Standard Names URI Tokenization Use of RDF

16 Copyright © 2012 Open Geospatial Consortium.

8.1.1 Standard Names

The standard names used in the CF conventions are a special case of a NetCDF standard term. They are designed to provide an unambiguous reference to the property that a variable measures. The CF standard name table provides a small amount of semantic information: canonical units, descriptive text, aliases and mappings onto a small number of similar codes. However, standard names are generally used in software simply to ensure that like is compared with like. Where possible, the variables in the document should have a standard name that reflects the property of the observation. If the standard name is taken from a specific vocabulary, the vocabulary should be recorded in the observation metadata and specified in the NetCDF global attributes see Section 10.2. Where standard names can be mapped onto URIs, a dictionary should be included see Sections 8.1.4 and 10.4.1 Standard names can be derived from either a repository of standard names, such as the CF name table, by tokenizing a property URI see Section 8.1.2 or by using data available from RDF see Section 8.1.3.

8.1.2 URI Tokenization

Enumerations for URIs or terms from a standardized vocabulary can be constructed by tokenizing either the xlink:title value from the link to the URI, the fragment of the URI, the last path segment of the URI or the label or id of an AbstractSWEType. Tokenizing replaces whitespace and other non-alphanumeric characters with underscores and converts camelcase to lower case separated with underscores. For example: Table 2 URI Tokenization Examples Term Tokenized http:www.opengis.netdeftimeseriesTypeWaterML2.0MinSucc min_succ Water Temperature water_temperature http:purl.oclc.orgNETssnxcfcf-propertyrainfall_amount rainfall_amount Tokenized URIs can be associated with the actual URI by use of a dictionary – see Section 8.1.4.

8.1.3 Use of RDF

In many cases, URIs can be resolved to semantic descriptions of the URI, usually in the form of RDF. In such cases, values derived from the RDF can be used to provide standard name tokens and other property-specific information. Copyright © 2012 Open Geospatial Consortium. 17 For example, the following RDF description of rainfall is extended from the CF properties ontology 3 , developed by the Semantic Sensor Networks working group. The rdfs:label property can be used to derive a suitable long name and the cf:standardName can be used for the CF standard name. dim:SurfaceDensity rdf:about=http:purl.oclc.orgNETssnxcfcf- propertyrainfall_amount rdfs:labelRainfall Amountrdfs:label rdfs:commentAmount means mass per unit area.rdfs:comment dc:sourceClimate and Forecast CFdc:source cf:standardNamerainfall_amountcf:standardName qu:generalQuantityKind dim:SurfaceDensity rdf:about=http:purl.oclc.orgNETssnxququantitysurfaceDensity qu:generalQuantityKind ssn:isPropertyOf cf-feature:Precipitation rdf:about=http:purl.oclc.orgNETssnxcfcf- featurerainfall ssn:isPropertyOf qu:propertyType qu:PropertyKind rdf:about=http:purl.oclc.orgNETssnxququantityscalar qu:propertyType dim:SurfaceDensity

8.1.4 Dictionaries and Vocabularies