In computing, linked data (often capitalized as Linked Data) is a method of publishing structured data so that it can be interlinked and become more useful through semantic queries. It builds upon standard Web technologies such as HTTP, RDF and URIs, but rather than using them to serve web pages for human readers, it extends them to share information in a way that can be read automatically by computers.
Linked data may also be open data, in which case it is usually described as linked open data (LOD).
Tim Berners-Lee outlined four principles of linked data in his "Linked Data" note of 2006, paraphrased along the following lines:
- Use URIs to name (identify) things.
- Use HTTP URIs so that these things can be looked up (interpreted, "dereferenced").
- Provide useful information about what a name identifies when it's looked up, using open standards such as RDF, SPARQL, etc.
- Refer to other things using their HTTP URI-based names when publishing data on the Web.
- All kinds of conceptual things, they have names now that start with HTTP.
- If I take one of these HTTP names and I look it up...I will get back some data in a standard format which is kind of useful data that somebody might like to know about that thing, about that event.
- When I get back that information it's not just got somebody's height and weight and when they were born, it's got relationships. And when it has relationships, whenever it expresses a relationship then the other thing that it's related to is given one of those names that starts with HTTP.
- Structured data using controlled vocabulary terms and dataset definitions expressed in Resource Description Framework serialization formats such as RDFa, RDF/XML, N3, Turtle, or JSON-LD
- Linked Data Platform
Linked open data
Linked Open Data (LOD) is Linked Data which is released under an open license, which does not impede its reuse for free.
The term "linked open data" has been in use since at least February 2007, when the "Linking Open Data" mailing list was created. The mailing list was initially hosted by the SIMILE project at the Massachusetts Institute of Technology.
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 September 2011 this had grown to 31 billion RDF triples, interlinked by around 504 million RDF links. A detailed statistical breakdown was published in 2014.
European Union projects
There are a number of European Union projects[when defined as?] involving linked data. These include the linked open data around the clock (LATC) project, the PlanetData project, the DaPaaS (Data-and-Platform-as-a-Service) project, and the Linked Open Data 2 (LOD2) project. Data linking is one of the main goals of the EU Open Data Portal, which makes available thousands of datasets for anyone to reuse and link.
- DBpedia – a dataset containing extracted data from Wikipedia; it contains about 3.4 million concepts described by 1 billion triples, including abstracts in 11 different languages
- FOAF – a dataset describing persons, their properties and relationships
- GeoNames provides RDF descriptions of more than 7,500,000 geographical features worldwide.
- 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
- Wikidata – a collaboratively-created linked dataset that acts as central storage for the structured data of its Wikimedia Foundation sister projects
Dataset instance and class relationships
- American Art Collaborative - consortium of US art museums committed to establishing a critical mass of linked open data on American art
- Authority control – about controlled headings in library catalogs
- Citation analysis – for citations between scholarly articles
- Linked data page
- Network model – an older type of database management system
- VoID – Vocabulary of Interlinked Datasets
- Web Ontology Language
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- "Tim Berners-Lee on the next Web".
- "Frequently Asked Questions (FAQs) - Linked Data - Connect Distributed Data across the Web".
- "COAR » 7 things you should know about…Linked Data".
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- "5 Star Open Data".
- "firstname.lastname@example.org Mail Archives".
- "SIMILE Project - Mailing Lists".
- Linking open data cloud diagram 2014, by Max Schmachtenberg, Christian Bizer, Anja Jentzsch and Richard Cyganiak. http://lod-cloud.net/
- "SweoIG/TaskForces/CommunityProjects/LinkingOpenData - W3C Wiki". esw.w3.org. Retrieved 22 March 2018.
- Fensel, Dieter; Facca, Federico Michele; Simperl, Elena; Ioan, Toma (2011). Semantic Web Services. Springer. p. 99. ISBN 3642191924.
- Max. "State of the LOD Cloud". linkeddatacatalog.dws.informatik.uni-mannheim.de. Retrieved 22 March 2018.
- "Linked open data around the clock (LATC)". latc-project.eu. Retrieved 22 March 2018.
- "Welcome to PlanetData! - PlanetData". planet-data.eu. Retrieved 22 March 2018.
- "DaPaaS". project.dapaas.eu. Retrieved 22 March 2018.
- Linking Open Data 2 (LOD2)[permanent dead link]
- "CORDIS FP7 ICT Projects – LOD2". European Commission. 2010-04-20.
- "LOD2 Project Fact Sheet – Project Summary" (PDF). 2010-09-01. Archived from the original (PDF) on 2011-07-20. Retrieved 2010-12-18.
- "Instance relationships amongst datasets". fu-berlin.de. Retrieved 22 March 2018.
- "Class relationships amongst datasets". archive.org. Retrieved 22 March 2018.
- Ahmet Soylu, Felix Mödritscher, and Patrick De Causmaecker. 2012. “Ubiquitous Web Navigation through Harvesting Embedded Semantic Data: A Mobile Scenario.” Integrated Computer-Aided Engineering 19 (1): 93–109.
- Linked Data: Evolving the Web into a Global Data Space (2011) by Tom Heath and Christian Bizer, Synthesis Lectures on the Semantic Web: Theory and Technology, Morgan & Claypool
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- Interview with Sören Auer, head of the LOD2 project about the continuation of LOD2 in 2011, June 2011
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