OWL 2 Web Ontology Language

Topics Introduction to OWL Usage of OWL Problems with OWL 1 Solutions from OWL 2

OWL Introduction Initially focus was on RDF and RDF Schema OWL is a family of three language variants 1. OWL Lite 2. OWL DL 3. OWL Full OWL is a standardized language.

Usage of OWL OWL Ontologies are used in areas such as e-Science, medicine, biology, geography, astronomy, defense, automotive and aerospace industries. OWL is extensively used in life sciences community.

Problems in OWL 1 Expressivity Issues Problems with syntax Deficiencies in the definition

Expressivity Limitations Lacks several constructs Community users introduced various patterns Still Incomplete Problem with 1) Qualified Cardinality Restriction 2) Relational Expressivity 3) Data type Expressivity 4) Keys

1) Qualified Cardinality Restriction(QCR) Ex: “Persons that have at least one child who is male” --- existential restriction (qualified) Ex: “Person with at least 3 children who are male”--- cardinality restriction(not qualified) Some situations where QCR is used i) Define a quadruped animal with exactly 4 parts that are legs. ii) Medical Committee consisting of at least 5 members, 2 are medically qualified, 1 is manager and 2 are public members QCR was used in DAML + OIL---predecessor of OWL

2)Relational Expressivity Propagation along properties Ex: “Abnormality of a part of an anatomical structure constitutes an abnormality of the structure as a whole”. Properties of Properties Ex: “If x is part of y and y is part of z then x is part of z” reflexive(every object is a part of itself) asymmetric (nothing is a part of one of its parts).

Data type Expressivity “British citizen must have a passport number which is an xsd: string” We cannot express 1) restrictions to a subset of datatype values 2) relationships between values of data properties on one object 3) relationships between values of data properties on different objects 4)aggregate functions

4) Keys OWL DL does not support Key Constraints on data properties. Ex: “US Citizens are uniquely identified by their SSN” This is not possible OWL Full supports this using inverse functional properties.

2) Syntax Issues OWL 1 uses 2 types of syntax 1. Abstract Syntax 2. OWL 1 RDF Both are difficult to parse.

1) Frame-Based Paradigm Axiom Ex: “Tiger is a Class and it is a subclass of Class Cat” Class(Tiger partial Cat) “Class Tiger is a subclass of Class Predator” Adding, SubClassOf(Tiger Predator) Ans: Cat and Predator

2) Alignment with DL Constructs DL allows only one class to appear in property restriction Ex: restriction( hasParent SomeValuesFrom(Person) allValuesFrom(Person)) This can be translated into intersectionOf( restriction(hasParent SomeValuesFrom(Person)) restriction(hasParent allValuesFrom(Person))

3) Types of Ontology Entities Ex: class(Person partial restriction(hasMother SomeValuesFrom(Woman)) Ex: DisjointClasses(Animal Plant) SubClassOf(Human Animla)

4) Problems with OWL 1 RDF Information is represented in triples Ex: A is UNION of B and C (A, owl:unionOf, _:x1) (_:x1, rdf:first, B) (_:x1, rdf:rest, _:x2) (_:x2, rdf:first, C) (_:x2, rdf:rest, rdf:nil) This is difficult to read

Metamodeling Ex: “Harry is an Eagle” “Eagles are endangered species” --Individual Harry is an instance of Class Eagle --Individual Eagle id an instance of Class Endangered Species. Eagle is Individual in one and Class in the another.

Imports and Versioning Imports one ontology from another URI must point to the location of imported ontology and the location should match with the name of imported ontology. Coupling of names and location is not possible when ontologies are moved.

Solutions from OWL 2 1)Qualified Number Restrictions QCR is implemented in OWL 2

2) Relational Expressivity Addressed using Complex Property Inclusion Axioms Ex: “If a contains b and b has a part c, then a also contains c” subPropertyOf( Propertychain(contains hasPart) contains) Ex: “If b is part of a and b contains c, then c is also a part of a” subProperty( Propertychain(hasPart hasPart) hasPart) ---Cycle of dependency between contains and hasPart OWL 2 supports transitive, reflexive and asymmetric.

3) Increasing Data type Expressivity OWL 2 supports owl:boolean, owl:string, xsd:integer, xsd:dateTime, xsd:hexBinary Build new data types by providing data type restriction construct. Ex: DatatypeRestriction(xsd:integer xsd:minInclusive 18)

4) Keys OWL 2 implements Easy Keys. Key Axioms are of the form HashKey(C P1,……Pn) This states that object or data type properties Pi are keys for instance of Class C. No two instances of C can coincide on values of Pi. Ex: HashKey(Person hasSSN)

2 assertions can be made PropertyAssertion(PSmith hasSSN “ ”) PropertyAssertion(PeterSmith hasSSN “ ”) ---Psmith and PeterSmith are same individuals. Easy Keys are not applied to individuals not known by name Ex: Jane is connected through marriedTo to an individual x that is an instance of Man and that has “ ” as the value of hasSSN.

ClassAssertion( SomeValuesFrom(marriedTo IntersectionOf( Man HasValue(hasSSN “ ”))) Jane) --We cannot conclude that ClassAssertion(Man Psmith)

MOF Metamodel It is a meta language and used for specifying other languages. It is also a Structural Specification presented using UML.

Typing and Declarations Ex: Declaration(Class(Plant)) Declaration(Class(Animal)) SubPropertyOf(P Q) Both P and Q must be declared, otherwise we cannot conclude whether they are objects or data type properties.

Metamodeling with Punning Ex: “Eagles are endangered species” “Harry is an Eagle” ClassAssertion(Eagle Harry) ClassAssertion(Endangered Eagle) This can be interpreted in OWL 2 as ClassAssertion(Class-Eagle Individual-Harry) ClassAssertion(Class-Endangered Individual-Eagle) ---This type of meta modeling is referred as Punning.

Annotation of Axiom Ex: “Humans are animals and attributes that statement to Peter” SubClassOf( Annotation(attributedTo “Peter”) Human Animal) ---Annotations do not affect the semantics of ontology but they do affect the structural equivalence. SubClassOf(Human Animal) This is semantically equivalent to the above one but not structurally.

Imports and Versioning OWL 2 allows redirection mechanism. OWL 2 contains Version URI which identifies the version of the ontology.

Conclusion Apart from addressing problems in OWL 1, goal of OWL 2 is to provide a robust platform for future development.

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