When a variable has an associated coordinate which is single-valued,
that coordinate may be represented as a scalar variable. Since there is no
associated dimension these scalar coordinate variables should be attached
to a data variable via the coordinates
attribute.
Under COARDS the method of providing a single valued coordinate was to add a dimension of size one to the variable, and supply the corresponding coordinate variable. The new scalar coordinate variable is a convenience feature which avoids adding size one dimensions to variables. Scalar coordinate variables have the same information content and can be used in the same contexts as a size one coordinate variable. Note however that use of this feature with a latitude, longitude, vertical, or time coordinate will inhibit COARDS conforming applications from recognizing them.
Once a name is used for a scalar coordinate variable it can not be used for a 1D coordinate variable. For this reason we strongly recommend against using a name for a scalar coordinate variable that matches the name of any dimension in the file.
Example 5.12. Multiple forecasts from a single analysis
dimensions:
lat = 180 ;
lon = 360 ;
time = UNLIMITED ;
variables:
double atime
atime:standard_name = "forecast_reference_time" ;
atime:units = "hours since 1999-01-01 00:00" ;
double time(time);
time:standard_name = "time" ;
time:units = "hours since 1999-01-01 00:00" ;
double lon(lon) ;
lon:long_name = "station longitude";
lon:units = "degrees_east";
double lat(lat) ;
lat:long_name = "station latitude" ;
lat:units = "degrees_north" ;
double p500
p500:long_name = "pressure" ;
p500:units = "hPa" ;
p500:positive = "down" ;
float height(time,lat,lon);
height:long_name = "geopotential height" ;
height:standard_name = "geopotential_height" ;
height:units = "m" ;
height:coordinates = "atime p500" ;
data:
time = 6., 12., 18., 24. ;
atime = 0. ;
p500 = 500. ;
In this example both the analysis time and the single pressure level are represented using scalar coordinate variables. The analysis time is identified by the standard name "forecast_reference_time" while the valid time of the forecast is identified by the standard name "time".