Ontology modellingand the semantic web

Asgeir Rekkavik

Deichmanske bibliotek

What does the wordsemantic mean?

• Semantics: The branch of linguistics concerned with meaning.(Shorter Oxford English dictionary)

• Semantics is the study of meaning.(Wikipedia 2013-10-16)

I love youI ♥ U

Different syntax, same semantics

What does ontology mean?

• Ontology: The science or study of being.(Shorter Oxford English dictionary)

• In computer science and information science, an ontology formally represents knowledge as a set of concepts within a domain, and the relationships between those concepts.(Wikipedia 2013-10-16)

What does ontology mean?

• The world can be described in many different ways: e.g. language, art etc.

• An ontology describes the world in a way that is formal, structured and unambiguous.

• Why? Because we want to describe it to computers.

Ann and Becky are sisters

Ann and Becky are mothers

Taxonomies

Taxonomies

• Hierarchical classification• Characteristics

• Generic relations (’is-a’ relations)• Directed graph• Nodes represent categories• Arrows represent broader/narrower relations

• Especially known from biology. Developed by Carl von Linné.

Taxonomies

Transitive relations

• If A is related to B and B is related to C,then A is related to C

• Examples:• If Ann is younger than Bob and Bob is younger

than Carl, then Ann is younger than Carl• If a wolf is a mammal and a mammal is an

animal, then a wolf is an animal.

Transitive relations

• Other transitive relations can exist between concepts, e.g. ’part-of’ relations

Different types of relations

• Generic (’is-a’, e.g. Cat - Animal)

• Partitive (’part-of’, e.g. Oslo - Norway)

• Instance (e.g. Socrates - Philospher)

• Equivalence (e.g. Dove – Pigeon)

• Associative (’the rest’)

Thesaurus

• Concepts are represented by terms

• Certain types of relations between concepts are formalized:• Generic, partitive and instance relations are all

formalized as ’broader / narrower’• Equivalence relations are formalized as ’use% / use

for ’• Some associative relations are formalized as ’see

also:’

Thesaurus hierarchy

Thesaurus

• Solar systemsNT: Planets

• PlanetsBT: Solar systemsNT: Gas giants

• Gas giantsBT: PlanetsNT: Jupiter

• JupiterBT: Gas giants

Protégé

• Free, open source ontology editor

• Developed by Stanford University and the University of Manchester

• Available from:

http://protege.stanford.edu

Ontology – key concepts

• Classes

• Instances (individuals)

• Properties

Classes

• Represent categories, sets of individual instances

• Are related to eachother through parent-child relationships (superclass-subclass)

• Only generic ’is a’-relations are allowed• Unlike in a taxonomy, multiple inheritence

is allowed.

Generic class hierarchy

Generic class hierarchy

Properties of classes

• Classes can be:

• Disjoint

(if n is a member of A, n is not a member of B)

(e.g. if Robin is a girl, then Robin is not a boy)

• Equivalent

(if n is a member of A, n is also a member of B and

if n is a member of B, n is also a member of A)

(e.g. Firstgraders Pupils born in 2007)

ExerciseCreate a taxonomy with these classes:

• Bicycle• Boat• Bulldog• Car• Cat• Colour• Dog• Dolphin• Flower• Man

• Oak• Person• Pet• Pinetree• Plant• Puppy• Rose• Whale• Woman• Zebra

Instances

• Individual entities that can populate any number of classes.

• An instance that is a member of a class, is necessarily also a member of all its superclasses.

ExerciseCreate these instances:

The semantic triple

• A semantic triple is a statement consisting of three parts:• an instance (subject)• a property that refers to that instance (predicate)• a value for that property (object)

George likes chocolate

s p o

Properties

• The instances are described through properties.

• There are two different types of properties:• Object property:

• Takes another instance as value• e.g. Alice knows Fred

• Datatype property• Takes a distinct datatype value, like a number, a string etc.• e.g. King Harald has year of birth 1937

• The property is the ”predicate” in the semantic triple.

Domain and Range

• The domain and range of a property determine what kind of instances it can be used for and what kind of values it can have.

• Domain• The class, whose instances can have the property• If domain is not set, domain=Thing

• Range• The class, whose instances can be value for an object

property• The type of data that is allowed as value for a datatype

property

Properties of properties

• Properties can be:• symmetric

(Martin has cousin Thomas) (Thomas has cousin Martin)

• asymmetric(Martin is father of Rosie) (Rosie can not be father of Martin)

• inverse(Martin is parent of Rosie) (Rosie is child of Martin)

• transitive(Rosie descends from Martin) and (Martin descends from Emma) (Rosie descends from Emma)

• functional (can have only one value)• inverse functional (value can be held by only one instance)• reflexive (instance takes itself as a value)

ExerciseObject properties

• Create the following object properties• owns• ownedBy• hasNeighbour

• Set domain and range

• Connect instances, so that:• Mr. Taylor owns Duchess• Mrs. Robertson owns Lassie• Mr. Taylor and Mrs. Robertson are neighbours

Restrictions

• Classes can be populated according to rules called restrictions.

• This is done by expressing that a class is equivalent to a certain set of instances.

• The set can be defined by• combining other classes with and/or/not

operators• using criteria based on desired properties for

the instances

Restrictions

• Add new class LivingThing• Use class expression editor to express

equivalence relation:LivingThing Animal or Plant or Person

• Add the class Gender• Add the individuals Male and Female• Add the property hasGender, domain: LivingThing• Express that:

• Lassie is female• Duchess is female• Moby Dick is male• Mr. Taylor is male• Mrs. Robertson is female• Thomas O’Malley is male

• Add classes FemaleBeing and MaleBeing• Use class expression editor to express

equivalence relations:

FemaleBeing ≡ hasGender value Female

MaleBeing ≡ hasGender value Male

Pet ≡ Animal and ownedBy some Person

What about this?

WildAnimal ≡ Animal and not (ownedBy some Person)

Open world assumption

• The truth-value of an assumption does not depend on whether it is known or not

• The absence of a statement therefore does not count as a negation of that statement

• Statements:• Mary is a woman• George is a man• Mary is an American citizen

• Question:• Is George an American citizen?

• Answers• Closed world assumption: "No"• Open world assumption: "Unknown"

Example ontologies

• Dublin Core metadata termshttp://purl.org/dc/terms/

• Bibo (Bibliographic ontology)http://purl.org/ontology/bibo/

• Core FRBRhttp://purl.org/spar/frbr/

• FOAF (Friend of a friend)http://xmlns.com/foaf/spec/