Identity (with guidance)

Resource URIs, subject identifiers, subject locators, source locators. Blank nodes.

Both topics and resources may use URI references (or URIrefs) as identifiers (the term URIref is used here in accordance with W3C usage to mean a URI with an optional fragment identifier). However, in Topic Maps there are two ways in which a URIref can be used to identify a subject:

  • directly, as the actual locator of the subject, in which case it is called a "subject locator"; or
  • indirectly, as the locator of an information resource that provides some human-interpretable indication of the subject, in which case it is called a "subject identifier". It is always clear, in both the model and the interchange syntax, whether the URIref is a subject locator or a subject identifier.

RDF does not make this distinction explicitly. The question therefore arises, when going from RDF to Topic Maps, whether to map the URIref of a resource to a subject locator or to a subject identifier; and, conversely, when going from Topic Maps to RDF, whether to map subject locators or subject identifiers (or neither, or both) to the URIrefs of resources.

Any solution which favours one type of identifier (say, subject identifiers) will lead to unnatural results with the identifiers of the other type. These guidelines therefore suggest a solution that retains some of the ambiguity of the RDF approach while at the same time preserving enough information to be able to perform roundtripping. The solution hinges on the assumption that topics with subject locators are explicitly or implicitly instances of the class InformationResource.

The classes and properties involved in the guidance for Identity are the following:

  • rdftm:InformationResource
  • rdftm:subjectIdentifier
  • rdfs:subPropertyOf
  • rdfs:subClassOf
  • owl:sameAs

The rules are as follows:

RDFTM

Resources become Topics. The guidance indicates if the resource's URI must be translated as a subject locator or a subject identifier. That is, it indicates whether the resource is an information resource or not. In order to specify the nature of the resource the class rdftm:InformationResource is used. It is also possible to define explicitly properties as being rdfs:subPropertyOf the rdftm:subjectIdentifier in order to translate their values as subject identifiers.

In particular:

  • If the resource is an instance of rdftm:InformationResource, the resource's URI becomes a subject locator. Any owl:sameAs properties become additional subject locators. The values of properties that are rdfs:subPropertyOf the property rdftm:subjectIdentifier become subject identifiers.
  • If the resource is not an instance of rdftm:InformationResource, the URI becomes a subject identifier. Any owl:sameAs properties become additional subject identifiers. The values of properties that are rdfs:subPropertyOf the property rdftm:subjectIdentifier become subject identifiers.
  • Blank nodes become topics with no identifier.

TMRDF

Topics becomes Resources. It is always clear if the URIs associated to a topic identifies the real subject (i.e. the topic is the subject itself) or information resources describing the subject.

Topics map to resources, and there are two possible cases: (i) the topic represents an information resources and (ii) the topic represents another kind of resource. In (i) the topic will have at least one subject locator, and optionally one or more subject identifiers. In (ii) the topic will have at least one subject identifier and no subject locator.

In particular,

  • If the topic has one or more subject locators, one subject locator (chosen at random) becomes the URI of the resource, and, if the topic is not an instance of rdftm:InformationResource, then the resource is typed as rdftm:InformationResource. Additional subject locators become owl:sameAs properties. Any subject identifier becomes a rdftm:subjectIdentifier property.
  • If the topic has one or more subject identifiers and no subject locators, one subject identifier (chosen at random) becomes the URI of the resource. Additional subject identifiers become owl:sameAs properties.
  • Source locators [item identifiers] are discarded.
  • Topics with neither subject identifier nor subject locator become blank nodes.

Revision: r1.8 - 19 Oct 2005 - 15:05 - ValentinaPresutti
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