Database Reference
In-Depth Information
9
OWL
9.1 INTRODUCTION
In the previous chapters, we covered RDF (Resource Description Framework) and
SPARQL, and explained how to organize Geographic Information into Linked
Data. In Chapter 8, we highlighted some of the difficulties of linking different RDF
datasets together due to the limited descriptions afforded by the RDF language. In
this chapter, we introduce the OWL Web Ontology Language (Dean and Schreiber,
2004; World Wide Web Consortium [W3C], 2009), which offers a way of express-
ing more detailed knowledge about the domain of interest and of inferring new
information from the set of statements we provide. This chapter sets out the main
concepts of the OWL language and discusses the various options for software tool
support. Chapter 10 takes you through the step-by-step process of using OWL to
author an ontology, so that you will be in a position to use your ontologies to inte-
grate data from different domains. This chapter does not attempt to provide a com-
plete description of OWL since there are many other excellent publications that do
this. Rather, it provides a flavor of the language and what it can and cannot be used
for. The chapter ends with some examples of modeling best practice with some
suggested design patterns.
OWL is not a single language but a whole family of related species. However,
of these different species, one is almost universally used: OWL DL (Description
Logic), so in this chapter we place our concentration here. To provide a more com-
plete picture, for those interested, Appendix A describes the different OWL species
and explains their differences and uses, and Appendix B provides details of the
three syntaxes (Rabbit, Manchester Syntax, and RDF/XML [eXtensible Markup
Language]) that we use in this chapter.
9.2 THE NATURE OF OWL
OWL, the Web Ontology Language, was first standardized as a recommendation
of the W3C in 2004, with a more recent update, OWL 2, in 2009. Its purpose is to
allow more expressive descriptions of knowledge than are possible with RDFS (RFD
Schema), using formal logic to encode the semantic meaning in the statements.
In the Semantic Web “layer cake” shown in
Figure 9.1
, OWL is shown as a layer
above RDFS, since it offers the knowledge modelers the opportunity to express more
complex and detailed statements of their knowledge and perform more logical infer-
ence than is possible with RDFS. This does not necessarily mean that it is
better
to use OWL than RDFS as there are many cases for which an ontology allowing a
simple set of triple statements will suffice. For example, it is likely to be more appro-
priate to use RDFS rather than OWL to describe the structure of a straightforward
dataset, such as a set of names and ages of people. In this example, the class is Person
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