PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58Slide 59Slide 60Slide 61Slide 62Slide 63Slide 64Slide 65Slide 66Slide 67Slide 68Slide 69Slide 70Slide 71Slide 72Slide 73Slide 74Slide 75Slide 76Slide 77Slide 78Slide 79Slide 80Slide 81Slide 82Slide 83Slide 84Slide 85Slide 86Slide 87Slide 88Slide 89Slide 90Slide 91Slide 92Slide 93Slide 94Slide 95Slide 96Slide 97Slide 98Slide 99Slide 100Slide 101Slide 102Slide 103Slide 104Slide 105Slide 106Slide 107Slide 108Slide 109Slide 110Slide 111Slide 112Slide 113Slide 114Slide 115Slide 116Slide 117Slide 118Slide 119Slide 120Slide 121Slide 122Slide 123Slide 124Slide 125Slide 126Slide 127Slide 128Slide 129Slide 130Slide 131Slide 132Slide 133Slide 134Slide 135Slide 136Slide 137Slide 138Slide 139Slide 140Slide 141Slide 142Slide 143Slide 144Slide 145Slide 146Slide 147Slide 148Slide 149Chapter 4 A Semantic Web Primer1Chapter 4Web Ontology Language: OWL Grigoris AntoniouFrank van HarmelenChapter 4 A Semantic Web Primer2Lecture Outline1. Basic Ideas of OWL 2. The OWL Language3. Examples4. The OWL Namespace5. Future ExtensionsChapter 4 A Semantic Web PrimerWhy do we need OWL?HTML is used to create Web pages - Represents both the semantics and presentation of the information- The tags used for semantics and the tags used for presentation are often intermixedXML allowed us to write our own markup language that was used to describe the semantics of the information onlyChapter 4 A Semantic Web PrimerWhy do we need OWL? (cont)One problem with XML is that the user has to define the meaning of the tags themselves and encode it in a schemaThere is no global representation for an XML element or schemaAlso, it is possible in XML to describe the same object using the same tags in multiple waysChapter 4 A Semantic Web PrimerWhy do we need OWL? (cont)To solve these problems, RDF was introducedRDF allows resources to be described using global URIsAlso, an RDF document is a graph, and not a tree like an XML document. This allows for one graph to uniquely represent an RDF document (using Turtle, that is )Chapter 4 A Semantic Web PrimerWhy do we need OWL? (cont)RDF allows us to make statements about individualsIt is possible in RDF to say that “John is a Professor”However, it is not possible in RDF to create a Professor classTherefore, we cannot create ontologies using RDF aloneChapter 4 A Semantic Web PrimerWhy do we need OWL? (cont)RDF Schema was introduced to allow for the creation of classes and ontologiesrdfs:Class, rdf:Property, rdfs:Containerrdfs:subClassOf, rdfs:subPropertyOf, rdfs:domain, rdfs:range, rdfs:seeAlso, rdfs:comment, etc.RDF Schema however, a weak ontology language, in that there is much that we cannot expressChapter 1 A Semantic Web Primer8The Semantic Web Layer TowerOWL and ontologiesOWL allows for the expression of richly defined ontologiesOntologies can be used to model the knowledge of a particular domain (Biomedical ontologies, Earth science ontologies, an ontology describing a university, and much more)Ontologies can be used to create a hierarchical classification of data based on semantics (ontology backed Web search engines)OWL and ontologies (cont)Ontologies in related domains, or modelling related objects, can be alignedThis is called ontology alignmentExample: the Texas Department of Transportation creates an ontology of the Texas Road System, and in Louisiana (a neighboring state), the Louisiana Department of Transportation creates an ontology of Louisiana roads.Why are ontologies important? (cont)If there is ever a car accident close to the Texas-Louisiana border, then first responders from both states may have to come to the scene of the accidentAligning the ontologies will allow for more effective data sharing by services used by first responders and other emergency crew to cordon the accident scene, allow the victims to receive medical care quickly, etc.Chapter 4 A Semantic Web Primer12Limitations of the Expressive Power of RDF Schema (2)Disjointness of classes–Sometimes we wish to say that classes are disjoint (e.g. male and female)Boolean combinations of classes–Sometimes we wish to build new classes by combining other classes using union, intersection, and complement–E.g. person is the disjoint union of the classes male and femaleChapter 4 A Semantic Web Primer13Limitations of the Expressive Power of RDF Schema (3)Cardinality restrictions–E.g. a person has exactly two parents, a course is taught by at least one lecturerSpecial characteristics of properties–Transitive property (like “greater than”)–Unique property (like “is mother of”)–A property is the inverse of another property (like “eats” and “is eaten by”)14Limitations of the Expressive Power of RDF SchemaLocal scope of properties –rdfs:range defines the range of a property (e.g. eats) for all classes –In RDF Schema we cannot declare range restrictions that apply to some classes only –E.g. We may declare a class called ‘Animal’ and a property called ‘eats’. The domain and the range of ‘eats’ might be ‘Animal’. –But we might have class called ‘Cow’ that is a subclass of ‘Animal’. Yet, cows only eat plantsChapter 4 A Semantic Web Primer15Requirements for Ontology LanguagesOntology languages allow users to write explicit, formal conceptualizations of domain modelsThe main requirements are:–a well-defined syntax –efficient reasoning support –a formal semantics –sufficient expressive power –convenience of expressionReasoning support in ontology languagesBoth RDF Schema and OWL support reasoningReasoning is the process by which new statements within an ontology are inferredThis requires (1: A language that expresses formal semantics (2: A program called a reasoner that makes the inferencesChapter 4 A Semantic Web Primer17Examples of Formal Semantics Class membership –If x is an instance of a class C, and C is a subclass of D, then we can infer that x is an instance of