Standards for the Representation of Knowledge on the Semantic Web

1
Standards for the Representation of Knowledge on
the Semantic Web

• Antoine ISAAC
• STITCH Project
• eScience Seminare
• Oct. 25th, 2007

2
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

3
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

4
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

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KB Illustrated Manuscripts
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KB Illustrated Manuscripts
7
Mandragore
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Mandragore
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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

10
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!

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The Result
12
An Ideal Situation
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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

14
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

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SW Problem The Web for Humans

• A city
• A flag
• The citys location
• Meaning

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SW Problem The Web for Humans
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SW Problem The Web for Computers?

• Characters
• Images
• Black boxes
• Markup
• Layout/Display

Where is meaning?
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SW Problem The Web for Computers?
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The Interoperability Problems in CH (Reminder)
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The Semantic Web Approach A Web of (Meta)data
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Footnote

• Why (meta)data?
• Because what is metadata for certain applications
  can indeed be the data for the Semantic Web
• No clear boundary

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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

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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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A Web of Resources
myVoc1Article
http//ex.org/files/file1
myVoc2Amsterdam
http//ex.org/files/file1par3
http//www.ned.nl/rep321
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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/file1par3
myVoc1defines
myVoc2Amsterdam
http//www.w3.org/TR/rdf-primer/
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Data in an RDF Graph
myVoc1Article
rdftype
http//ex.org/files/file1
myVoc2Amsterdam
myVoc1partOf
myVoc1defines
http//www.ned.nl/rep321
http//ex.org/files/file1par3
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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

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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)

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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/
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RDF Schema (RDFS)

• Assigning domain and ranges of properties
• defines is applied to resources of type
  Document
• myVoc1defines rdfsdomain myVoc1Document
• Creating hierarchies of classes and properties
• Article is a subclass of the class Document
• myVoc1Article rdfssubClassOf myVoc1Document
• Labels and informal specifications
• My Article has the literal article as a label
  for display
• myVoc1Article rdfslabel article

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Ontological Information
rdfssubClassOf
myVoc1Article
myVoc1Document
rdftype
http//ex.org/files/file1
myVoc2Amsterdam
myVoc1partOf
myVoc1defines
http//ex.org/files/file1par3
http//www.ned.nl/rep321
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RDF Schema (RDFS)

• (Some) constructs are equipped with formal
  semantics
• R rdftype C1
• C1 rdfssubClass C2
• -gt X rdftype C2
• P rdfsdomain C,
• R1 P R2
• -gt R1 rdftype C

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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

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Reasoning
rdfssubClassOf
myVoc1Article
myVoc1Document
rdftype
rdftype
http//ex.org/files/file1
myVoc2Amsterdam
myVoc1partOf
myVoc1defines
http//ex.org/files/file1par3
http//www.ned.nl/rep321
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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/
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Tools to build RDFS/OWL ontologies
http//protege.stanford.edu/
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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

38
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

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Building on Top of the Web

• Web-based resources allow distribution/sharing of
  
• document
• description vocabularies
• (meta)data

http//www.geo.org/voc/
(par3, defines, Amsterdam)
http//www.kb.nl/eDepot
http//www.ned.nl/rep321
different owners locations
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Why is it Useful?

• Based on open standards
• W3Cs URI, XML, RDF, RDFS, OWL

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Footnote Building on top of XML

• RDF can be encoded as XML data

ltrdfDescription rdfabouthttp//www.ned.nl/doc3
21gt ltmyVoc1defines rdfresource
http//www.geo.org/Amsterdam/gt lt/rdfDescriptiongt
ltrdfDescription rdfabouthttp//www.geo.org/Th
e_Netherlandsgt ltmyVoc2hasCapital
rdfresourcehttp//www.geo.org/Amsterdam/gt lt/rd
fDescriptiongt

• RDF/XML is the reference syntax, but others are
  possible

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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

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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

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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!

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Porting KOSs to the Semantic Web

• How about metadata values from Knowledge
  Organisation Schemes?
• E.g. dcsubject 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

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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 rdfssubClassOf
  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

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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/
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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)

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SKOS Example
skosConceptScheme
rdftype
skosConcept
http//www.iconclass.nl/
rdftype
skosinScheme
http//www.iconclass.nl/s_11F
skosprefLabel
skosbroader
the Virgin Mary_at_en
la Vierge Marie_at_fr
skosprefLabel
http//www.iconclass.nl/s_11
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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/interopera
  bility 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

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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/
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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

53
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)

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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

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Back to the Problem Semantic Alignment

• SW languages give appropriate representation
  means
• Equivalence/specialisation links for properties
  and classes
• myVocauteur rdfssubPropertyOf dccreator
• myVocArticle owlequivalentClass yourVocArtikel
• Identity link between individuals
• vuaisaac owlsameAs kbAntoineIsaac
• (yet unstable) SKOS mapping links between
  subjects
• iconclassbirds exactMatch swdvogel

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Back to the Problem Semantic Alignment

• But they dont do the job for us!
• The links have to be created somehow
• This is another story

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Thank you!
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Links

• Tutorials on the Semantic Web, with application
  examples http//www.w3.org/People/Ivan/CorePresent
  ations/
• STITCH project http//stitch.cs.vu.nl/
• Other SW projects dealing with Cultural Heritage
• MuseumFinland http//www.museosuomi.fi/
• eCulture http//e-culture.multimedian.nl/