SKOS and Linked Data

Antoine Isaac

ISKO, London, Sept. 14th 2010

Personal background

• Europeana• Web & Media Lab, Vrije Universiteit Amsterdam

• W3C Library Linked Data group• (2006-2009) W3C Semantic Web Deployment group

SKOS

Towards a web of culture data

Government data again?

http://standards.esd.org.uk/

Government data again?

SKOS

Simple Knowledge Organization System

Scope: knowledge organization systems (KOS) such as thesauri, classification systems, subject heading lists…

SKOS is for representings KOSs in RDF in a simple way

Representing semantics

The formal way: OWL Semantic Web ontology language

Used for ontologies that enable machine reasoning• Mother is a class• It is the intersection of the classes Woman and Parent• Parent is the class of entities of type Person that are

related to at least one other resource of type Person using the child property

…

SKOS is not for formal ontologies

• Turning KOSs into ontologies is possible, but KOSs– are large– have softer “semantics”

Parent RelatedTerm Child

– have often a focus on terminological informationChild UsedFor Offspring

• Softer semantics can be useful as such for many applications!Semantic search, annotation…

Basic SKOS

A set of features common to various KOS types and useful for many applications

• Concepts• Lexical properties • Semantic relations • Notes

Thesaurus example

Animals

catsUF (used for) domestic catsRT (related term) wildcatsBT (broader term) animalsSN (scope note) used only for domestic cats

domestic catsUSE cats

wildcats

Concepts and labels

catsUF (used for) domestic cats

skos: = http://www.w3.org/2004/02/skos/core#rdf: = http://www.w3.org/1999/02/22-rdf-syntax-ns#ex: = http://example.org/

Note: multilingual labels

Semantic relations

catsRT (related term) wildcatsBT (broader term) animals

A SKOS graphanimalscats

UF domestic catsRT wildcatsBT animalsSN used only for domestic

catsdomestic cats

USE catswildcats

SKOS mappingsSKOS allows bridging across KOSs from different contexts

KOS 1:animalscatswildcats

KOS 2:animalhumanobject

Other features

• Concept groupingskos:Collection, skos:member…

• Concept documentationskos:example…

• SKOS-XL: extension for more complex representation of labels

Note: SKOS can be extended as any other RDF vocabulary

Semantics for SKOS

There are some basic constraints on SKOS dataE.g., a concept has only one prefLabel per language

Semantics for SKOS

There are rules to infer new factsE.g., broader and narrower are inverse of each other

Semantics for SKOS

Minimal semantic commitment:• SKOS must cope with existing information and not

infer new data beyond what KOS publishers intend

References

SKOS Reference http://www.w3.org/TR/skos-reference SKOS Primer http://www.w3.org/TR/skos-primer SKOS homepage http://www.w3.org/2004/02/skosSKOS wiki http://www.w3.org/2001/sw/wiki/SKOS SKOS mailing list public-esw-thes@w3.org

Benefits of SKOS?

Easily fitting KOSs into the Semantic Web & Linked Data vision

• Web-oriented representation• Re-use & sharing of concepts and their descriptions• Linking between concepts from different contexts

Demo

Subject heading lists as SKOS linked data• American LCSH http://id.loc.gov• French RAMEAU: http://stitch.cs.vu.nl/rameau• German SWD: http://d-nb.info/gnd/ • mapped using manual links from the MACS project

http://macs.cenl.org

Starting from http://id.loc.gov/authorities/sh85014310#concept

Linked Data?

1. Use URIs as names for things2. Use HTTP URIs so that people can look up those names3. When someone looks up a URI, provide useful information

using standards (RDF, SPARQL)4. Include links to other URIs, so that they can discover more

things

Tim Berners-Lee, http://linkeddata.org/

SKOS Implementations

Miles, Bechhofer, SKOS Implementation Report, May 19th 2009http://www.w3.org/2006/07/SWD/SKOS/reference/20090315/implementation.html

Some landmark SKOS implementations

• Swedish National Library’s Libris catalogue and thesaurus http://libris.kb.se/ • Library of Congress’ vocabularies, including LCSH http://id.loc.gov/ • DNB’s Gemeinsame Normdatei (incl. SWD subject headings) http://d-nb.info/gnd/ • BnF’s RAMEAU subject headings http://stitch.cs.vu.nl/ • OCLC’s DDC classification http://dewey.info/ and VIAF http://viaf.org/ • STW economy thesaurus http://zbw.eu/stw • National Library of Hungary’s catalogue and thesauri http://oszkdk.oszk.hu/resource/DRJ/404 (example)• Wikipedia categories through Dbpedia http://dbpedia.org/ • New York Times subject headings http://data.nytimes.com/ • IVOA astronomy vocabularies http://www.ivoa.net/Documents/latest/Vocabularies.html• GEMET environmental thesaurus http://eionet.europa.eu/gemet • Agrovoc http://aims.fao.org/ • Linked Life Data http://linkedlifedata.com/ • Taxonconcept http://www.taxonconcept.org/ • UK Public sector vocabularies http://standards.esd.org.uk/ (e.g., http://id.esd.org.uk/lifeEvent/7 )

http://www.w3.org/2001/sw/wiki/SKOS/Datasets

KOS Alignments?

Quite many of them are linked to some other resource• LCSH, SWD and RAMEAU interlinked through MACS mappings• GND -> DBpedia, VIAF• Libris -> LCSH• Agrovoc -> CAT, NAL, SWD, GEMET• NYT -> freebase, DBpedia, Geonames• dbPedia links are overwhelming

Hungary, STW, TaxonConcept, GND…

Issues: semantic alignment of SKOS data

• In a linked data environment, the most interesting applications are the ones that cross contexts

• Mapping data between concept schemes is still scarce• More efforts are needed

Automatic or manual or a mixture of both

Issues: linking KOS data to other resources

• KOSs become valuable when they bring a “semantic layer” over other resourcesE.g. books and the topics they are about

• Existing links are often only implicit in the data—using labels of concepts not their identifiers

• Semantic annotation with KOS should be a main targetCf. Drupal’s taxonomy plug-in

Conclusion

Publication and linking of linked KOS data is still work in progress, but

• We are now in position of realizing the issues and working towards solving them

• We can start building applications that make use of the wealth of data already available or yet to comeMany KO resources to re-use and link to

Thank you!

aisaac@few.vu.nl

Acknowledgements

• Participants of the Semantic Web Deployment working groupAlistair Miles, Sean Bechhofer, Ed Summers, Tom Baker, Guus Schreiber…

• Library of Congress• French National Library• German National Library• MACS team (Swiss National Library)• EuropeanaConnect project