Continuing the theme of reflecting on SKOS, the question of organisation is next. SKOS provides an RDF vocabulary for describing Knowledge Organisation Systems and there’s an assumption that SKOS is RDF from the ground up. The use of RDF brings advantages, but there are also limitations, in particular when we consider issues of containment. This is something that I wrestled with in the past when building the OWL-API libraries to support OWL . In the RDF/XML serialisations of OWL, there was no explicit connection between the axioms stated in an ontology and the Ontology object itself. This can cause difficulties in the face of
owl:imports as there was also no explicit link between the location where an RDF graph that represents and ontology is retrieved from and the URI of the Ontology itself. This was partly solved by the use of physical and logical URIs, but the question of containment is still there.
There is a similar, but perhaps more easily stated issue with SKOS. Consider, for example, the following fragment from the IVOAT thesaurus :
asteroid is a narrower term of
rotating body. In the SKOS version of this thesarus, we have two concepts,
http://www.ivoa.net/rdf/Vocabularies/IVOAT#asteroid with triples asserting the appropriate labels, the fact that these concepts occur in the IVOAT scheme and the narrower relationship.
What we don’t have here, however, is the assertion that the narrower relationship occurs within the ConceptScheme. The same also holds of the labels — the labelling of the concept is not explicitly bound to the concept scheme.
Now, this isn’t really a failing of SKOS, but is rather a consequence of the use of RDF for the representation. Solutions to this could involve reification (bleuurgh) or the use of named graphs to identify the triples associated with a ConceptScheme. At the time of the SKOS Recommendation, however, no standard was available.
Does this really matter? Is it an issue? So far, a lot of SKOS publication seems to be organisations exposing their own vocabularies, with instances of
skos:Concept appearing in a single
skos:ConceptScheme with semantic relationships asserted “within” that scheme and thus under the control of the Scheme “owner”. That may not be too difficult to manage. Things will get more interesting once we have greater use of the SKOS mapping relationships , which are intended for use between Concepts in different ConceptSchemes. Such mappings are likely to present different and potentially conflicting points of view or opinions, and we will then require more details of the provenance of the assertions.
- OWL API http://owlapi.sourceforge.net/
- IVOAT Thesaurus http://www.ivoa.net/rdf/Vocabularies/IVOAT
- SKOS Mapping Properties, http://www.w3.org/TR/skos-reference/#mapping