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Figure 2. Topic maps-based ontology modeling
that the information is “about”; whereas RDF
defines information resources and attaches a meta-
data structure to them. The Library of Congress
Subject Headings (LCSH) in the domain of library
sciences are an example of a subject language that
is used for creating Topic Maps (Pepper, 2002b).
TheAnglo-American Cataloguing Rules (AACR)
is an example of a document language used for
creating RDF (Pepper, 2002b).
One of the important guidelines to consider
when choosing between Topic Maps and RDF
is whether the subjects are addressable or non-
addressable. The subject of every assertion in an
RDF model is a resource, identified by a URI. The
subject of every assertion in a Topic Map is a topic,
representing a subject, which may be addressable
or non-addressable (Pepper, 2002b). Address-
able subjects are identified by their URIs (as in
RDF); non-addressable subjects are identified by
the URIs of one or more subject indicators. This
important distinction is not present in RDF.
In RDF, assertions have a direction (Garshol,
2002). The direction of the statement “John ordered
the Belgian Waffles—ordered (John, the Belgian
Waffles)” (1) is different from the direction of
the statement “The Belgian Waffles ordered
John—ordered (The Belgian Waffles, John)”
(2), which in turn is different from “The Belgian
(Garshol, 2002). Garshol asserts that RDF only
relates one thing to another, while Topic Maps
can relate any number of things. In Topic Maps,
users can discern between the relationships that are
represented which makes it easier to build complex
relationships. Figure 2 and Figure 3 illustrate the
different structures of the representation in Topic
Maps and RDF.
migrAtion seleCtion
guidelines
Topic Maps and RDF have many similarities. They
both have data models and their syntaxes are based
on XML. Topic Maps are subject-centric whereas
RDF is resource-centric (Pepper, 2002b). In other
words, Topic Maps mainly focus on the subjects
Figure 3. RDF-based ontology modeling
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