background image
34
DigiCULT
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 http://www.m-i.org/schemas/
images.
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
property:
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 http://www.m-i.org, 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
mechanisms.
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
http://www.m-i.org/schemas/images) 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
rdfs:domain
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 SearchWebServices.com 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.'
13
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-
Services.com, these are only potential benefits, i.e.
they depend on the level of actual uptake of RDF.
13
whatis.com:
searchWebServices.com
Definitions - Resource
Description Framework,
http://searchwebservices.
techtarget.com/sDefinition/
0,,sid26_gci213545,00.html
(last updated: July 27, 2001).