Standards for the
Representation of Knowledge
on the Semantic Web
Antoine ISAAC
STITCH Project
eScience Seminare
Oct. 25th, 2007
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 useful?
• Porting existing metadata to the Semantic Web
• SKOS
• Conclusion: SW and semantic alignment
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 useful?
• Porting existing metadata to the Semantic Web
• SKOS
• Conclusion: SW and semantic alignment
Standards for the Representation of Knowledge on the Semantic Web
The Interoperability Problem in Cultural Heritage
• STITCH
• SemanTic Interoperability To access Cultural Heritage
• Here, CH at large (libraries, archive)
• 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…
Standards for the Representation of Knowledge on the Semantic Web
KB Illustrated Manuscripts
Standards for the Representation of Knowledge on the Semantic Web
KB Illustrated Manuscripts
Standards for the Representation of Knowledge on the Semantic Web
Mandragore
Standards for the Representation of Knowledge on the Semantic Web
Mandragore
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 schemas
• “Something could be used”
• Dublin Core for simple MD publication
Standards for the Representation of Knowledge on the Semantic Web
The Interoperability Problems
From syntactic to semantic (continued)
• Different conceptual vocabularies for description
• Knowledge Organisation Schemes (KOSs)
• thesauri, classification schemes, subject heading lists…
• “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: classes, terms, concepts…
• Even worse: there are reasons for this!
Standards for the Representation of Knowledge on the Semantic Web
The Result
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
-…
Standards for the Representation of Knowledge on the Semantic Web
An Ideal Situation
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
-…
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 useful?
• Porting existing metadata to the Semantic Web
• Conclusion: SW and semantic alignment
Standards for the Representation of Knowledge on the Semantic Web
Why the Semantic Web can be Relevant
• 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”
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
Standards for the Representation of Knowledge on the Semantic Web
SW Problem: The Web for Humans
Standards for the Representation of Knowledge on the Semantic Web
SW Problem: The Web for Computers?
• Characters
• Images
Black boxes
• Markup
Layout/Display
Where is
meaning?
Standards for the Representation of Knowledge on the Semantic Web
SW Problem: The Web for Computers?
Standards for the Representation of Knowledge on the Semantic Web
The Interoperability Problems in CH (Reminder)
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
-…
MDS 2
- Field 1
- Field 1.1
- Field 1.2
- Field 1.2.1
- Field 1.3
- Field 2
-…
Standards for the Representation of Knowledge on the Semantic Web
The Semantic Web Approach: A Web of (Meta)data
Article
The_Netherlands
Document
subClassOf
type
hasCapital
file1
Amsterdam
partOf
type
defines
City
par3
Standards for the Representation of Knowledge on the Semantic Web
Footnote
• Why “(meta)data”?
• Because what is metadata for certain applications can
indeed be the data for the Semantic Web
• No clear boundary
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 useful?
• Porting existing metadata to the Semantic Web
• Conclusion: SW and semantic alignment
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)
Standards for the Representation of Knowledge on the Semantic Web
A Web of Resources
myVoc1:Article
http://ex.org/files/file1
myVoc2:Amsterdam
http://ex.org/files/file1#par3
http://www.ned.nl/rep321
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? Typed links between resources
• How? RDF (Resource Description Framework)
Data model, with descriptions encoded as “triples”
subject – predicate (property) – object
http://ex.org/files/file1#par3
myVoc1:defines
http://www.w3.org/TR/rdf-primer/
myVoc2:Amsterdam
Standards for the Representation of Knowledge on the Semantic Web
Data in an RDF “Graph”
myVoc1:Article
rdf:type
http://ex.org/files/file1
myVoc2:Amsterdam
myVoc1:partOf
myVoc1:defines
http://ex.org/files/file1#par3
http://www.ned.nl/rep321
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 useful?
• Porting existing metadata to the Semantic Web
• Conclusion: SW and semantic alignment
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 graph
“building blocks”
• What? Ontologies
• “Formal definitions of shared conceptual vocabularies”
• Giving semantics for properties and classes
• How? RDFS /OWL (Ontology Web Language)
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
• Give semantics to vocabulary elements
• Doing so using RDF itself
http://www.w3.org/TR/rdf-primer/
Standards for the Representation of Knowledge on the Semantic Web
RDF Schema (RDFS)
• Assigning domain and ranges of properties
• “defines” is applied to resources of type “Document”
• myVoc1:defines rdfs:domain myVoc1:Document
• Creating hierarchies of classes and properties
• “Article” is a subclass of the class “Document”
• myVoc1:Article rdfs:subClassOf myVoc1:Document
• Labels and informal specifications
• My “Article” has the literal article as a label for display
• myVoc1:Article rdfs:label “article”
Standards for the Representation of Knowledge on the Semantic Web
Ontological Information
rdfs:subClassOf
myVoc1:Article
myVoc1:Document
rdf:type
http://ex.org/files/file1
myVoc2:Amsterdam
myVoc1:partOf
myVoc1:defines
http://ex.org/files/file1#par
3
http://www.ned.nl/rep321
Standards for the Representation of Knowledge on the Semantic Web
RDF Schema (RDFS)
• (Some) constructs are 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
Standards for the Representation of Knowledge on the Semantic Web
The Semantic Web (4/4)
• Pointing at resources: URIs
• Enabling structured assertions: RDF
• Using “building blocks” with precise semantics
• Controlling existing facts, inferring new ones
Part of the tasks are delegated from the user to inference
engines that use the formal semantics of ontologies
Standards for the Representation of Knowledge on the Semantic Web
Reasoning
rdfs:subClassOf
myVoc1:Article
rdf:type
myVoc1:Document
rdf:type
http://ex.org/files/file1
myVoc2:Amsterdam
myVoc1:partOf
myVoc1:defines
http://ex.org/files/file1#par3
http://www.ned.nl/rep321
Standards for the Representation of Knowledge on the Semantic Web
Web Ontology Language (OWL)
• Same function as RDFS, but more possibilities
• Characteristics of properties
• Inverse(defines, isDefinedBy)
• Restriction on property usage
• EquivalentClass(Definition,
• restriction(defines minCardinality(1)))
• Combination and exclusion of classes and properties
• DisjointClasses(Persons, Document)
• Inherits from AI research and Description Logics
• Different levels of complexity: Lite, DL, Full
http://www.w3.org/TR/owl-guide/
Standards for the Representation of Knowledge on the Semantic Web
Tools to build RDFS/OWL ontologies
http://protege.stanford.edu/
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 useful?
• Porting existing metadata to the Semantic Web
• Conclusion: SW and semantic alignment
Standards for the Representation of Knowledge on the Semantic Web
Why is it Useful?
• RDF model is simple: just triples!
• Semantics is 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
• Vocabularies can be re-used
• RDF does not enforce the use of a specific ontology
• Their meaning (incl. formal semantics) is shareable
Standards for the Representation of Knowledge on the Semantic Web
Building on Top of the Web
• Web-based resources allow
distribution/sharing of
• document
• description vocabularies
http://www.geo.org/voc/
• (meta)data
(par3, defines, Amsterdam)
http://www.kb.nl/eDepot
http://www.ned.nl/rep321
different
owners & locations
Standards for the Representation of Knowledge on the Semantic Web
Why is it Useful?
• Based on open standards
• W3C’s URI, XML, RDF, RDFS, OWL
Standards for the Representation of Knowledge on the Semantic Web
Footnote: Building on top of XML
• RDF can be encoded as XML data
<rdf:Description rdf:about=”http://www.ned.nl/doc321”>
<myVoc1:defines 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/XML is the reference syntax, but others are
possible
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 useful?
• Porting existing (meta)data to the Semantic Web
• SKOS
• Conclusion: SW and semantic alignment
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 semantic annotation
• Converting existing data bases to SW format
• The SW as a place to publish and exchange existing data
• In CH case: porting legacy metadata is crucial
Standards for the Representation of Knowledge on the Semantic Web
Porting CH Metadata to the Semantic Web
• Requirement: an ontology to create SW
representations for metadata
• “Ontologised” metadata schemas
• A first candidate: Dublin Core
• Well-established set of metadata elements
• Already coming in RDFS!
Standards for the Representation of Knowledge on the Semantic Web
Porting KOSs to the Semantic Web
• How about metadata values from Knowledge
Organisation Schemes?
• E.g. dc:subject values (terms, keywords, classes…)
• DC does not address the problem of KOS representation
• Why is it important?
• Taxonomies, thesauri, etc. give (informal) semantics
• Commitment on how objects are described and accessed
• Useful for designing applications
• Their heterogeneity is a primary source of interoperability
problems
Standards for the Representation of Knowledge on the Semantic Web
Porting KOSs to the Semantic Web
• A first solution: converting KOSs to formal ontologies
• “Ontologisation” of terms/concepts into RDFS/OWL 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!
• Different status: the concept of a car (a subject in a KOS)
vs. the class of cars (a set of objects in the world)
• We need a model for elements of the realm of subjects
Standards for the Representation of Knowledge on the Semantic Web
SKOS (Simple Knowledge Organisation System)
• Model to represent KOSs on the Semantic Web in a
simple way
• Comparable to Dublin Core, for conceptual vocabularies
Reminder: there are many models/formats for KOSs
• But also common features, used by typical applications
• Especially lexical information and semantic links
• Cf. SKOS Use Cases and Requirements
http://www.w3.org/TR/skos-ucr/
http://www.w3.org/2004/02/skos/
Standards for the Representation of Knowledge on the Semantic Web
SKOS (Simple Knowledge Organisation System)
• SKOS offers building blocks to create XML/RDF data
representing KOS content
• Concepts and ConceptSchemes
• Lexical properties (prefLabel, altLabel)
• Semantic relations (broader, related)
• Notes (scopeNote, definition)
• …
Standards for the Representation of Knowledge on the Semantic Web
SKOS: Example
skos:ConceptScheme
rdf:type
skos:Concept
http://www.iconclass.nl/
rdf:type
skos:inScheme
http://www.iconclass.nl/s_11F
skos:prefLabel
“the Virgin [email protected]
“la Vierge [email protected]
skos:prefLabel
skos:broader
http://www.iconclass.nl/s_11
Standards for the Representation of Knowledge on the Semantic Web
SKOS: Benefits
• It is possible to produce standard SW representations
of KOSs
• SKOS is simple
• Use of formats like XML makes conversion (relatively) easy
• It is useful for enhancing re-usability/interoperability
of application components
• E.g. browsing, query reformulation…
• & Creating links to resources outside of a given scheme
• Metadata links between objects and concepts
• Semantic correspondences between concept schemes
Standards for the Representation of Knowledge on the Semantic Web
SKOS: Limitations
• Not everything can be represented in SKOS!
E.g. for Iconclass, difficulty to represent all types of auxiliaries
• Keys, structural digits…
• SKOS is still work in progress (though quite stable)
• W3C Semantic Web Deployment Working Group
http://www.w3.org/2006/07/SWD/
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 useful?
• Porting existing metadata to the Semantic Web
• SKOS
• Conclusion: SW and semantic alignment
Standards for the Representation of Knowledge on the Semantic Web
What have we seen?
• There are important interoperability problems in CH
• Semantic Web provides standards to represent data:
• Structured, but simple
RDF
• Web-enabled
URIs
• Using custom ontologies
RDF(S)
• Defined with semantics usable by computers
RDFS/OWL
• SW enables representation of CH data
• Including the original semantics (SKOS)
Standards for the Representation of Knowledge on the Semantic Web
Back to the Problem: Semantic Alignment
• Different ontologies/individuals should be aligned
• Using the same resources to join SW graphs together
• Using the same vocabularies and semantics
• But recognizing equivalent resources at data creation
time is difficult
• There is (and will be) no such thing as a single one ontology!
• A posteriori semantic alignment is needed
Standards for the Representation of Knowledge on the Semantic Web
Back to the Problem: Semantic Alignment
• SW languages give appropriate representation means
• Equivalence/specialisation links for properties and classes
• myVoc:auteur rdfs:subPropertyOf dc:creator
• myVoc:Article owl:equivalentClass yourVoc:Artikel
• Identity link between individuals
• vu:aisaac owl:sameAs kb:AntoineIsaac
• (yet unstable) SKOS mapping links between subjects
• iconclass:birds exactMatch swd:vogel
Standards for the Representation of Knowledge on the Semantic Web
Back to the Problem: Semantic Alignment
• But they don’t do the job for us!
• The links have to be created somehow
• This is another story…
Standards for the Representation of Knowledge on the Semantic Web
Thank you!
Standards for the Representation of Knowledge on the Semantic Web
Links
• Tutorials on the Semantic Web, with application
examples
http://www.w3.org/People/Ivan/CorePresentations/
• STITCH project
http://stitch.cs.vu.nl/
• Other SW projects dealing with Cultural Heritage
• MuseumFinland
• eCulture
http://www.museosuomi.fi/
http://e-culture.multimedian.nl/
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