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Semantic Transformation /1:
The RDF Data Model
Progressing towards semantic interoperability, the
metadata in the XML documents are now transformed
into RDF statements corresponding to the RDF data
model.With these so-called RDF `triples', the XML
metadata elements are mapped to the RDF classes and
properties, which are defined by the RDF Schema of
the FMS initiative (see section /3).
`XML is nothing more than a way to standardize data
formats....This is not to underplay XML's importance.
A data-format standard makes all of the more glamorous
technologies possible, and RDF is the leading example of
the benefit that comes once the data format has been
standardized. Many proclaim that RDF is really the
XML's killer app, and with good reason. Despite all this,
RDF remains somewhat obscure.This is mainly because at
its core RDF is very abstract, very dry, and very academic.'
Uche Ogbuji: An introduction to RDF (2000),
RDF Data Model
In order to make Web resources semantically
interoperable, we need resources that provide
machine-understandable information about them-
selves. In the Semantic Web architecture, these
statements are built by using the Resource
Description Framework (RDF).
RDF defines a data model for the statements
describing typed relationships between uniquely
identified sources. RDF distinguishes between:
| resources: familiar examples are, for example, a
Web page, electronic document or digital image,
but in RDF also entities that are not `network
retrievable', e.g. museums, curators or bound
medieval manuscripts, can be resources.
| properties: these identify a specific aspect,
characteristic, attribute, or relation used to
describe the resource.
| statements: these associate a value for a named
property with the resource.
Hence, RDF provides a model for describing
relationships between resources in terms of named
properties and values.The RDF data model
intrinsically supports only binary relations. Its base
element is the `triple', which takes the form of
subject, predicate, object: a resource (the subject) is
linked to another resource (the object) through an
arc labelled with a third resource (the predicate).
The semantics of a triple clearly depends on the
property used as predicate.
A convenient way to visualise this is to draw nodes
for subject and object and an arrow between them
for the predicate (see graphic 2). In this labelled
directed graph, subject and predicate (property) are
Uniform Resource Identifiers (URIs), and the object
is either a URI or a literal (which is drawn as a box).
Everything in RDF can be represented by a graph
with nodes and arcs, and the data model allows for
using the same URI as a node and as an arc label.To
represent RDF statements in a machine-processable
way, RDF builds on XML.With RDF/XML, a
specific XML markup language, RDF information
can be represented and ex-changed between
With these two triples we state that
#ColumnMiniature is a subclass of #Miniature,
and that #Miniature is a subclass of #Image.
The predicate in our statements is the
rdfs:subClassOf property which is predefined
in the RDF Schema namespace*
The classes #Image, #Miniature and
#ColumnMiniature would also need to be
defined in our RDF Schema namespace*
*For details on how to use RDF Schema for
defining your domain ontology see section /3.
In a nutshell, RDF Schema is a `higher-level'
language which is itself defined using RDF!
Graphic 2: RDF Data Model