70 4 The Semantic Web and Bioinformatics Applications
speculative, and one could certainly make other choices, all of which would
result in a consistent conversion to RDF. Converting from a DTD to RDF is
further complicated by implicit classes. When converting the Medline DTD
to RDF, it is necessary to introduce an RDF class for the date, yet there is no
such element in the Medline DTD. In general, XML element types can cor-
respond to either RDF classes or RDF properties, and both RDF classes and
RDF properties can be implicit. In other words, XML DTDs and schemas are
missing important information about the concepts being represented.
One specifies an RDF ontology using RDF itself. The fact that a resource is
an RDF class, for example, is stated using an ordinary RDF. For example, one
possibility for the classes and properties of the RDF ontology corresponding
to the Medline DTD is shown in figure 4.3. There are two namespaces used
by RDF. The first one is RDF, and the second is RDF Schema (RDFS). The RDF
namespace is sufficient for specifying ordinary facts, while RDFS is necessary
for specifying an RDF ontology.<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
<rdfs:Class rdf:ID="MedlineCitation"/>
<rdf:Property rdf:ID="Owner"/>
<rdf:Property rdf:ID="Status"/>
<rdf:Property rdf:ID="MedlineID"/>
<rdf:Property rdf:ID="PMID"/>
<rdf:Property rdf:ID="DateCreated"/>
<rdfs:Class rdf:ID="Date"/>
<rdf:Property rdf:ID="Year"/>
<rdf:Property rdf:ID="Month"/>
<rdf:Property rdf:ID="Day"/>
<rdf:Property rdf:ID="ArticleTitle"/>
</rdf:RDF>Figure 4.3 One possible way to represent the Medline DTD of figure 2.2 using an
RDF ontology.The Medline citation in figure 2.1 is already almost in a form that is com-
patible with RDF. All that is needed is to add aDateelement as shown in
figure 4.4. However, RDF gives one the freedom to represent the same infor-