latitude and longitude variables that are identified via the coordinates attribute. The use of the axis attribute with values X and Y is recommended for the coordinate variables(see Chapter 4, Coordinate Types ).

It is sometimes not practical to specify the latitude-longitude location of data which is representative of geographic regions with complex boundaries. For this purpose, provision is made in Section 6.1.1, “Geographic Regions” for indicating the region by a standardized name.

dimensions:
  lat = 18 ;
  lon = 36 ;
  pres = 15 ;
  time = 4 ;
variables:
  float xwind(time,pres,lat,lon) ;
    xwind:long_name = "zonal wind" ;
    xwind:units = "m/s" ;
  float lon(lon) ;
    lon:long_name = "longitude" ;
    lon:units = "degrees_east" ;
  float lat(lat) ;
    lat:long_name = "latitude" ;
    lat:units = "degrees_north" ;
  float pres(pres) ;
    pres:long_name = "pressure" ;
    pres:units = "hPa" ;
  double time(time) ;
    time:long_name = "time" ;
    time:units = "days since 1990-1-1 0:0:0" ;