4 Web Ontology Language: OWL
4.1 Introduction
1 Main requirements for ontology language
a well-defined syntax
efficient
reasoning support
a formal semantics
sufficient
expressive power
convenience of expression
2 Mapping to a known logical formalism
A Formal semantics and reasoning support are usually provided by mapping an ontology language to a known logical formalism, and by using automated reasoners that already exist for those formalisms.
OWL is (partially) mapped on a description logic, and makes use of existing reasoners such as FaCT and RACER.
3 Limitations of the Expressive Power of RDF Schema
local
scope of properties
In RDF Schema we cannot declare range restrictions that apply to some classes only. For example, we cannot say that cows eat only plants, while other animals may eat meat, too.
Disjointness
of classes
不能指定两个类(如male和female)没有交集.
Boolean
combinations of classes
无法通过交,联合,补等集合操作来定义新的类.
Cardinality
restrictions
不能指定一个属性可以有或应该有几个值.
Special
characteristics of properties
不能指定a property is transitive, unique, inverse of another property
4. Comparison between OWL language Species
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OWL Full |
OWL DL |
OWL Lite |
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Expressive power |
High |
Medium |
Low |
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uses all the OWL languages primitives. It also allows the combination of these primitives in arbitrary ways with RDF and RDF Schema. |
essentially application of OWL’s constructor’s to each other is disallowed, thus ensuring that the language corresponds to a well studied description logic. |
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Reasoning support |
Low |
Medium |
Hign |
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Upward-compatibility with RDF |
Any legal RDF document is also a legal OWL Full document; Any valid RDF/RDF Schema conclusion is also a valid OWL Full conclusion. |
An RDF document will in general have to be extended in some ways and restricted in others before it is a legal OWL DL document. Every legal OWL DL document is a legal RDF document. |
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5. upward compatibility between these three sublanguages
• Every legal OWL Lite ontology is a legal OWL DL ontology.
• Every legal OWL DL ontology is a legal OWL Full ontology.
• Every valid OWL Lite conclusion is a valid OWL DL conclusion.
• Every valid OWL DL conclusion is a valid OWL Full conclusion.
6. OWL still uses RDF and RDF Schema to a large extent
• All varieties of OWL use RDF for their syntax.
• Instances are declared as in RDF, using RDF descriptions and typing information.
• OWL constructors like owl:Class, and owl:DatatypeProperty, and owl:ObjectProperty are specialisations of their RDF counterparts.
7. Full downward compatibility for OWL
The advantage of full downward compatibility for OWL (that any OWL-aware processor will also provide correct interpretations of any RDF Schema document) is only achieved for OWL Full, at the cost of computational intractability.
4.2 The OWL Language
1. Syntactic form for OWL language
(1) RDF/XML(primary, we use it in this book)
(2) An XML-based syntax5 that does not follow the RDF conventions and is thus more easily read by human users.
(3) An abstract syntax, used in the language specification document.
(4) A graphic syntax based on the conventions of UML.
2. Class Elements
subclass
<owl:Class rdf:ID="associateProfessor">
<rdfs:subClassOf rdf:resource="#academicStaffMember"/>
</owl:Class>
disjoint
<owl:Class rdf:about="#associateProfessor">
<owl:disjointWith rdf:resource="#professor"/>
<owl:disjointWith rdf:resource="#assistantProfessor"/>
</owl:Class>
equivalent
<owl:Class rdf:ID="faculty">
<owl:equivalentClass rdf:resource="#academicStaffMember"/>
</owl:Class>
There are two predefined classes, owl:Thing and owl:Nothing.
The former is the most general class, which contains everything (everything is a thing), and the latter is the empty class. Thus every class is a subclass of owl:Thing and a superclass of owl:Nothing.
3. Property Elements
Type of Properties
(1) Object
Property
对象之间的关系,比如Teach
<owl:ObjectProperty rdf:ID="isTaughtBy">
<rdfs:domain rdf:resource="#course"/>
<rdfs:range rdf:resource="#academicStaffMember"/>
<rdfs:subPropertyOf rdf:resource="#involves"/>
</owl:ObjectProperty>
(2) Data
type Property
属性,比如phone
<owl:DatatypeProperty rdf:ID="age">
<rdfs:range rdf:resource="http://www.w3.org/2001/XMLSchema
#nonNegativeInteger"/>
</owl:DatatypeProperty>
4. Property Restrictions
(universal quantification) Requires first-year courses to be taught by professors only
<owl:Class rdf:about="#firstYearCourse">
<rdfs:subClassOf>
<owl:Restriction>
<owl:onProperty rdf:resource="#isTaughtBy"/>
<owl:allValuesFrom rdf:resource="#Professor"/>
</owl:Restriction>
</rdfs:subClassOf>
</owl:Class>
(existential quantification) All academic staff members must teach at least one undergraduate course
<owl:Class rdf:about="#academicStaffMember">
<rdfs:subClassOf>
<owl:Restriction>
<owl:onProperty rdf:resource="#teaches"/>
<owl:someValuesFrom rdf:resource="#undergraduateCourse"/>
</owl:Restriction>
</rdfs:subClassOf>
</owl:Class>
defines
cardinality restrictions
<owl:Class rdf:about="#course">
<rdfs:subClassOf>
<owl:Restriction>
<owl:onProperty rdf:resource="#isTaughtBy"/>
<owl:minCardinality rdf:datatype="&xsd;nonNegativeInteger">
1
</owl:minCardinality>
</owl:Restriction>
</rdfs:subClassOf>
</owl:Class>
owl:Restriction defines an anonymous class which has no ID, is not defined by owl:Class, and has only local scope: it can only be used in the one place where the restriction appears.
5. Special Properties
<owl:ObjectProperty rdf:ID="hasSameGradeAs">
<rdf:type rdf:resource="&owl;TransitiveProperty" />
<rdf:type rdf:resource="&owl;SymmetricProperty" />
<rdfs:domain rdf:resource="#student" />
<rdfs:range rdf:resource="#student" />
</owl:ObjectProperty>
6. Boolean Combination
<owl:Class rdf:ID="peopleAtUni">
<owl:unionOf rdf:parseType="Collection">
<owl:Class rdf:about="#staffMember"/>
<owl:Class rdf:about="#student"/>
</owl:unionOf>
</owl:Class>
4.3 Examples
4.3.1 An African Wildlife Ontology
4.3.2 A Printer Ontology
4.4 OWL in OWL
A part of the definition of OWL in terms of
itself.
4.5 Future Extensions
!