Foundations of Semantic Web Chapter 4 - OWL

Foundations of Semantic Web Chapter 4 - OWL Kent Robin Haugen & Severin Sverdvik

Web Ontology Language (OWL) ● W3C recommended standard for the modeling of ontologies ● Design issues: expressivity of the language ← → efficient reasoning (i.e. scalability) ● OWL species ○ OWL Full ○ OWL DL ○ OWL Lite

OWL Syntax and Intuitive Semantics ● OWL ontology is basically expressed in terms of classes and properties. ● Much more complex relationships between these classes and properties can be expressed than with RDF(S)

For instance ● Every project has at least one participant ● Projects are always internal or external projects ● Gisela Schillinger and Anne Eberhardt are the secretaries of Rudi Studer ● The superior of my superior is also my superior

The header of an OWL Ontology ● Contains information about ○ namespaces ○ versioning ○ annotations

● No direct impact on knowledge expressed by the ontology

The header of an OWL Ontology ● Can contain some general information about the ontology

Classes, Roles, and Individuals ● Classes, properties and individuals are basic building blocks in OWL ● Properties are also called Roles ● Individuals are RDF instances of classes

Classes Class gets assigned to the name "Professor"

is equivalent to

Classes ● Two predefined classes ○ owl:Thing ■ Every Class is a subclass of Thing

○ owl:Nothing ■ Nothing is a subclass of every other class

○ owl:Class subclass of rdfs:Class

Individuals ● Individuals declared to be instances of classes equivalent to

Roles (Properties) Two roles in OWL ● Abstract ● Concrete ● Both are subproperties of rdf: Property

Abstract ● Connects individuals with individuals

● expresses which organization(s) a given person is affiliated with

Concrete ● Connects individuals with data values (i. e. elements of datatypes)

assigns first names to persons

XML datatypes for OWL

Role assignment

● Possible to assign two affiliations to one person

Simple Class Relations ● Subclassing

Simple Class Relations ● Logical Inference by transitivity

Relations between individuals

Relations between individuals

Closed Classes ● If for instance the Class has only two members. Closed Classes can be used Teenager = person(age:13-19)

OWL 2 Profiles ● Sublanguages of OWL2 are called profiles ● OWL 2DL, OWL1 DL, OWL1 are all examples of profiles of OWL2.

Standard OWL2 profiles ● OWL2 ○ ○ ○ ○ ○

DL EL QL RL Full

OWL2 EL ● Fairly restricted version ● Good computational properties ● Allows polynomial time algorithms for all standard inferences ● Designed for ontologies that include very large class and role hierarchies using only a small amount of OWL features

OWL2 QL ● Allows conjunctive query answering ● Designed for data-driven applications ● Convenient for vendors of RDF stores to include OWL support

OWL2 RL ● Good for rule-based reasoning ● RL can be seen as a restriction of OWL 2 DL

OWL2 Full ● OWL2 equivalent of OWL1 really, used for conceptual modelling in cases where automated reasoning is not required

Thank you