Ontology modelling and the semantic web
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Transcript of Ontology modelling and the semantic web
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/