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Linked Data is a sub-topic of the Semantic Web. The term Linked Data is used to describe a method of exposing, sharing, and connecting data via dereferenceable URIs on the Web.
Tim Berners-Lee outlined four principles of Linked Data in his Design Issues: Linked Data note, paraphrased along the following lines:
- Use URIs to identify things.
- Use HTTP URIs so that these things can be referred to and looked up ("dereference") by people and user agents.
- Provide useful information (i.e., a structured description — metadata) about the thing when its URI is dereferenced.
- Include links to other, related URIs in the exposed data to improve discovery of other related information on the Web.
Tim Berners-Lee gave a presentation on Linked Data at the TED 2009 conference.
Linking Open Data Community Project
The goal of the W3C Semantic Web Education and Outreach group's Linking Open Data community project is to extend the Web with a data commons by publishing various open datasets as RDF on the Web and by setting RDF links between data items from different data sources. In October 2007, datasets consisted of over two billion RDF triples, which were interlinked by over two million RDF links. By May 2009 this had grown to 4.2 billion RDF triples, interlinked by around 142 million RDF links.
Dataset Instance and Class Relationships
Clickable diagrams that show the individual datasets and their relationships within the DBpedia-spawned LOD cloud, as shown by the figures to the right, are:
- DBpedia - a dataset containing extracted data from Wikipedia; it contains about 2.18 million concepts described by 218 million triples, including abstracts in 11 different languages (see the very DBpedia resource associated to the present wikipedia page)
- DBLP Bibliography - provides bibliographic information about scientific papers; it contains about 800,000 articles, 400,000 authors, and approx. 15 million triples
- GeoNames provides RDF descriptions of more than 6,500,000 geographical features worldwide.
- Revyu - a Review service consumes and publishes Linked Data, primarily from DBpedia.
- riese - serving statistical data about 500 million Europeans (the first linked dataset deployed with XHTML+RDFa)
- UMBEL - a lightweight reference structure of 20,000 subject concept classes and their relationships derived from OpenCyc, which can act as binding classes to external data; also has links to 1.5 million named entities from DBpedia and YAGO
- Sensorpedia - A scientific initiative at Oak Ridge National Laboratory using a RESTful web architecture to link to sensor data and related sensing systems.
- FOAF - a dataset describing persons, their properties and relationships
- OpenPSI for the OpenPSI project a community effort to create UK government linked data service that supports research
Use case demos
- Linked Data Web architecture note by Tim Berners-Lee
- Christian Bizer, Tom Heath, Tim Berners-Lee (to appear). Linked Data - The Story So Far. Special Issue on Linked Data, International Journal on Semantic Web and Information Systems (IJSWIS)
- Linked Data at the ESW Wiki
- Linking Open Data project wiki home page
- List of public Linked Data sources
- Tutorial: How to publish Linked Data on the Web - Chris Bizer, Richard Cyganiak, Tom Heath
- White Paper: How to publish Linked Data on the Web using OpenLink Virtuoso
- Linked Data on the Web - Chris Bizer, Tom Heath, Kingsley Idehen, Tim Berners-Lee. In Proceedings WWW2008, Beijing, China.
- Interlinking Open Data on the Web - Chris Bizer, Tom Heath, Danny Ayers, Yves Raimond. In Proceedings Poster Track, ESWC2007, Innsbruck, Austria.