ontology engineering: representation in owl

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  • 1. Ontology representation Guus Schreiber, VU Ontology Engineering course
  • 2. RDF/OWL Ontology language used in this course OWL = W3C Ontology Web Language RDF is basis of representation Triples = binary relation format Turtle is the triple syntax format most frequently used 2
  • 3. Background readingOWL Primer, W3C Recommendation, October 2009, http://www.w3.org/TR/owl2-primer/. Section 4 contains the basic constructs Secs. 5-8 describe the advanced features (not all of these will be used in the course). The Turtle syntax is easiest to read. You can hide the other in the beginning of the document. 3
  • 4. Some notes about syntax Protg hides most of the syntax details OWL has quite a few syntax formats If you want to look at OWL syntax we recommend that you use the Turtle syntax Background reading on Turtle): h http://wikitravel.org/en/Wikitravel:Turtle_RDF 4
  • 5. Class Central grouping construct Its instances are called members Classes can have multiple sub-classes Classes can have multiple super-classes Root class is conventionally called Thing: the super-class of all classes Nothing is a sub-class of all classes 5
  • 6. Property Properties define relationships RDF/OWL properties have a direction Artist creates Artwork Compare with UML! Two types of properties Object property: relationship between two classes Datatype property: relationship between a class and a value space (integers, strings, dates) Terminology: Subject = left hand of relation Object = right hand of relation 6
  • 7. Property hierarchy Properties may have sub-properties hasChild hasDaugther hasSon Logically sub-properties represent relationships between subsets of the super- property 7
  • 8. hasChild Judith Peter Bob Mary SuehasDaugther Peter Judith Mary Sue hasSon Peter Bob 8
  • 9. Domain and range Artist creates Artwork Artist is the domain Class of allowed values at the left side (origin( of the relationship Artwork is the range Class of allowed values at the right side (destination) of the relationship 9
  • 10. Property characteristics (1) Functional property For each subject this relation has at most one object hasBiologicalMother Inverse-functional property For each object this relation has are most one subject hasStudentNumber 10
  • 11. Property characteristics (2) Symmetric property IF i1 p i2 THEN i2 p i1 Example: friend Asymmetric property IF i1 p i2 THEN NOT i2 p i1 Example: parent 11
  • 12. Property characteristics (3) Transitive property IF i1 p i2 AND i2 p i3 THEN i1 p i3 Example: partOf If Amsterdam is a part of North-Holland and North-Holland is a part of The Netherlands, then Amsterdam is a part of the The Netherlands. 12
  • 13. Property characteristics (4) Inverse property P1 inverseOf p2 implies that IF i1 p1 i2 THEN i2 p2 i1 Example: hasPart is the inverse of partOf, so if Amsterdam is a part of North-Holland, then North-Holland must have Amsterdam as one of its parts. 13
  • 14. Property characteristics (5) Reflexive property FORALL p HOLDS i p i Example: for the property knows holds that everybody knows him/herself Irreflexive property FORALL p MUST NOT HOLD I p I Example: for the parent relation holds that no one can be his own parent 14
  • 15. Individual Instances of classes Rembrandt is and individual and member of the Artist class Note: Protg-OWL supports meta-classes (classes which members are classes) poorly! Enumerated class: a class for which all individual members can be listed Da Ponte operas of Mozart: Nozze di Figaro, Cosi fan tutte, Don Giovanni 15
  • 16. Equality and inequality of individuals Equality example: two people (with different URLs) are actually the same: ex:Jim sameAs ex:James Inequality example: two people are different Ex1:Jim differentFrom ex2:Jim Important on the Web! Difference between closed and open world 16
  • 17. Cardinality restrictions of properties Defines how many relationships of a certain type there can be for a particular subject Examples: Person marriedTo max 1 Course hasTutor min 1 Person hasParent exactly 2 17
  • 18. Value restrictions of properties Defines to what objects a subject can be related through a particular relation Examples Wine producedBy only Vineyard Wine is only produced by vineyards RedWine color value Red Red wines have a red color Bicycle hasPart some Wheel Bicycles consist, amongst others, of at least one wheel 18
  • 19. Equivalent classes & properties: simple States that two classes are the same, for example two classes in different ontologies wn-en:Dog = wn-it:Cane You can do the same for properties ex1:hasPart = ex2:hasComponent Question: do you think equivalence occurs frequently? 19
  • 20. Interlude: the notion of class extension OWL is derived from description logic Description logic takes an extensional view of classes: Two classes are the same if - and only if- they have the same class extension The class extension is the set of members of the class Question: does this correspond to your intuition? 20
  • 21. Equivalent classes: complex (1) A class as the union of other classes Parent = Mother or Father In terms of class extensions: The class extension of the class Parent is the union of the class extensions of the classes Mother and Father OWL calls such formulas class expressions (term used in Protg) 21
  • 22. Equivalent classes: complex (2) A class as the intersection of other classes Mother =Woman and Parent In words: members of the Mother class must be members of both the Woman and the Parent class You can build even more complex expressions: Mother = Woman and some hasChild 22
  • 23. Class expressions Statements such as Woman and Parent and Woman and some hasChild are called class expressions Description logic treats class expressions as anonymous classes i.e. concepts with no symbol, cf. the concept triad 23
  • 24. Equivalent classes: complex (3) Defining a class as the negation (complement) of other classes ChildlessPerson = Person and not Parent In words: a childless person is a member of the person class who does not belong to the extension of the parent class 24
  • 25. Necessary class definitions Description logic is used for classification reasoning A necessary definition state a constraint which must be true for class membership, but is not enough to classify it as a member of the class Example: red