background image
drawn from illustrated medieval manuscripts, we
first need to define a class of things that are images.
In RDF Schema, a class is any resource having an
rdf:type property whose value is the RDFS-defined
resource rdfs:class.
So, using the basic RDF data model we define:
mi:Image [resource] rdf:type [property] rdfs:Class
[value].The self-defined prefix mi (for medieval
images) stands for the URI reference of our RDF
Schema namespace
In our image collection we have various special
kinds of digitised images, such as column miniatures,
decorated initials, schematic drawings, etc.To distin-
guish, for example, the miniatures, first we need to
define a general class Miniature and subclasses of
miniatures, e.g. a subclass ColumnMiniature:
mi:Miniature rdf:type rdfs:Class
mi:ColumnMiniature rdf:type rdfs:Class
Secondly, we need to define that
mi:ColumnMiniature is a subclass of mi:Miniature,
and that mi:Miniature is a subclass of mi:Image,
for which we use the predefined rdfs:subClassOf
mi:Miniatures rdfs:subClassOf mi:Image
mi:ColumnMiniature rdfs:subClassOf mi:Miniature
As the rdfs:subClassOf property is transitive, this
means that mi:ColumnMiniature is also implicitly a
subclass of mi:Image.
Graphic 2 on page 32 visualises this with the nodes
and arcs of the basic RDF data model.
Defining properties
In order to make the meaning of our metadata (i.e.
`type') explicit, we need to be capable of declaring
specific properties that characterise the classes of
things we hold at, e.g. digital
images of medieval column miniatures.
Basically, RDF schema defines properties in terms
of the classes of resources to which they apply.This
is the role of the rdfs:domain and rdfs:range
The range constraint defines the class or set of classes
whose instances can be values of a particular pro-
perty. If we want to define the property mi:hasType,
we must describe this resource (which we locate at with an
rdf:type property whose value is rdf:Property:
mi:ColumnMiniature [resource] rdf:type [property]
rdf:Property [value].
The following RDF statements indicate that
mi:ColumnMiniature is a class, mi:hasType is a proper-
ty, and RDF statements using the mi:hasType pro-
perty have instances of mi:ColumnMiniature as values:
mi:ColumnMiniature rdf:type rdfs:Class
mi:hasType rdf:type rdf:Property
mi:hasType rdfs:range mi:ColumnMiniature
The domain constraint restricts the set of classes
whose instances may have a particular property
attached to them. If we want to indicate that the
property mi:hasType applies to instances of class
mi:ColumnMiniature, we would write:
mi:ColumnMiniature rdf:type rdfs:Class
mi:hasType rdf:type rdf:Property
mi:has Type rdfs:domain mi:ColumnMiniature
Benefits of RDF
In a definition of RDF,
some benefits of RDF are mentioned:
| `By providing a consistent framework, RDF
will encourage the providing of metadata about
Internet resources.
| Because RDF will include a standard syntax for
describing and querying data, software that exploits
metadata will be easier and faster to produce.
| The standard syntax and query capability will allow
applications to exchange information more easily.
| Searchers will get more precise results from
searching, based on metadata rather than on
indexes derived from full text gathering.
| Intelligent software agents will have more
precise data to work with.'
This is a well-crafted listing of RDF benefits, from
provision and exchange of better metadata to agents
working with them, hopefully for the benefit of
humans. But, as explicitly stated by SearchWeb-, these are only potential benefits, i.e.
they depend on the level of actual uptake of RDF.
Definitions - Resource
Description Framework,
(last updated: July 27, 2001).