The SKOS vocabulary is intended for the representation of simple
knowledge organisation schemes such as thesauri, term lists and
controlled vocabularies. SKOS is designed to fit into existing
Semantic Web standards, and to achieve this, an RDF Schema which
defines the SKOS vocabulary has been produced.
The development of this schema has highlighted issues relating to OWL
species which are relevant to current work on OWL 2. In particular,
the current schema is in OWL Full. It would be desirable if the SKOS
vocabulary could be represented in OWL DL (or the equivalent of OWL DL
in the OWL 2 space).
Concepts in a SKOS vocabulary are defined as instances of the
owl:Class skos:Concept. They may be related to other skos:Concepts
using object properties such as skos:broader, skos:narrower and
skos:related -- known as SKOS semantic relations. SKOS also provides a
set of lexical labelling properties, such as skos:prefLabel and
skos:altLabel. These are datatype properties, with RDF plain literals
as values. Documentation properties including skos:note,
skos:changeNote, skos:definition are also provided, allowing the
decoration of concepts with additional information. In some
circumstances, the expected value of a documentation property is a
literal value (e.g. a simple textual note). In other circumstances,
the value may be an object, for example the value of skos:changeNote
may be a structured object containing information about a change along
with provenance or date information. Documentation properties are
defined as subproperties of skos:note.
An additional use case for SKOS is as an annotation vocabulary for OWL
ontologies. For example the SKOS vocabulary itself uses
skos:changeNote and skos:definition to attach provenance and
documentation information to the SKOS vocabulary terms. This then
pushes the ontology out of OWL DL as object properties are being
applied to classes. We envisage that SKOS properties will also be used
to attach lexical information to OWL ontologies (for example,
providing alternate names or labels for classes). Again, with the
current situation, this results in OWL Full ontologies.
One solution to this would be the provision of rich annotations. The
SKOS documentation properties are essentially annotations and would
ideally be represented as such, but with the addition of subproperty
relationships between the properties (i.e, skos:note as a
superproperty). The lexical labelling properties could also be
considered as annotation properties. SKOS is intended to be
extensible, however, so again allowing the possibility of providing
subproperties of the lexical labelling properties is desirable.
Support for punning, both in terms of class/instance and data/object
property would also provide solutions to some of these issues. Rich
annotations would, however, be our preference.