Semantic Web: From Representations to Applications

Guus SchreiberFree University AmsterdamCo-chair W3C Web Ontology Working GroupCo-chair W3C Semantic Web Best Practices

and Deployment Working Group

Overview

Representations: Reflections on the making of OWL and its

relation to RDF Using representations

Best practices (as far as we know then now) to help application developers

Applications Examples from the SWBPD weblog

Disclaimer

This presentation describes work of many different people, including many participating in respective W3C Working Groups as well as others

W3C Web Ontology Working Group Chartered to develop the Ontology

Vocabulary for the Semantic Web. Starting point: DAML+OIL Started in November 2001 Factions:

logicians (Description Logic, KIF) knowledge/ontology engineers RDF developers

OWL Recommendation published 10 February 2004

Working group communication Mailing lists

working-group list: 8,000 messages in two years public comments list: 600 messages in 18

months Telecons

60 telecons of 60-90 minutes with 10-30 people simultaneous scribing in IRC (chat) channel

Face-to-face meetings five two-day meetings during first 15 months

All proceedings in the public domain:http://www.w3.org/2001/sw/WebOnt

Key issue to be resolved: 5.3 Semantic Layering

OWL is expected to be semantically compatible with RDF(S).

Problems were foreseen with aligning a DL-style model theory with the RDF model theory

The Semantic Layering debate

Excerpt from a telecon debate:

"You are not creating a semantic web, but semantic islands with high fences"

"But your are creating a semantic swamp, with crocodiles and snakes"

What do you prefer?

Consensus on Semantic Layering

OWL Full ("Large OWL", "Great Horned OWL") Free mixing of OWL and RDF = high expressivity Non-standard formalization Tractability not guaranteed

OWL DL ("Fast OWL") Maximize expressiveness while retaining tractability Standard formalization Same language constructs as OWL Full Constraints on RDF/OWL vocabulary use

Correspondence theorem links the two styles of semantics: entailments in OWL DL also hold in OWL Full.

RDF/OWL schema constructs RDFS Schema

(sub)classes, (sub)properties, domain, range, datatypes (using XML Schema)

OWL Lite cardinality 0/1, local property restrictions,

inverse/transitive/symmetric properties, (in)equality of classes/instances

OWL DL enumeration, disjunction, conjunction, negation,

hasValue OWL Full

meta-modeling

Is RDF/OWL just another datamodelling/KR language?

Key differences: All classes/properties/individuals have a URI as

identifier RDF/XML exchange syntax enables

interoperabilityXML features

UTF-8 character set Support for multilinguality Use of XML Schema datatypes: numeric, date,

time, etc.For the rest: RDF/OWL is state-of-the-art

concept language

Semantic Web Best Practices and Deployment Working Group Started 1 March 2004 => early days Co-chairs David Wood and Guus Schreiber 30+ participants Objective: support for semantic-web

application developer Focus on “low hanging fruit” Publication of key ontologies/vocabularies,

development guidelines, ontology-design patterns, repositories, links to related techniques, ……

High expectations, not much effort (yet)

Issues for publishing ontologies:good and bad ontologies?!

Good ontologies are used Good ontologies represent some form

of consensus in a community Good ontologies are maintained Good ontologies do not need to be

complex Good ontologies may contain

“mistakes”

ontology = community consensus

N.B.It is a contradiction in terms to talk about “creating my own ontology”!

Source: Financial Times, e-procurement, Oct. 2000

Thesauri and ontologies

ISWC’03 Semantic Web Challenge showed that thesauri are important resources for SW applications

Typically weak semantic structure Approach in Best Practices WG:

Phase 1: “as-is” conversion Phase 2: additional ontological

interpretations/extensions

Human-readable syntax?!<owl:Class rdf:ID="MozartDaPonteOpera"> <owl:equivalentClass> <owl:Class> <owl:oneOf rdf:parseType="Collection"> <Opera rdf:about="#NozzDiFigaro"/> <Opera rdf:about="#DonGiovanni"/> <Opera rdf:about="#CosiFanTutte"/> </owl:oneOf> </owl:Class> </owl:equivalentClass></owl:Class>

OWL abstract syntax

Used in a (very) sloppy fashion in this presentation

Developed for specifying the OWL DL semantics

UML Profile for OWL Under

development at OMG

Not trivial, e.g. RDF/OWL properties are different from UML associations

N3 Turtle syntax See note by Dave Beckett (Bristol)

:pressureInHg a owl:ObjectProperty; rdfs:domain :DiastolicBloodPressure; rdfs:range xsd:nonNegativeInteger.

@prefix owl: <http://www.w3.org/2002/07/owl#>.

@prefix xsd: <http://www.w3.org/2001/XMLSchema#>.

Ontology engineering patterns Best practices for frequently occurring

modeling problems WG documents outline alternatives with

pros and cons Currently three notes published:

Classes as values N-ary relations Specification of value sets

Planned: Part-of, numeric constraints, QCRs

Representing value sets Intuitive representation of color value set:

class/datatype color with instances/values “red”, “white”, etc.

But suppose we want to talk about a subtype of “red”, e.g. “vermillion red”

Pattern: Represent values as subclass hierarchy of value type This preserves flexibility Use anonymous instances as property values

“This porcelain vase has as color some value of vermillion red”

See note by Alan Rector http://www.w3.org/TR/swbp-specified-values/

Classes as values Common problem when using a hierarchy

of concepts for indexing purposes Example: indexing books with concepts

from the ACM computer-science subject hierarchy

See draft technical note by Noy: Four options with different merits

See note by Natasha Noy:http://www.w3.org/TR/swbp-classes-as-values/

Numeric constraints and user-defined XML Schema datatypes Example: “an elevated diastolic blood

pressure is a diastolic blood pressure of 90 Hg or more”

Currently no simple way to represent this in OWL

User-defined XML Schema datatypes could provide a solution

Currently not possible for detailed technical reasons

SWBPD task force is active in trying to solve this problem (Jeremy Carroll, HP)

Representing a numeric constraintthrough a datatypeClass(DiastolicBloodPressure)Propert(pressureInHg domain(DiastolicBloodPressure) range(xsd:nonNegativeInteger))Class(ElevatedDiastolicBloodPressure subClassOf(DiastolicBloodPressure) subClassOf(Restriction onProperty(pressureInHg) allValuesFrom(ex:NinetyPlus))

Plus corresponding definition for the ex:NinetyPlus user-defined datatype

Pervasive issue: metamodelling

OWL DL requires strict separation of classes and instances

But on the Semantic Web my instances may be your classes!

Metamodelling features especially required in vocabulary/ontology mapping and/or interpretation

Cf. Protégé metamodelling facilities

RDF in XHTML

How to mark up your (X)HTML page? Various proposals under discussion in

RDF-in-XHTML Task Forcehttp://lists.w3.org/Archives/Public/public-rdf-in-xhtml-tf/2004Oct/

Link typing using “rel” attribute?! Consequences for HTTP GET?!

Other work (planned) in the W3C Best Practices WG: a selection

Tutorial pagehttp://www.w3.org/2001/sw/BestPractices/Tutorials

Tools page May just build on work of others, e.g. see DAML

tool-assessment studyhttp://semwebcentral.org/

Publication of vocabularies/ontologies WordNet is first on the list Units and measures is likely next target

Links to MPEG, Topic Maps

Applications and demo’s

Webloghttp://esw.w3.org/mt/esw/archives/cat_applications_and_demos.html

Four examples1. AKTive Space: CS research in the UK2. DOPE: Drug Ontology Project at Elsevier3. Building Finder (USC)4. Finnish Museums on the Web

AKTive Space AKT project (Shadbolt et al. ), winner

Semantic Web Challenge 2003) Integration of heterogeneous sources

Papers, researchers, projects 430 Mb in total

RDF/OWL used for syntactic interoperability Storage/access issues

Schema mapping is required Referential integrity is a problem

Use of owl:sameAs Use automatic techniques in combination with

user approval

DOPE: thesaurus-based search of large document repositories Stuckerschmidt et al. (2003) EMTREE thesaurus (MesSH-based) Documents

5M Medline abstracts 500K full-text articles of Elsevier

Automatic document indexing RDF used for syntactic interoperability

RDF wrapper for SOAP-based access to documents Disambiguation of search terms Visualization of search results through semantic

categories Needed to prevent information overflow

Building Finder: integrating image analysis and textual sources

Knoblock et al. (USC/ISI) Multiple heterogeneous sources

Satellite images (Microsoft Terraservice) Road map info (US) Address information (white pages)

Image analysis techniques to map satellite data to road map

RDF used for syntactic interoperability

Finnish Museums on the Semantic Web

Hyvonen et al. (2004) Multiple museum collections Indexed with multiple ontologies

Artifact, material, actor, location, time, event

RDF used for syntactic interoperability Ontologies used for query

specialization/generalization

Cultural heritage collections: possible use case A person is interested

in Fauve paintings There is a digital

collection with images of paintings of Andre Derain

The Derain images match the query, despite the fact that “Fauve does not appear in the annotation.

Application issues (1) Public domains are promising application

areas Medicine, cultural heritage, digital libraries Many existing vocabularies & annotations Application-pull

Information integration/presentation is prime use case

Multimedia is important focus Requires multi-disciplinary approach

Application issues (2)

Free access to vocabularies / ontologies is a real problem AAT, EuroWordNet

Similar applications can be built for company intranets

NOTE for academics: be conscious of unfair criticism of application papers

More information

See home page of the Best Practices WG:

http://www.w3.org/2001/sw/BestPractices/

All proceedings are public

Related European effort:IST Knowledge Web network

http://knowledgeweb.semanticweb.org Objectives (selection):

Research integration Summer schools Educational material Showcase applications Industrial dissemination

Started 1-1-2004 and runs for four years