Knowledge Representation on the Semantic Web

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22 Οκτ 2013 (πριν από 3 χρόνια και 1 μήνα)

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74.793 Natural Language and Speech Processing

Professor: Dr. Christel Kemke

Winter 2004


Knowledge Representation on the Semantic Web


by

Femi G. Olumofin

Department of Computer Science

University of Manitoba,

Winnipeg, Canada



March 8, 2004

Tuesday, October 22, 2013

2

Outline


Introduction


The Semantic Web Layers


Resource Description Framework (RDF)


Ontology


OWL Web Ontology Language


Semantic Web Current Activity


Conclusion

Tuesday, October 22, 2013

3

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]


Tuesday, October 22, 2013

4

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

Tuesday, October 22, 2013

5

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].


Tuesday, October 22, 2013

6

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].

Tuesday, October 22, 2013

7

The Semantic Web Layers

RDF
-
based

XML for syntax

URI for naming

Research
Issues

OWL (W3C
Recommendation
since February,
2004), OIL, DAML

Tuesday, October 22, 2013

8

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

Tuesday, October 22, 2013

9

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

Predicate

Subject

Object

Tuesday, October 22, 2013

10

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=" http://www.w3.org/DesignIssues/Semantic">


<exterms:authour rdf:resource=“
http://www.w3.org/People/Berners
-
Lee/
”>


</rdf:Description>


</rdf:RDF>


Statement

Property

Resourc
e

Value

http://purl.org/dc/elements/1.1/author


http://www.w3.org/DesignIssues/Semantic

http://www.w3.org/People/Berners
-
Lee/

Tuesday, October 22, 2013

11

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

Tuesday, October 22, 2013

12

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

Tuesday, October 22, 2013

13

The Resource Description Framework
-

Limitations



The Semantic Web needs support for
ontologies
.


Tuesday, October 22, 2013

14

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]

Tuesday, October 22, 2013

15

Ontology: Languages for the Semantic Web


Ontology Language Layer is on top of the Resource
Description Framework (RDF)


DAML
-

D
ARPA
A
gent
M
arkup
L
anguage


OIL
-

O
ntology
I
nference
L
ayer


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].

Tuesday, October 22, 2013

16

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.

Tuesday, October 22, 2013

17

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 StudentID

rdfs:subClassOf


#Student

owl:Restriction

#StudentID

1

owl:Class

owl:cardinality


owl:onProperty


rdfs:subClassOf


Tuesday, October 22, 2013

18

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?

Tuesday, October 22, 2013

19

OWL Web Ontology Language
-

Example

Tuesday, October 22, 2013

20

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.


Tuesday, October 22, 2013

21

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

Tuesday, October 22, 2013

22

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/

Tuesday, October 22, 2013

23

Q&A