Table of Contents
The previous section contained several examples in which measurements from scattered sites were grouped using a single dimension. Coordinates of the site locations can be provided using auxiliary coordinate variables, but it is often desirable to identify measurement sites by name, or some other unique string. Other purposes for string identifiers are also described in Section 6.1.1, “Geographic Regions”, and Section 7.3.3, “ Statistics applying to portions of cells ”.
The list of string identifiers plays an analogous role to a coordinate variable, hence we have chosen to use the coordinates attribute to provide the name of the variable that contains the string array. An application processing the variables listed in the coordinates attribute can recognize a labeled axis by checking whether or not a given variable contains character data. If a character variable has only one dimension (the length of the string), it is regarded as a string-valued scalar coordinate variable, analogous to a numeric scalar coordinate variable (see Chapter 5, Coordinate Systems.)
Character strings can be used to provide a name or label for each element of an axis. This is particularly useful for discrete axes (section 4.5). For instance, if a data variable contains time series of observational data from a number of observing stations, it may be convenient to provide the names of the stations as labels for the elements of the station dimension (Section H.2, “Time Series Data”). Example H.1, “Point data.” illustrates another application for labels.
Character strings labelling the elements of an axis are regarded as
string-valued auxiliary coordinate variables. The coordinates attribute
of the data variable names the variable that contains the string
array. An application processing the variables listed in the coordinates
attribute can recognize a string-valued auxiliary coordinate variable
because it contains an array of character data. The inner dimension (last
dimension in CDL terms) is the maximum length of each string, and the
other dimensions are axis dimensions. If a character variable has only
one dimension (the maximum length of the string), it is regarded as a
string-valued scalar coordinate variable, analogous to a numeric scalar
coordinate variable (see Section 5.7, “Scalar Coordinate Variables”)
Example 6.1. Several parcel trajectories
Consider a set of ocean floats that follow parcel trajectories and simultaneously measure temperature at fixed times. We wish to identify the floats by name. The temperature data is a function of parcel (i.e., float) and time. The location of each sample is also a function of parcel and time, so the position information is stored in a multidimensional coordinate variable.
dimensions:
parcel = 15 ; // number of trajectories
times = 20 ;
max_len_parcel_name = 64 ; // max length of trajectory name
variables:
float temperature(parcel,times) ;
temperature:coordinates = "parcel_name lat lon" ;
float times(times) ;
char parcel_name(parcel,max_len_parcel_name) ;
float lon(parcel,times) ;
float lat(parcel,times) ;
When data is representative of geographic regions which can be identified by names but which have complex boundaries that cannot practically be specified using longitude and latitude boundary coordinates, a labeled axis should be used to identify the regions. We recommend that the names be chosen from the list of standardized region names whenever possible. To indicate that the label values are standardized the variable that contains the labels must be given the standard_name attribute with the value region.
Example 6.2. Northward heat transport in Atlantic Ocean
Suppose we have data representing northward heat transport across a set of zonal slices in the Atlantic Ocean. Note that the standard names to describe this quantity do not include location information. That is provided by the latitude coordinate and the labeled axis:
dimensions:
times = 20 ;
lat = 5
lbl = 1 ;
strlen = 64 ;
variables:
float n_heat_transport(time,lat,lbl);
n_heat_transport:units="W";
n_heat_transport:coordinates="geo_region";
n_heat_transport:standard_name="northward_ocean_heat_transport";
double time(time) ;
time:long_name = "time" ;
time:units = "days since 1990-1-1 0:0:0" ;
float lat(lat) ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
char geo_region(lbl,strlen) ;
geo_region:standard_name="region"
data:
geo_region = "atlantic_ocean" ;
lat = 10., 20., 30., 40., 50. ;