The UML for the MD_Keywords object includes keyword, type, and thesaurusName fields. A minimum of one keyword field is mandatory. The type field groups keywords into like categories using KeywordTypeCodes, and the thesaurusName field provides a citation to the authoritative source of keywords. Using common types to classify keywords is critical for consistent discovery, particularly using faceted searches. The MD_KeywordClass object is new with ISO 19115-1. This object was added in order to allow unambiguous specification of keywords from ontologies. The MD_KeywordClass enables users to define categories of keywords that extend or are orthogonal to the standardized KeywordTypeCodes, and are associated with an ontology that allows additional semantic query processing. In contrast the thesaurus citation specifies a collection of instances from some ontology, but is not an ontology. It might be a list of places that include rivers, mountains, counties and cities. There might be a Laconte county, the city of Laconte, the Laconte River, and Mt. Laconte; when searching it is useful for the user to be able to restrict the search to only rivers. |