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Property features are defined by the property axioms. The basic form expresses the existence only. For example,
in a smartphone ontology, the property hasTouchscreen can be declared to express a major feature of mobile phones
(see Listing 7-76).
Listing 7-76. A Property Declaration in OWL
<owl:ObjectProperty rdf:ID="hasTouchscreen"/>
OWL property axioms can also define additional characteristics. OWL supports RDF Schema constructs
such as rdfs:subPropertyOf , rdfs:domain , and rdfs:range . Relations to other properties can be expressed by
owl:equivalentProperty and owl:inverseOf (Listing 7-77).
Listing 7-77. Two Equivalent Smartphone Properties (Accelerometer and G-sensor)
<owl:ObjectProperty rdf:ID="hasAccelerometer">
<owl:ObjectProperty rdf:ID="hasGsensor"/>
Global cardinality constraints are defined by owl:FunctionalProperty and owl:InverseFunctionalProperty .
Logical property features are defined by owl:SymmetricProperty and owl:TransitiveProperty [158] (Listing 7-78).
Listing 7-78. A Symmetric Property in OWL
<owl:SymmetricProperty rdf:ID="hasLiveConnection">
<rdfs:domain rdf:resource="#Smartphone"/>
<rdfs:range rdf:resource="#Smartphone"/>
OWL provides precise declarations for expressing relationships, even if they are evident. For example, the
property hierarchy of two smartphone features can be expressed in functional syntax, as presented in Listing 7-79.
Listing 7-79. Property Hierarchy in OWL
SubObjectPropertyOf( :hasGeotagging :hasCamera )
Similar to RDF, OWL provides classes to group resources. There are six different class descriptions in OWL:
Class identifier (URI reference). A named instance of
owl:Class , a subclass of rdfs:Class . 8
Listing 7-80 shows an example.
Listing 7-80. A Class Identifier in OWL
<owl:Class rdf:ID="Handheld"/>
8 In OWL Lite and OWL DL. In OWL Full they are equivalent.
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