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Standards for the Representation of Knowledge on the Semantic Web
Antoine ISAACSTITCH Project
Offene Archivierbare FormateOct. 25th, 2007
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Standards for the Representation of Knowledge on the Semantic Web
Agenda
• Interoperability problems in Cultural Heritage
• An introduction to the Semantic Web• The problem
• RDF
• RDFS/OWL
• Why is it interesting?
• Porting existing metadata to the Semantic Web• SKOS
• Conclusion: SW and semantic alignment
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Standards for the Representation of Knowledge on the Semantic Web
Agenda
• Interoperability problems in Cultural Heritage
• An introduction to the Semantic Web• The problem
• RDF
• RDFS/OWL
• Why is it interesting?
• Porting existing metadata to the Semantic Web• SKOS
• Conclusion: SW and semantic alignment
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Standards for the Representation of Knowledge on the Semantic WebThe Interoperability Problem in Cultural
Heritage
• STITCH• SemanTic Interoperability To access Cultural Heritage
• Here, CH at large (incl. Digital Libraries)
• Trend: simultaneous access to different collections• The European Library, Memory of the Netherlands
• Problem: how to access seamlessly different collections?
• Traditional solution: using object metadata • But…
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Standards for the Representation of Knowledge on the Semantic WebKB Illustrated Manuscripts
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Standards for the Representation of Knowledge on the Semantic WebKB Illustrated Manuscripts
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Standards for the Representation of Knowledge on the Semantic Web
Mandragore
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Standards for the Representation of Knowledge on the Semantic Web
Mandragore
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Standards for the Representation of Knowledge on the Semantic Web
The Interoperability Problems
From syntactic to semantic
• Different formats• “We have a solution”
• XML as a standard for data exchange
• Different metadata schemes• “Something is coming”
• Dublin Core for MD exchange
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Standards for the Representation of Knowledge on the Semantic Web
The Interoperability Problems
From syntactic to semantic (continued)
• Different conceptual vocabularies for description• “Do you really want to discuss about it now?”
• No standard vocabulary• DDC, UDC, SWD, LCSH, AAT, Iconclass and myriads of
others…
• Not even a common model for these Knowledge Organization Schemes (KOSs)
• thesauri, classification schemes, subject heading lists…
• Even worse: there are reasons for this!
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Standards for the Representation of Knowledge on the Semantic Web
MDS 2- Field 1
- Field 1.1- Field 1.2
- Field 1.2.1- Field 1.3
- Field 2- …
MDS 1- Field 1
- Field 1.1- Field 2
- Field 2.1- Field 2.2- …
The result
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Standards for the Representation of Knowledge on the Semantic Web
MDS 1- Field 1
- Field 1.1- Field 2
- Field 2.1- Field 2.2- …
MDS 2- Field 1
- Field 1.1- Field 1.2
- Field 1.2.1- Field 1.3
- Field 2- …
An Ideal Situation
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Standards for the Representation of Knowledge on the Semantic Web
Agenda
• Interoperability problems in Cultural Heritage
• An introduction to the Semantic Web• The problem
• RDF
• RDFS/OWL
• Why is it interesting?
• Porting existing metadata to the Semantic Web
• Conclusion: SW and semantic alignment
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Standards for the Representation of Knowledge on the Semantic Web
Why thinking of the Semantic Web?
• Cf Semantic Web activity page at W3C• http://www.w3.org/2001/sw/
• “The Semantic Web provides a common framework that allows data to be shared and reused”
• “The Semantic Web is a web of data”
• “It is about common formats for integration and combination of data drawn from diverse sources”
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Standards for the Representation of Knowledge on the Semantic Web
SW Problem: The Web for Humans
• A city
• A flag
• The city’s location
Meaning
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Standards for the Representation of Knowledge on the Semantic Web
SW Problem: The Web for Humans
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Standards for the Representation of Knowledge on the Semantic Web
SW Problem: The Web for Computers?
• Characters
• Images
Black boxes
• Markup
Layout/Display
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Standards for the Representation of Knowledge on the Semantic Web
SW Problem: The Web for Computers?
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Standards for the Representation of Knowledge on the Semantic Web
The Interoperability Problems in CH (reminder)
MDS 2- Field 1
- Field 1.1- Field 1.2
- Field 1.2.1- Field 1.3
- Field 2- …
MDS 1- Field 1
- Field 1.1- Field 2
- Field 2.1- Field 2.2- …
MDS 2- Field 1
- Field 1.1- Field 1.2
- Field 1.2.1- Field 1.3
- Field 2- …
MDS 1- Field 1
- Field 1.1- Field 2
- Field 2.1- Field 2.2- …
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Standards for the Representation of Knowledge on the Semantic WebThe Semantic Web Approach: A Web of
(Meta)data
subject
Amsterdam
par3
file1
Article
type
partOf
DocumentsubClassOfThe_Netherlands
hasCapital
City
type
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Standards for the Representation of Knowledge on the Semantic Web
A footnote
• Why “(meta)data”?
• Because what is metadata for certain applications can indeed be the data for the Semantic Web
• Boundary is blurred
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Standards for the Representation of Knowledge on the Semantic Web
Agenda
• Interoperability problems in Cultural Heritage
• An introduction to the Semantic Web• The problem
• RDF
• RDFS/OWL
• Why is it interesting?
• Porting existing metadata to the Semantic Web
• Conclusion: SW and semantic alignment
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Standards for the Representation of Knowledge on the Semantic Web
The Semantic Web (1/4)
• Pointing at resources• What? Knowledge objects, everything that we may
want to refer to (including documents)
• How? Uniform Resource Identifiers (incl. URLs)
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Standards for the Representation of Knowledge on the Semantic Web
A Web of Resources
myVoc2:Amsterdam
http://ex.org/files/file1#par3
http://ex.org/files/file1
myVoc1:Article
http://www.ned.nl/rep321
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Standards for the Representation of Knowledge on the Semantic Web
The Semantic Web (2/4)
• Pointing at resources: URIs
• Creating structured assertions involving resources• What? Structured assertions with typed links
• How? RDF (Resource Description Framework)
Factual knowledge encoded as “triples”subject – predicate (property) – object
myVoc1:subject
myVoc2:Amsterdam
http://ex.org/files/file1#par3
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Standards for the Representation of Knowledge on the Semantic Web
Data in an RDF “graph”
myVoc1:subject
myVoc2:Amsterdam
http://ex.org/files/file1#par3
http://ex.org/files/file1
myVoc1:partOf
myVoc1:Article
rdf:type
http://www.ned.nl/rep321
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Standards for the Representation of Knowledge on the Semantic Web
Agenda
• Interoperability problems in Cultural Heritage
• An introduction to the Semantic Web• The problem
• RDF
• RDFS/OWL
• Why is it interesting?
• Porting existing metadata to the Semantic Web
• Conclusion: SW and semantic alignment
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Standards for the Representation of Knowledge on the Semantic Web
The Semantic Web (3/4)
• Pointing at resources: URIs
• Enabling structured assertions: RDF
• Giving machine-understandable semantics to “building blocks” • What? Ontologies
• “Formal definitions of shared conceptual vocabularies”
• Giving semantics for properties and classes
• How? RDFS /OWL (Ontology Web Language)
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Standards for the Representation of Knowledge on the Semantic Web
RDF Schema (RDFS)
• Meta-language to create vocabularies• “Article” is an (RDFS) Class
• Denotes a type, a collection of resources (individuals)
• “subject” is an (RDFS) Property
• Giving semantics to vocabulary elements• My “Article” has the literal article as a label for
display• myVoc1:Article rdfs:label “article”
• “Article” is a subclass of the class “Document”• myVoc1:Article rdfs:subClassOf myVoc1:Document
• “subject” is applied to resources of type “Document”• myVoc1:Article rdfs:domain myVoc1:Document
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Standards for the Representation of Knowledge on the Semantic Web
RDF Schema (RDFS)
• Different kind of constructs• Assigning domain and ranges of properties
• Creating hierarchies of classes and properties
• Labels and informal specifications
• (Some) Equipped with formal semantics• R rdf:type C1, C1 rdfs:subClass C2 -> X rdf:type C2
• P rdfs:domain C, R1 P R2 -> R1 rdf:type C
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Standards for the Representation of Knowledge on the Semantic Web
Web Ontology Language (OWL)
• Same function as RDFS, but more possibilities, e.g.• Characteristics of properties
• Inverse(hasAuthor, authorOf)
• Restriction on property usage• SubClassOf(Books, restriction(hasISBN minCardinality(1)))
• Combination and exclusion of classes and properties• DisjointClasses(Persons, Books)
• Inherits from AI research and Description Logics
• Comes in different levels of complexity:• Lite, DL, Full
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Standards for the Representation of Knowledge on the Semantic Web
Tools to build RDFS/OWL ontologies
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Standards for the Representation of Knowledge on the Semantic Web
Ontological information
myVoc1:subject
myVoc2:Amsterdam
http://ex.org/files/file1#par3
http://ex.org/files/file1
myVoc1:Article
rdf:type
myVoc1:partOf
myVoc1:Documentrdfs:subClassOf
http://www.ned.nl/rep321
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Standards for the Representation of Knowledge on the Semantic Web
The Semantic Web (4/4)
• Pointing at resources: documents, knowledge objects
• Enabling structured assertions
• Using “building blocks” with precise semantics
• Controlling existing facts, inferring new onesPart of the tasks are delegated from the user to
inference engines that use the formal semantics of ontologies
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Standards for the Representation of Knowledge on the Semantic Web
Ontological information
myVoc1:subject
myVoc2:Amsterdam
http://ex.org/files/file1#par3
http://ex.org/files/file1
myVoc1:Article
rdf:type
myVoc1:partOf
myVoc1:Documentrdfs:subClassOf
http://www.ned.nl/rep321
rdf:type
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Standards for the Representation of Knowledge on the Semantic Web
RDFS/OWL and Semantic Interoperability
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Standards for the Representation of Knowledge on the Semantic Web
Agenda
• Interoperability problems in Cultural Heritage
• An introduction to the Semantic Web• The problem
• RDF
• RDFS/OWL
• Why is it interesting?
• Porting existing metadata to the Semantic Web
• Conclusion: SW and semantic alignment
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Standards for the Representation of Knowledge on the Semantic Web
Why is it interesting?
• RDF model is simple• Just triples
• There is meaning exploitable by computers
• Resources are universal, hence shareable• One resource for one object, used in different places
• Vocabularies for (meta)data are made of resources• They can be re-used in different applications
• RDF does not enforce the use of a specific ontology
• Their meaning (incl. formal semantics) is shareable
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Standards for the Representation of Knowledge on the Semantic Web
Building on top of the Web
• Web-based resources allow distribution/sharing of • document
• vocabulary
• (meta)data
(par3, subject, Amsterdam)
differentowners & locations
http://www.kb.nl/eDepot
http://www.geo.org/voc/
http://www.ned.nl/rep321
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Standards for the Representation of Knowledge on the Semantic Web
Why is it interesting?
• Using open standards• W3C’s URI, XML, RDF, RDFS, OWL
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Standards for the Representation of Knowledge on the Semantic Web
Footnote: Building on top of XML
<rdf:Description rdf:about=”http://www.ned.nl/doc321”> <myVoc1:subject rdf:resource=” http://www.geo.org/Amsterdam”/></rdf:Description><rdf:Description rdf:about=”http://www.geo.org/The_Netherlands”> <myVoc2:hasCapital rdf:resource=”http://www.geo.org/Amsterdam”/></rdf:Description>
• RDF can be encoded as XML data• RDF/XML is the reference syntax, but others are
possible
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Standards for the Representation of Knowledge on the Semantic Web
Agenda
• Interoperability problems in Cultural Heritage
• An introduction to the Semantic Web• The problem
• RDF
• RDFS/OWL
• Why is it interesting?
• Porting existing (meta)data to the Semantic Web• SKOS
• Conclusion: SW and semantic alignment
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Standards for the Representation of Knowledge on the Semantic Web
Problem: Data Population
• How will Semantic Web data will be created?• Creation of “born-semantic” data?
• Automatic or manual (tagging)
• Converting existing databases to SW format• Fits the vision of the SW as a place to exchange data
• In the CH situation: porting legacy metadata is fundamental
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Standards for the Representation of Knowledge on the Semantic Web
Porting CH Metadata to the Semantic Web
• Requirement: an ontology to create SW-enabled representations for metadata• “Ontologized” metadata schema
• A first candidate: Dublin Core for metadata schema• Well-established set of metadata elements
• Already coming in RDFS!
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Standards for the Representation of Knowledge on the Semantic Web
Porting KOSs to the Semantic Web
• How about metadata values from Knowledge Organization Schemes?• E.g. dc:subject values (terms, keywords, classes…)
• DC does not address the problem of KOS representation
• Why is it important?• Their heterogeneity is a primary source of
interoperability problems
• They are provided with (informal) semantics• Taxonomies, associative networks can be exploited in
many applications
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Standards for the Representation of Knowledge on the Semantic Web
Porting KOSs to the Semantic Web
• A first solution: converting KOSs to formal ontologies• Ontologization of terms/concepts into classes
• Problem: KOSs are generally no full-fledged ontologies• Iconclass: “Group of Birds” rdfs:subClassOf “Birds”?
• There is much work needed to have semantics fit!
• The concept of a car (reference=a subject in a KOS)
vs. the class of cars (reference=a set of objects in the world)
• Things in ontologies and KOSs don’t have the same epistemological status
• We need a model for elements of the realm of subjects
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Standards for the Representation of Knowledge on the Semantic Web
Representing KOSs – Requirements
Many different models and formats to represent vocabularies
• Need for standard formats to develop standardized tools and methods• Semantic correspondences
• Browsing/information retrieval tools using vocabularies
• Need to represent features commonly used by these tools• Especially lexical information and semantic links
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Standards for the Representation of Knowledge on the Semantic Web
SKOS (Simple Knowledge Organisation System)
• Model to represent KOSs (thesauri, classification schemes) on the Semantic Web in a simple way• Comparable to Dublin Core, for conceptual vocabularies
• SKOS offers building blocks to create XML/RDF data• Concepts and ConceptSchemes
• Lexical properties (prefLabel, altLabel)
• Semantic relations (broader, related)
• Notes (scopeNote, definition)
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Standards for the Representation of Knowledge on the Semantic Web
SKOS: Iconclass Example
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Standards for the Representation of Knowledge on the Semantic Web
SKOS: Limitations
• SKOS is a standard• Simple• Meant for information exchange and re-use
• Not everything can be represented!E.g. for Iconclass, difficulty to represent all types of
auxiliaries• Keys, structural digits…
• It is still work in progress• W3C Semantic Web Deployment Working Group
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Standards for the Representation of Knowledge on the Semantic Web
Agenda
• Interoperability problems in Cultural Heritage
• An introduction to the Semantic Web• The problem
• RDF
• RDFS/OWL
• Why is it interesting?
• Porting existing metadata to the Semantic Web• SKOS
• Conclusion: SW and semantic alignment
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Standards for the Representation of Knowledge on the Semantic Web
What have we seen?
• TODO
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Standards for the Representation of Knowledge on the Semantic Web
Back to the Problem: Semantic Alignment
• Different ontologies/individuals should be aligned at the semantic level• Using the same resources to join SW graphs together
• Using the same vocabularies and semantics
• But: difficulty to recognize equivalent resources at data creation time• There is (and will be) no such thing as a single one
ontology!
• A posteriori semantic alignment is needed
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Standards for the Representation of Knowledge on the Semantic Web
Back to the Problem: semantic alignment
• Fortunately, SW languages give appropriate means• Equivalence/specialization links for properties and classes
• myVoc:auteur rdfs:subPropertyOf dc:creator
• myVoc:Article owl:equivalentClass yourVoc:Artikel
• Identity link for individuals• vu:aisaac owl:sameAs kb:AntoineIsaac
• (yet unstable) SKOS mapping links for subjects• iconclass:birds exactMatch swd:vogel
• But they don’t do the job for us!• The links have to be created somehow
• This is another story…
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Standards for the Representation of Knowledge on the Semantic Web
Thank you!
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Standards for the Representation of Knowledge on the Semantic Web
Vocabulary alignment
• Find correspondences between vocabulary elements• “klassieke ruïnes” ≈ “landschap met ruïnes”
• “maagd Maria” = “Heilige Moeder”
• STITCH aim: doing it (semi-)automatically• Vocabularies are big
• They evolve over time
• Using techniques from Semantic Web research domain• Problem comparable to ontology alignment
• Techniques already investigated there• Linguistics, statistics
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Standards for the Representation of Knowledge on the Semantic Web
Automatic alignment techniques
• Lexical • Structural• Statistical• Background knowledge
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Standards for the Representation of Knowledge on the Semantic Web
Lexical alignment
• Labels of entities, textual definitions
tumorbrainLong tumor LongMore specific than
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Standards for the Representation of Knowledge on the Semantic Web
Automatic Alignment Techniques
• Lexical • Structural• Statistical• Background knowledge
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Standards for the Representation of Knowledge on the Semantic Web
Statistical alignment
• Object information (e.g. book indexing)
Thesaurus 1
Thesaurus 2
Collectionof books
“DutchLiterature”
“Dutch”
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Standards for the Representation of Knowledge on the Semantic Web
Automatic Alignment Techniques
• Lexical • Structural• Statistical• Background knowledge
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Standards for the Representation of Knowledge on the Semantic Web
Backgroundknowledge
Alignment using shared background knowledge
• Using a shared conceptual reference to find links
Thesaurus 1 Thesaurus 2
“Calendar”
“Publication”
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Standards for the Representation of Knowledge on the Semantic Web
Alignment: no universal solution
• No single technique gives an ideal solution
• Different techniques have to be selected/combined, depending on the application case• Poor vs. rich semantic structure
• Extensive vs. limited lexical coverage
• Existence of collections described by several vocabularies
• Alignment is a difficult research problem
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Standards for the Representation of Knowledge on the Semantic Web
Conclusions : Alignement
• Les techniques simples permettent d'obtenir des résultats rapides• 12300 concepts de Mandragore “accessibles” depuis
Iconclass
• Leur fiabilité ne permet pas de les considérer comme sources uniques• Combinaison avec travail manuel (vérification, complétion)
• L’alignement sémantique est toujours un problème de recherche difficile• Aucune technique n’est parfaite• Il faut sélectionner/combiner, en fonction des cas applicatifs
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Standards for the Representation of Knowledge on the Semantic Web
Demo
• http://prauw.cs.vu.nl/rp33333/MANDRA-SV-ICE-mandraNewNONE , amphibiens
• Blé
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Standards for the Representation of Knowledge on the Semantic Web
Conclusions : Représentation
• Il est possible de produire des représentations WS standardisées (SKOS) des vocabulaires conceptuels• Et des méta-données qui les utilisent
• Les techniques existantes pour accéder aux méta-données et vocabulaires (OAI-PMH, XML) facilitent le travail
• C’est utile • Réutilisation/interopérabilité des composants applicatifs
utilisant les vocabulaires
• Facilité de la représentation de liens avec des éléments extérieurs au vocabulaire représenté
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Standards for the Representation of Knowledge on the Semantic Web
Links
• STITCH http://stitch.cs.vu.nl• Demo collections
• BNF Mangragore http://mandragore.bnf.fr• KB illuminated manuscripts http://www.kb.nl/manuscripts/
• Library-originated integration projects:• MSAC search interface http://sigma.nkp.cz• MACS project http://macs.cenl.org
• Semantic web links• Semantic Web at W3C http://www.w3.org/2001/sw/• SKOS http://www.w3.org/2004/02/skos/
• Semantic Web projects dealing with Cultural Heritage• MuseumFinland http://www.museosuomi.fi/ • eCulture
http://e-culture.multimedian.nl/
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Standards for the Representation of Knowledge on the Semantic WebDemo (1)
Subject vocabulary, collection 1
Subjects
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Standards for the Representation of Knowledge on the Semantic Web
Demo (2)
Hierarchical path from root to selected
subject
Possible specialization for selected subject
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Standards for the Representation of Knowledge on the Semantic Web
Document from Collection 2
Semantic alignment of subjects activated
Demo (3)
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Standards for the Representation of Knowledge on the Semantic Web
Demo (4)
Subject from voc2 aligned to voc1:amphibians”
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