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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Ontologies for the Semantic Web
Michael Lutz
Slides based on Co-ode OWL Tutorial (University of Manchester)http://co-ode.man.ac.uk/resources/tutorials/intro/slides/
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Overview
• Why do we need ontologies and the Semantic Web?
• What are ontologies ontology definitions properties
• Ontology languages OWL & Description Logics Reasoning
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Impossible (?) using the Syntactic Web…
• Complex queries involving background knowledge Find information about “animals that use sonar
but are not either bats or dolphins”, e.g., Barn owl
• Locating information in data repositories Travel enquiries Prices of goods and services
• Finding and using web services Visualise surface interactions between two proteins
• Delegating complex tasks to web agents Book me a holiday next weekend somewhere
warm, not too far away, and where they speakFrench or English
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Consider a typical web page:
• Markup consists of: • rendering information
(e.g., font size and colour)
• Hyper-links to related content
• Semantic content is accessible to humans but not (easily) to computers…
What is the Problem?
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
What information can we see…
• WWW2002
• The eleventh international world wide web conference
• Sheraton waikiki hotel, Honolulu, hawaii, USA, 7-11 may 2002
• 1 location. 5 days. learn. interact
• Registered participants coming from: australia, canada, chile denmark, france, germany, …
• Register now
• On the 7th May Honolulu will provide the backdrop of the eleventh international world wide web conference. This prestigious event …
• Speakers confirmed Tim Berners-Lee (Tim is the well known inventor of the
Web, …) …
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
…
… …
What information can a machine see…
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
<name> </name>
<location> </location><date> </date><slogan> </slogan><participants>
</participants>
<introduction>
… </introduction>
<speaker> </speaker><bio> </bio>…
XML markup with “meaningful” tagsThe Solution?
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
< > </ >< >
</ >< > </ >< > …</ >< >
, … </ >
< >
… </ >< > </ >< > </
>…
Machine sees…
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Need to Add “Semantics”
• External agreement on meaning of annotations e.g., Dublin Core: agree on the meaning of a set of
annotation tags but this approach is inflexible and only a limited
number of things can be expressed
• Use ontologies to specify meaning of annotations Ontologies provide a vocabulary of terms New terms can be formed by combining existing ones Meaning (semantics) of such terms is formally
specified Can also specify relationships between terms
in multiple ontologies
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Ontology in Computer Science
• An ontology is an engineering artifact: constituted by a specific vocabulary used to describe
a certain reality, plus a set of explicit assumptions regarding the intended
meaning of the vocabulary.
• Thus, an ontology describes a formal specification of a certain domain: Shared understanding of a domain of interest Formal and machine manipulable model of a domain
of interest
“An explicit specification of a conceptualisation” [Gruber93]
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Ontology Components
• Names for important concepts in the domain Elephant is a concept whose members are a kind of
animal Herbivore is a concept whose members are exactly
those animals who eat only plants or parts of plants
• Background knowledge/constraints on the domain Adult_Elephants weigh at least 2,000 kg All Elephants are either African_Elephants or
Indian_Elephants No individual can be both a Herbivore and
a Carnivore
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
A semantic continuum
[Mike Uschold, Boeing Corp]
Shared human consensus
Implicit
Text descriptions
Pump: “a device for moving a gas or liquid from one place or container to another”
Informal(explicit)
Semantics hardwired; used at runtime
Formal(for humans)
Semantics processed and used at runtime
(pump has (superclasses (…))
Formal(for machines)
• Less ambiguity• Better interoperation• More robust – less
hardwiring• More difficult
Further to the right
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Ontology Design and Deployment
• In the Semantic Web there should be tools and services to help users: Design and maintain high quality ontologies Store (large numbers) of instances of ontology classes,
e.g. annotations from web pages Answer queries over ontology classes and instances,
e.g.- Find more general/specific classes- Retrieve annotations/pages matching a given
description Integrate and align multiple ontologies
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
• Clash of intuitions: Domain Experts vs. Logicians Transparency & predictability vs.
Rigour & Completeness Neophytes caught in the muddled middle
• The knowledge acquisition “bottleneck”
• Assuring quality & managing change
• Confusion of terminology and usage
• Interdisciplinarity: Linguistics, Cognitive science, Software engineering, Philosophy
• A jumble of syntaxes
Why Ontology Engineering is hard…
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
• “Class” “Concept” “Category” “Type” are often used synonymously, but more precisely…
- categories are in the world- concepts are in the mind- classes/types are in the ontology
• “Instance” “Individual”
• “Entity” “object” can be class or individual
• “Property” “Slot” “Relation” “Relationtype” “Attribute” Semantic link type” “Role”
Vocabulary
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
OntologiesOntologies
Software agents
Software agents Problem-
solving methods
Problem-solving
methods Domain-independent applications
Domain-independent applications
DatabasesDatabasesDeclarestructure
Knowledgebases
Knowledgebases
Providedomain
description
The “Semantic
Web”
The “Semantic
Web”
An Ontology should be just the Beginning
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Ontology Languages
• Wide variety of languages for “Explicit Specification” Graphical notations, e.g. UML, Semantic networks,
Topic Maps (see http://www.topicmaps.org/), RDF Logic based, e.g. Description Logics (e.g., OWL), Rules
(e.g., RuleML, LP/Prolog), First Order Logic (e.g., KIF), Conceptual graphs, higher order and non-classical logics
Probabilistic/fuzzy
• Degree of formality varies widely Increased formality makes languages more amenable
to machine processing(e.g., automated reasoning)
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
• Objects/Instances/Individuals Elements of the domain of discourse Equivalent to constants in FOL
• Types/Classes/Concepts Sets of objects sharing certain characteristics Equivalent to unary predicates in FOL
• Relations/Properties/Roles Sets of pairs (tuples) of objects Equivalent to binary predicates in FOL
Language Primitives
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Web Ontology Language Requirements
• Desirable features for Web Ontology Language: Extends existing Web standards, e.g. XML, RDF,
RDFS Easy to understand and use should be based on
familiar KR idioms Formally specified Of “adequate” expressive power Possible to provide automated reasoning support
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
OWL Language
• Three species of OWL OWL full is union of OWL syntax and RDF OWL DL restricted to FOL fragment ( DAML+OIL) OWL Lite is “easier to implement” subset of OWL DL
• OWL DL based on SHIQ Description Logic
• OWL DL benefits from many years of DL research Well defined semantics Formal properties well understood (complexity,
decidability) Known reasoning algorithms Implemented systems (highly optimised)
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
(In)famous “Layer Cake”
Data Exchange
Semantics+reasoning
Relational Data?
?
???
???
???
TimBL, 2000 (http://www.w3.org/2000/Talks/1206-xml2k-tbl/slide10-0.html)
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
References
• Daconta, M.C., Obrst, L.J. & Smith, K.T. (2003): The Semantic Web. A Guide to the Future of XML, Web Services, and Knowledge Management, John Wiley & Sons
esp. chapter 8: Understanding Ontologies
• McGuinness, D.L. & van Harmelen, F. (Eds.) (2004): OWL Web Ontology Language Overview, available from http://www.w3.org/TR/owl-features/
• Baader, F., Calvanese, D., McGuinness, D., Nardi, D. & Patel-Schneider, P. (2003): The Description Logic Handbook, Cambridge University Press
esp. introductory chapters 1 & 2
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TU Wien – April 24-29, 2006 Semantics and Ontologies in GI Services
Questions???
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