Post on 14-Dec-2015
74.793 Natural Language and Speech ProcessingProfessor: Dr. Christel Kemke
Winter 2004
Knowledge Representation on the Semantic Web
by
Femi G. OlumofinDepartment of Computer Science
University of Manitoba,Winnipeg, Canada
March 8, 2004
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Outline
Introduction
The Semantic Web Layers
Resource Description Framework (RDF)
Ontology
OWL Web Ontology Language
Semantic Web Current Activity
Conclusion
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Introduction – The Current Web
The current Web represents knowledge as a global repository of resources interconnected by hyperlinks (href).
“Accessed by specifying URI addresses, searching, and following links to find other related resources” [1]
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Introduction – The Current Web
Human do the difficult part Linking, and interpreting
Machines do the easy part Presentation (of requested resource)
Humans can interpret because we know the meaning (or semantics) of web contents
If we add semantic annotations to web resources, then machines would be able to interpret!
Syntactic Web -> Semantic Web
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Introduction – The Vision of the Semantic Web
To develop facilities (enabling standards and technologies) to express information on the Web in a form that can be shared and processed by machines and human
“The Semantic Web is a web of data, in some ways like a global database” [2].
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Introduction – What is the Semantic Web?
Definition: “The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries. It is a collaborative effort led by W3C with participation from a large number of researchers and industrial partners. It is based on the Resource Description Framework (RDF), which integrates a variety of applications using XML for syntax and URIs for naming.” [10].
"The Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabling computers and people to work in cooperation." [3].
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The Semantic Web Layers
RDF-based XML for syntax
URI for naming
Research Issues OWL (W3C
Recommendation since February,
2004), OIL, DAML
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The Resource Description Framework (RDF)
Provides a base language for representing information about web resources
Provides a common framework for encoding, exchange, and reuse of structured metadata (or semantic annotation)
Uses Web identifiers (URI: URL, URN) to identify resources
Uses XML for the exchange and processing of metadata
Metadata is machine understandable Recently became a W3C Recommendation as of
February 10, 2004
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The RDF Data Model
Represents knowledge as statements in the form of a directed labeled graph of the triple <Subject, Predicate, Object>
Subject Resource: An entity; a class name or "any addressable
unit of information or service"[16] Predicate
Property, attribute, characteristics, verb Object
Value, Literal Defines resource in terms of property and value
PredicateSubject Object
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The RDF Model - Example
Represent “Tim Berners-Lee is the author of ‘Semantic Web Road map’ at http://www.w3.org/DesignIssues/Semantic”
<?xml version="1.0"?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:exterms=" http://purl.org/dc/terms#"> <rdf:Description rdf:about=" <rdf:Description rdf:about="
http://www.w3.org/DesignIssues/Semantic">http://www.w3.org/DesignIssues/Semantic"> <exterms:authour rdf:resource=“http://www.w3.org/People/Berners-<exterms:authour rdf:resource=“http://www.w3.org/People/Berners-
Lee/”>Lee/”> </rdf:Description> </rdf:RDF>
Statement
PropertyResource
Value
http://purl.org/dc/elements/1.1/author
http://www.w3.org/DesignIssues/Semantic http://www.w3.org/People/Berners-Lee/
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The RDF Model - Example
Extend representation to include “Tim Berners-Lee email [timbl @ w3.org] and address [200 Technology Square, Cambridge MA 02139]
http://purl.org/dc/elements/1.1/author
http://www.w3.org/DesignIssues/Semantic http://www.w3.org/People/Berners-Lee/
Address timbl@w3c.org
200 Technology Square Cambridge MA 02139
http://www.w3.org/2000/10/swap/pim/contact#address http://.../contact#mailbox
http://.../contact/address#cityandstate
http://.../contact/address#street
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The Resource Description Framework- Limitations
RDF provides framework for describing web resources using metadata
Programs can only understand web resources if they understand the meaning of the metadata and its variations in different RDF descriptions (e.g., Personnel vs. Employee, matriculation # vs. student id, LastName vs. Surname)
RDF is similar to defining table attributes in RDBMS
External agreement on meaning of annotations for a domain is required for interoperability and machine understandability
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The Resource Description Framework- Limitations
The Semantic Web needs support for ontologies.
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Ontology
Ontology “refers to the science of describing the kinds of entities in the world and how they are related” [5]
Ontology provides sources of precisely defined knowledge (vocabulary), which can be shared across applications and humans.
An Ontology consists of a hierarchical description of important concepts in a domain and possible constraints
“An explicit specification of a conceptualisation” [6]
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Ontology: Languages for the Semantic Web
Ontology Language Layer is on top of the Resource Description Framework (RDF)
DAML - DARPA Agent Markup Language OIL - Ontology Inference Layer DAML+OIL - "semantic markup language for
Web resources“ OWL - Web Ontology Language (revision of
DAML+OIL and is based on Description Logics) OWL is a recent W3C Recommendation
(February 2004) [7, 8].
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OWL Web Ontology Language
Designed to enable machine interpretability of Web contents (not for human interpretation).
Three sublanguages of OWL:
OWL Lite supports simple classification hierarchy and simple constraints. It is the easiest to implement and even provides a quick migration path for thesauri and other taxonomies.
OWL DL offers maximum expressiveness while retaining computational completeness and decidability. Full formalism of DL supported.
OWL Full offers maximum expressiveness with full syntactic liberty of the RDF without computational guarantees.
OWL Full
OWL DL
OWL Lite
RDF, etc.
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OWL Web Ontology Language
Offers extended vocabulary and a formal semantics for defining classes and properties, including cardinality (e.g. minCardinality, maxCardinality,
cardinality) equality (e.g. equivalentClass) relationships between classes (e.g. disjointWith,
unionOf) characteristics of properties (e.g. FunctionalProperty)
Example: Indicate that student has only one StudentIDrdfs:subClassOf
#Student owl:Restriction
#StudentID
1
owl:Class
owl:cardinality
owl:onProperty
rdfs:subClassOf
rdf:type
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OWL Web Ontology Language - Example
A CS Graduate Student is either in the Masters or PhD programme. The Masters programme is made up of Course work and Thesis. A student has to take at least four courses and the 74.722 course.
What is the OWL representation for this ontology?
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OWL Web Ontology Language - Example
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W3C Semantic Web Group Current Activity
W3C Lunched Phase 2 of the Semantic Web Activity! – February 25, 2004
Two new Working Groups have been formed. Best Practices and Deployment WG: “focused on
providing consensus-based guidance - including practical deployment recommendations, engineering guidelines, ontology / vocabulary development practices, educational material and effective demonstrations, designed to facilitate Semantic Web deployment.” [10]
RDF Data Access Working Group: Tasked with developing an RDF query language (with interoperability as SQL did for relational databases)
Activities continue in other Work Groups of the Semantic Web: RDF Core and Web Ontology WGs, the Semantic Web Interest Group, and the Semantic Web Coordination Group.
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Conclusion
The Web can only reach its full potential when programs can seamlessly process the vast information represented on the World Wide Web (WWW).
The Semantic Web, which is an extension of the current Web, is intended to address this need.
The W3C Semantic Web Activity group is driving the Semantic Web through: A global vision of a Layered architecture of standards and
technologies that achieves the Semantic Web Development of frameworks and languages to support the
semantic annotation of web resources and the development of ontologies.
And recently, provision of pedagogical support for seamless adoption of the standards and technologies
Development Tools and Industry solutions are already emerging based on the W3C Semantic Web Activity Group efforts
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References
[1] Marja-Riitta Koivunen and Eric Miller, W3C Semantic Web Activity. The proceedings of the Semantic Web Kick-off Seminar in Finland, November 2, 2001
[2] Tim Berners-Lee, Semantic Web Road map, September 1998. Available: http://www.w3.org/DesignIssues/Semantic.html
[3] Tim Berners-Lee, James Hendler, Ora Lassila, The Semantic Web, Scientific American, May 2001 http://www.w3.org/2002/07/swint.
[4] Eric Miller, Digital Libraries and the Semantic Web. Available: http://www.w3.org/2001/09/06-ecdl/slide17-0.html
[5] Michael K. Smith, Chris Welty,and Deborah L. McGuinness, W3C Recommendation: OWL Web Ontology Language Guide, February 2004. Available: http://www.w3.org/TR/owl-guide/
[6] M. Uschold, M. Gruninger, Ontologies: Principles, Methods and Applications. Knowledge Engineering Review 11(2) (1996).
[7] Web Ontology Language Home Page http://www.w3c.org/2001/sw/WebOnt/[8] Christel Kemke, 74.419 Artificial Intelligence Course Notes on Semantic Web, 2003[9] James Hendler, Tim Berners-Lee, and Eric Miller. Integrating Applications on the
Semantic Web, Journal of the Institute of Electrical Engineers of Japan, Vol 122(10), October, 2002, p. 676-680. Available: http://www.w3.org/2002/07/swint
[10] Semantic Web Home Page http://www.w3.org/2001/sw/ [11] RDF Home Page http://www.w3.org/RDF/[12] Web Ontology Language Home Page http://www.w3c.org/2001/sw/WebOnt/[13] Description Logics Home Page http://dl.kr.org/[14] DAML Home Page http://www.daml.org/[15] OIL Home Page http://www.ontoknowledge.org/oil/[16] W3C Home Page http://www.w3c.org/
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Q&A