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