Ontology Engineering: representation in OWL


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Ontology Engineering: representation in OWL

  1. 1. Ontology representation Guus Schreiber, VU Ontology Engineering course
  2. 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. 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. 4. Some notes about syntax Protégé 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. 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. 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. 7. Property hierarchy Properties may have sub-properties hasChild hasDaugther hasSon Logically sub-properties represent relationships between subsets of the super- property 7
  8. 8. hasChild Judith Peter Bob Mary SuehasDaugther Peter Judith Mary Sue hasSon Peter Bob 8
  9. 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. 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. 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. 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. 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. 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. 15. Individual Instances of classes – Rembrandt is and individual and member of the Artist class Note: Protégé-OWL supports meta-classes (classes which members are classes) poorly! Enumerated class: a class for which all individual members can be listed – Da Ponte opera’s of Mozart: Nozze di Figaro, Cosi fan tutte, Don Giovanni 15
  16. 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. 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. 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. 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. 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. 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 Protégé) 21
  22. 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. 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. 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. 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 wine must have a red color – But: not all red things are red wines 25
  26. 26. Necessary class definitions in description logic The red-wine constraint is expressed as follows: – Red wine is subclass of the class of all red things More formally: necessary conditions are stated as a constraint that the class in question (red wine) is a subclass of the anonymous class represented by the class expression (class of all red things) This explains how Protégé uses the “Superclasses” – Test this now; you’ll get the hang of it 26
  27. 27. Sufficient class definitions Sufficient definitions allow us to classify an individual as a member of a class Example: if we know someone belongs to the Woman and Parent class, we also know she belongs to the Mother class The equivalent-class expression represent sufficient definitions 27
  28. 28. Annotations Annotations are used to add metadata to classes and properties Annotations play no role in classification Example annotations: – Human-readable label (in multiple languages) – Time created – See also Dublin Core elements 28
  29. 29. Class exercise: comparison with UML class diagrams Make list of – Similarities – Differences Don’t forget that OWL is a Web language! 29
  30. 30. Guidelines for naming concepts Keep original names, where possible Prefer more specific names over general ones Don’t be afraid of long names if you have to invent a name Use a consistent format – Here: ClassName propertyName – But alternatives are fine 30
  31. 31. Class room exercise “Artefacts are created by humans. Art works are a kind of artefact. If a person has created an art work we call him/her an artist.” Exercise: create with the help of Protégé an OWL ontology for this domain description. 31