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 ([time-series-data]). There are several other uses for labels in CF. For instance, Northward heat transport in Atlantic Ocean shows the use of labels to indicate geographic regions.
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 has a type of char or string.
If the variable has a type of char, the inner dimension (last dimension in CDL terms) is the maximum length of each string, and the other dimensions are axis dimensions.
If an auxiliary coordinate variable has a type of string and has no dimensions, or has a type of char and has only one dimension (the maximum length of the string), it is a string-valued scalar coordinate variable (see [scalar-coordinate-variables]).
As such, it has the same information content and can be used in the same contexts as a string-valued auxiliary coordinate variable of a size one dimension.
This is a convenience feature.
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.
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 ;
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" ;
string geo_region(lbl) ;
geo_region:standard_name="region"
data:
geo_region = "atlantic_ocean" ;
lat = 10., 20., 30., 40., 50. ;
A taxon is a named level within a biological classification, such as a class, genus and species. Quantities dependent on taxa have generic standard names containing the phrase "organisms_in_taxon", and the taxa are identified by auxiliary coordinate variables.
The taxon auxiliary coordinate variables are string-valued.
The plain-language name of the taxon must be contained in a variable with standard_name of biological_taxon_name.
A Life Science Identifier (LSID) may be contained in a variable with standard_name of biological_taxon_lsid.
This is a URN with the syntax "urn:lsid:<Authority>:<Namespace>:<ObjectID>[:<Version>]".
This includes the reference classification in the <Authority> element and these are restricted by the LSID governance.
It is strongly recommended in CF that the authority chosen is World Register of Marine Species (WoRMS) for oceanographic data and Integrated Taxonomic Information System (ITIS) for freshwater and terrestrial data.
WoRMS LSIDs are built from the WoRMS AphiaID taxon identifier such as "urn:lsid:marinespecies.org:taxname:104464" for AphiaID 104464.
This may be converted to a URL by adding prefixes such as https://www.lsid.info/.
ITIS LSIDs are built from the ITIS Taxonomic Serial Number (TSN), such as "urn:lsid:itis.gov:itis_tsn:180543".
The biological_taxon_name auxiliary coordinate variable included for human readability is mandatory.
The biological_taxon_lsid auxliary coordinate variable included for software agent readability is optional, but strongly recommended.
If both are present then each biological_taxon_name coordinate must exactly match the name resolved from the biological_taxon_lsid coordinate.
If LSIDs are available for some taxa in a dataset then the biological_taxon_lsid auxiliary coordinate variable should be included and missing data given for those taxa that do not have an identifier.
A skeleton example for taxonomic abundance time series.
dimension:
time = 100 ;
string80 = 80 ;
taxon = 2 ;
variables:
float time(time);
time:standard_name = "time" ;
time:units = "days since 2019-01-01" ;
float abundance(time,taxon) ;
abundance:standard_name = "number_concentration_of_biological_taxon_in_sea_water" ;
abundance:coordinates = "taxon_lsid taxon_name" ;
char taxon_name(taxon,string80) ;
taxon_name:standard_name = "biological_taxon_name" ;
char taxon_lsid(taxon,string80) ;
taxon_lsid:standard_name = "biological_taxon_lsid" ;
data:
time = // 100 values ;
abundance = // 200 values ;
taxon_name = "Calanus finmarchicus", "Calanus helgolandicus" ;
taxon_lsid = "urn:lsid:marinespecies.org:taxname:104464", "urn:lsid:marinespecies.org:taxname:104466" ;
In some situations a dimension may have alternative sets of coordinates values. Since there can only be one coordinate variable for the dimension (the variable with the same name as the dimension), any alternative sets of values have to be stored in auxiliary coordinate variables. For such alternative coordinate variables, there are no mandatory attributes, but they may have any of the attributes allowed for coordinate variables.
Levels on a vertical axis may be described by both the physical coordinate and the ordinal model level number.
float xwind(sigma,lat); xwind:coordinates="model_level"; float sigma(sigma); // physical height coordinate sigma:long_name="sigma"; sigma:positive="down"; int model_level(sigma); // model level number at each height model_level:long_name="model level number"; model_level:positive="up";