In computing, linked data (often capitalized as Linked Data) is structured data which is interlinked with other data so it becomes 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 only for human readers, it extends them to share information in a way that can be read automatically by computers. Part of the vision of linked data is for the Internet to become a global database.
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.
Linked open data
Linked open data is linked data that is open data. Tim Berners-Lee gives the clearest definition of linked open data in differentiation with linked 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 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
- Global Research Identifier Database (GRID) – an international database of 89,506 institutions engaged in academic research, with 14,401 relationships, models two types of relationships: a parent-child relationship that defines a subordinate association, and a related relationship that describes other associations
- 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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- Linking open data cloud diagram 2014, by Max Schmachtenberg, Christian Bizer, Anja Jentzsch and Richard Cyganiak. http://lod-cloud.net/
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- Linking Open Data 2 (LOD2)
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- 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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- The Web Turns 20: Linked Data Gives People Power, part 1 of 4, by Mark Fischetti, Scientific American 2010 October 23
- Linked Data Is Merely More Data – Prateek Jain, Pascal Hitzler, Peter Z. Yeh, Kunal Verma, and Amit P. Sheth. In: Dan Brickley, Vinay K. Chaudhri, Harry Halpin, and Deborah McGuinness: Linked Data Meets Artificial Intelligence. Technical Report SS-10-07, AAAI Press, Menlo Park, California, 2010, pp. 82–86.
- Moving beyond sameAs with PLATO: Partonomy detection for Linked Data – Prateek Jain, Pascal Hitzler, Kunal Verma, Peter Z. Yeh, Amit Sheth. In: Proceedings of the 23rd ACM Hypertext and Social Media conference (HT 2012), Milwaukee, WI, USA, June 25–28, 2012.
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- The Flap of a Butterfly Wing - semanticweb.com Richard Wallis (February 2012)