74.793 Natural Language and Speech Processing Professor: Dr. Christel Kemke Winter 2004 Knowledge...

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

Tuesday, April 18, 2023 2

Outline

Introduction

The Semantic Web Layers

Resource Description Framework (RDF)

Ontology

OWL Web Ontology Language

Semantic Web Current Activity

Conclusion


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]


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


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].


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].


The Semantic Web Layers

RDF-based XML for syntax

URI for naming

Research Issues OWL (W3C

Recommendation since February,

2004), OIL, DAML


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


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


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/


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 [email protected]

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


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


The Resource Description Framework- Limitations

The Semantic Web needs support for ontologies.


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]


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].



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.



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


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?


OWL Web Ontology Language - Example


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.


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


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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