background image
other domain ontologies,
6
and remote applications
or intelligent software agents can refer to it when
they interact to provide a certain information service.
The degree of formalisation of concepts and their
relations varies considerably between different
domains of knowledge. At the lower end one finds
lexicons and simple taxonomies (i.e. an ordered
classification system where terms are related
hierarchically). At the middle level one might place
thesauri, i.e. controlled vocabularies that are
structured to show relationships between terms and
concepts, and, for example, allow for retrieving them
from a database. At the high end of formalisation of
knowledge there are axiomatised logic theories. Such
theories include rules to ensure the well-formedness
and logical validity of statements expressed in the
language of the scientific discipline.
In the cultural heritage sector, a powerful IT-
supported example of a hierarchical classification
system is Iconclass.This supports the documentation
of images, in particular art historical images, by
providing a systematic collection of 28,000 ready-
made definitions of objects, persons, events,
situations and abstract ideas that can be the subject of
an image.The definitions consist of an alphanumeric
classification code and its textual correlate.
7
For example, for the image we have described in
XML in graphic 1, the Iconclass definition is:
71A3421 Eve emerges from Adam's body.The
Medieval Illustrated Manuscripts Website of the
Koninklijke Bibliotheek,The Hague, has an Iconclass
Browser in place
8
that provides the hierarchical path
for this concept in the classification system:
An outstanding example of a controlled
vocabulary is the Art & Architecture Thesaurus
(AAT), one of the Getty Research Institute's
Vocabulary Databases. It is a structured vocabulary
of more than 125,000 terms and other information
about concepts that are used for describing fine art,
architecture, decorative arts, archival materials, and
material culture.
9
The W3C's Resource Description Framework
(RDF) Schema Specification 1.0, in a section on
its scope, mentions concept navigation, and states:
`Thesauri and library classification schemes are
well known examples of hierarchical systems for
representing subject taxonomies in terms of the
relationships between named concepts.The RDF
Schema specification provides sufficient resources
for creating RDF models that represent the logical
structure of thesauri (and other library classification
systems).'
10
Yet, for realising a full-blown cultural heritage
ontology for the Semantic Web, there are currently
limitations on both sides. On the one hand, hier-
archical classification systems and structured
vocabularies do not lend themselves easily to
rich inter-linking of conceptual `trees'.
A major step further in this direction is the
"CIDOC object-oriented Conceptual Reference
Model" (CRM).
11
This provides an ontology of 81
classes and 130 properties, which describes in a
formal language concepts and relations relevant to
the documentation of cultural heritage.
On the other hand, RDF Schema has limitations
when it comes to expressing complex ontological
relationships. New languages based on description
logics are being developed.These include DAML+
OIL and the upcoming Web Ontology Language
(OWL), which are capable of fully describing
ontologies.
Also worth highlighting is that tools for ontology
building are proliferating at the present time.These
ontology editors need to be carefully assessed as their
capabilities differ considerably.
12
DigiCULT 31
7 Bible
71 Old Testament
71A Genesis from the creation to the expulsion from paradise,
and later years of Adam and Eve
71A3 creation of man; the Garden of Eden (Genesis 1:26-2)
71A34 creation of Eve
71A342 Eve is fashioned from Adam's rib
71A3421 Eve emerges from Adam's body
6
Upper-level ontologies
describe the basic concepts and
relationships invoked when
information about any domain is
expressed in natural language.
7
For in-depth information, see
the official Iconclass Website:
http://www.iconclass.nl
8
Medieval Illustrated
Manuscripts Website,
http://www.kb.nl/kb/
manuscripts/ browser/.
The subject access system for
the Website was conceived by
Mnemosyne Partners, building on
the Iconclass classification system
nd technologies. See the valuable
information they provide at:
http://www.mnemosyne.org/
business/mss/tempex.html
9
A detailed description
of the AAT is provided at:
http://www.getty.edu/research/
tools/vocabulary/aat/about.html
10
http://www.w3.org/TR/
000/CR-rdf-schema-20000327/
11
See the information box
at the end of the Forum
discussion and the sources
mentioned in the Semantic
Web Terms and Reading List.
12
Michael Denny: Ontology
Building: A Survey of
Editing Tools (06-11-2002)
http://www.xml.com/pub/a/
2002/11/06/ontologies.html