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Semantic Transformation /2: Creating and
Validating the RDF Statements
In the mapping process, an editor tool (in the FMS
case, a tool developed by the project team called
Meedio) receives as input the XML documents and
assists in transforming them into semantically valid
RDF statements (instance descriptions).The tool
serves as instance editor, which provides a convenient
way of finding and selecting from the XML metadata
elements correct instance values for a particular
property.The editor tool also serves as semantic
metadata validator.When the museum cataloguer
saves the set of RDF statements corresponding to an
XML document, the semantics of these statements
are validated against the property constraints of the
FMS ontology.The result of a successful mapping and
validation process is a unique set of RDF triples,
called the RDF card.
RDF Mapping Rules
When RDF is used to define the meaning of XML
metadata elements, a set of mapping rules is created.
A mapping rule is a template of RDF triples where
XPath expressions are used to identify the actual
element values. XPath is a language for addressing
parts of an XML document, and was designed for
use in XML parsing software (XSLT, XPointer, and
others).
When applying such a rule to an XML document,
the XPath expressions are instantiated with matching
element values. If the rule matches, the RDF temp-
late evaluates to a set of RDF triples where XPath
expressions are substituted by the corresponding
values of the XML elements.
For example, by applying the template rule
<image5kb78d38i/type, mi:hasCreator,
/image5kb78d38i/creator>
to the XML document described in the section
on XML the following result would be obtained:
<Image Miniature, mi:hasCreator, `Alexander
Master`> .
mi:hasCreator is an example of a RDF property.
Such properties are explained in section /3: RDF
Schema.
Note: Due to the limited space permitted, we
do not address issues of term mapping.This is an
important aspect of the mapping and validation
process carried out in the FMS project.Working with
metadata from different museums, they need to deal
with partly different terminologies.Their technical
solution to synonymous terms (i.e. different terms
referring to the same concepts) is to attach synonym
sets to the FMS ontology classes.With situations
where polysemous terms occur (i.e. the same terms
refer to different concepts), the editor tool cannot
cope, and the cataloguer needs to select the correct
interpretation.
Semantic Transformation /3:
The RDF Schema (RDFS)
The shared ontology for the textiles domain is
created by using Resource Description Framework
Schema (RDFS). An RDF Schema is a tool for
indicating the classes of resources one wants to
describe as well as for defining the properties used to
describe those resources. Furthermore, class/sub-class
relationships and property/sub-property relationships
can be defined.The museums are mapping their
metadata to the classes and properties defined by the
RDF Schema of the FMS initiative.Thereby, they are
making the meaning of the metadata explicit and
representing them in a harmonised uniform way.
RDF Schema
In section Semantic Transformation /1 we have
described the data model provided by RDF for
expressing statements about Web resources. But we
also need a vocabulary for the RDF statements,
namely classes and properties defined with RDF
Schema (RDFS).
In brief, the RDF Schema mechanism provides a
pre-defined vocabulary, a basic type system that can
be used in creating domain-specific schemas. Its role
is to allow for declaring metadata properties (e.g. for
`type', `subject' or `creator'), to define the classes of
resources they may be used with, to restrict possible
combinations, and to detect violations of those
restrictions.
Defining classes
With RDF Schema (RDFS),Web resources can
be defined as instances of one or more classes. In
addition, classes can be organised in a hierarchical
fashion. As we hold a collection of digital images
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