Foundations of Semantic Web Chapter 4 - OWL

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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







Boolean class constructors

OWL provides language elements for

logical operators

● AND

● OR

● NOT


AND (conjunction)

● owl:intersectionOf

● the conjunction of two classes consists of

exactly those objects which belong to

both classes







OR (disjunction)

● owl:unionOf

● the disjunction of two classes means that

an object belongs to at least on of the

two classes.










NOT (negation)

● owl:complementOf

● the complement of a class consists of

exactly those objects which are not

members of the class itself.








● No faculty member can be a publication


Role restrictions

● Universal quantifier









● All examiners of an exam must be

professors


Role restrictions

● Existential quantifier









● There exist at least one examiner of an

exam


Role restrictions

● Max two examiners






2





● Min two examiners

○ minCardinality

● Exactly two

○ cardinality


Role Relationships

● Roles may have subproperties:

For instance, it can be declared that an

examinator of an event is also present at

the event.

● Two roles can be declared to be inverse

of each other


Role Characteristics

● Roles may be given certain

characteristics like transitivity or

symmetry.

● Example:




Types of inferences

● Classes can be declared to have different

properties, for instance two classes may

be defined as disjoint

● Modelling errors may be produced by

this, for instance if you say "A

isSubClassOf B" and "A isDisjointWith B",

this will return;

owl:Nothing


Owl Species

OWL can be divided into sublanguages:

OWL FULL

OWL DL

OWL Lite


OWL Full

OWL Full may be used and mixed freely

with all valid RDF.

● Cannot be used for software inferences.

● Can be used for purposes like translation


OWL DL and Lite

OWL DL is a restricted form of, that can

be used by software to make inferences

OWL Lite is an even more restricted form

that was intended to be an easy-toimplement

version of OWL DL.


OWL 2

● From the creators of OWL(WWWC); an

improvement of OWL


OWL 2 DL

● A backwards compatible extension of

OWL 1 DL

● Added type separation, punning and

declarations

● Relaxes old requirement that class

names, role names and individual names

must be distinct


Disjoint Classes

● Introduces option to declare several

classes to be mutually disjoint:


Role Characteristics and Relationshis

OWL 2 adds option to declare roles as:

○ Asymmetric

○ Reflexive

○ Transitive


Inverse Roles

● Like in OWL 1, except the inverse of a

role may now be referred to without

naming it:


Role Chains

● Allows for role chains:

○ Father has son, father has brother ->

Brother of father is uncle of son

● Most substatial improvement of OWL2


Quality Cardinal Restrictions

● Qualified cardinality:

○ For example, we can specify that one exam

may have at most two examinators


The Self Construct

● Allows for individuals to be related to

themselves under a given role:

○ For instance, a person committing suicide is

killedBy himself.


Negated Role Assignments

● We may now declare that two individuals

are NOT related by a given role.


Datatypes

● Most XML Schema datatypes are already

supported in OWL1.

OWL introduces some new ones:

○ owl:real - the set of real numbers

○ owl:rational

○ rdf:plainLiteral

○ xsd:dateTimeStamp - datetime w/ timezone

● Supports use of constraining facets to restrict

datavalues -> 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

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