View source for Linked data

{{Short description|Structured data and method for its publication}}
{{distinguish|Linked data structure}}
[[File:Wikidata in the Linked Open Data cloud 2020-08-20.svg|alt=A network of over a thousand circles clustered into groups and linked with lines into a web.|thumb|300x300px|Wikidata in the Linked Open Data Cloud, as at August 2020. Databases indicated as circles (with wikidata indicated as ‘WD’), with grey lines linking databases in the network if their data is aligned. Generated from https://lod-cloud.net/datasets . ]]
[[File:Screenshot_from_2021-05-17_12-26-27.png|alt=DBpedia as the most interlinked LOD dataset and crystallization point of the Linked Open Data Cloud since 2008|thumb|300x300px|DBpedia as the most interlinked LOD dataset and crystallization point of the Linked Open Data Cloud since 2008. Image from 2021, generated from [https://lod-cloud.net/ https://lod-cloud.net].]]
In [[computing]], '''linked data''' is structured data which is interlinked with other data so it becomes more useful through [[semantic query|semantic queries]]. It builds upon standard [[World Wide Web|Web]] technologies such as [[Hypertext Transfer Protocol|HTTP]], [[Resource Description Framework|RDF]] and [[uniform resource identifier|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]].<ref>{{Cite web|title=Linked Data as JSON|url=http://jsld.org/|access-date=2020-12-04|website=Linked Data as JSON|language=en}}</ref>

[[Tim Berners-Lee]], director of the [[World Wide Web Consortium]] (W3C), coined the term in a 2006 design note about the [[Semantic Web]] project.<ref name=DesignIssues>{{cite web |url=http://www.w3.org/DesignIssues/LinkedData.html
 |title=Linked Data |work=Design Issues |author=Tim Berners-Lee |author-link=Tim Berners-Lee |date=2006-07-27
 |publisher=[[W3C]] |access-date=2010-12-18}}</ref>

Linked data may also be [[open data]], in which case it is usually described as Linked Open Data.<ref>{{Cite web|url=https://www.ontotext.com/knowledgehub/fundamentals/linked-data-linked-open-data/|title=What are Linked Data and Linked Open Data?|website=Ontotext|language=en-US|access-date=2019-05-08}}</ref>

==Principles==
In his 2006 "Linked Data" note, [[Tim Berners-Lee]] outlined four principles of linked data, paraphrased along the following lines:<ref name=DesignIssues />
#[[Uniform Resource Identifier]]s (URIs) should be used to name and identify individual things.
#[[Hypertext Transfer Protocol|HTTP]] URIs should be used to allow these things to be looked up, interpreted, and subsequently "dereferenced".
#Useful information about what a name identifies should be provided through open standards such as [[Resource Description Framework|RDF]], [[SPARQL]], etc.
#When publishing data on the Web, other things should be referred to using their HTTP URI-based names.

Tim Berners-Lee later restated these principles at a 2009 [[TED (conference)|TED conference]], again paraphrased along the following lines:<ref>{{cite web|url=http://www.ted.com/talks/tim_berners_lee_on_the_next_web.html|title=Tim Berners-Lee on the next Web|access-date=2009-03-15|archive-date=2011-04-10|archive-url=https://web.archive.org/web/20110410204952/http://www.ted.com/talks/tim_berners_lee_on_the_next_web.html|url-status=dead}}</ref>
#All conceptual things should have a name starting with [[HTTP]].
#Looking up an HTTP name should return useful data about the thing in question in a standard format.
#Anything else that that same thing has a relationship with through its data should also be given a name beginning with HTTP.

==Components==
Thus, we can identify the following components as essential to a global Linked Data system as envisioned, and to any actual Linked Data subset within it:

* [[Uniform resource identifier|URI]]s
* [[HTTP]]
* [[Structured data]] using [[controlled vocabulary]] terms and dataset definitions expressed in [[Resource Description Framework]] [[serialization]] formats such as [[RDFa]], [[RDF/XML]], [[Notation 3|N3]], [[Turtle (syntax)|Turtle]], or [[JSON-LD]]
* [[Linked Data Platform]]
* [[CSV (file format)|CSV]]-W<ref> https://www.w3.org/2013/csvw/wiki/Main_Page.html </ref>

==Linked open data==
'''Linked open data''' are linked data that are [[open data]].<ref>{{cite web|url=http://linkeddata.org/faq|title=Frequently Asked Questions (FAQs) - Linked Data - Connect Distributed Data across the Web|access-date=2014-12-29|archive-date=2015-11-18|archive-url=https://web.archive.org/web/20151118060145/http://linkeddata.org/faq|url-status=dead}}</ref><ref>{{cite web|url=https://www.coar-repositories.org/activities/repository-observatory/second-edition-linked-open-data/7-things-you-should-know-about-open-data/|title=COAR » 7 things you should know about…Linked Data|access-date=2015-12-29|archive-url=https://web.archive.org/web/20151118085816/https://www.coar-repositories.org/activities/repository-observatory/second-edition-linked-open-data/7-things-you-should-know-about-open-data/|archive-date=2015-11-18|url-status=dead}}</ref><ref>{{cite web|url=http://openorg.ecs.soton.ac.uk/wiki/Linked_Data_Basics_for_Techies#Open_Linked_Data|title=Linked Data Basics for Techies|access-date=2015-12-29|archive-date=2021-05-05|archive-url=https://web.archive.org/web/20210505205603/http://openorg.ecs.soton.ac.uk/wiki/Linked_Data_Basics_for_Techies#Open_Linked_Data|url-status=dead}}</ref> Tim Berners-Lee gives the clearest definition of linked open data as differentiated from linked data.
 {{Quote |
text=Linked Open Data (LOD) is Linked Data which is released under an open license, which does not impede its reuse for free. |author=Tim Berners-Lee |title=Linked Data<ref name=DesignIssues /><ref>{{cite web|url=http://5stardata.info/en|title=5 Star Open Data}}</ref>
}}

Large linked open data sets include [[DBpedia]], [[Wikibase]], [[Wikidata]] and {{ill|Open ICEcat|uk||nl}}.

=== 5-star linked open data ===
[[File:5-star deployment scheme for Open Data.png|thumb|378x378px|Deployment scheme for Linked Open Data<ref>{{Cite web|title=5-star Open Data|url=http://5stardata.info/en/|access-date=2021-03-07|website=5stardata.info|language=en}}</ref>]]
In 2010, [[Tim Berners-Lee]] suggested a 5-star scheme for grading the quality of open data on the web, for which the highest ranking is Linked Open Data:<ref>{{Cite web|title=What is 5 Star Linked Data? {{!}} Webize Everything Community Group|url=https://www.w3.org/community/webize/2014/01/17/what-is-5-star-linked-data/|access-date=2021-03-07|website=www.w3.org|language=en-US}}</ref>

* 1 star: data is openly available in some format.
* 2 stars: data is available in a structured format, such as [[Microsoft Excel file format]] (.xls).
* 3 stars: data is available in a non-proprietary structured format, such as [[Comma-separated values]] (.csv).
* 4 stars: data follows [[World Wide Web Consortium|W3C]] standards, like using [[Resource Description Framework|RDF]] and employing [[Uniform Resource Identifier|URIs]]. 
* 5 stars: all of the others, plus links to other Linked Open Data sources.

===History===
The term "linked open data" has been in use since at least February 2007, when the "Linking Open Data" mailing list<ref>{{cite web|url=http://lists.w3.org/Archives/Public/public-lod/|title=public-lod@w3.org Mail Archives}}</ref> was created.<ref>{{cite web|url=http://www.w3.org/wiki/SweoIG/TaskForces/CommunityProjects/LinkingOpenData/NewsArchive|title=SweoIG/TaskForces/CommunityProjects/LinkingOpenData/NewsArchive}}</ref> The mailing list was initially hosted by the [[SIMILE]] project<ref>{{cite web|url=http://simile.mit.edu/mail.html|title=SIMILE Project - Mailing Lists}}</ref> at the [[Massachusetts Institute of Technology]].

===Linking Open Data community project===
[[File:LOD Cloud 2014-08.svg|thumb|The above diagram shows which Linking Open Data datasets are connected, as of August 2014.  This was produced by the Linked Open Data Cloud project, which was started in 2007.  Some sets may include copyrighted data which is freely available.<ref>Linking open data cloud diagram 2014, by Max Schmachtenberg, Christian Bizer, Anja Jentzsch and Richard Cyganiak. http://lod-cloud.net/</ref>]]

[[File:LOD Cloud - 2017-02-20.png|thumb|The same diagram as above, but for February 2017, showing the growth in just two and a half years]]

[[File:LOD Cloud - 2024-12-31.png|thumb|The LOD cloud in December 2024]]

The goal of the W3C Semantic Web Education and Outreach group's Linking Open Data community project is to extend the Web with a [[Knowledge commons|data commons]] by publishing various [[open knowledge|open]] [[dataset]]s as RDF on the Web and by setting [[Resource Description Framework|RDF]] links between data items from different data sources. In October 2007, datasets consisted of over two billion RDF [[Semantic triple|triples]], which were interlinked by over two million RDF links.<ref>{{cite web|url=http://esw.w3.org/topic/SweoIG/TaskForces/CommunityProjects/LinkingOpenData|title=SweoIG/TaskForces/CommunityProjects/LinkingOpenData - W3C Wiki|website=esw.w3.org|access-date=22 March 2018}}</ref><ref>{{cite book |last1=Fensel |first1=Dieter |last2=Facca |first2= Federico Michele |last3=Simperl |first3=Elena |last4=Ioan |first4=Toma |title=Semantic Web Services |year=2011 |publisher=Springer|isbn=978-3642191923 |pages=99}}</ref>  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.<ref>{{cite web|url=http://linkeddatacatalog.dws.informatik.uni-mannheim.de/state/|title=State of the LOD Cloud|last=Max|website=linkeddatacatalog.dws.informatik.uni-mannheim.de|access-date=22 March 2018}}</ref>

===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,<ref>{{cite web|url=http://latc-project.eu/|title=Linked open data around the clock (LATC)|website=latc-project.eu|access-date=22 March 2018|archive-url=https://web.archive.org/web/20180919095411/https://latc-project.eu/|archive-date=19 September 2018|url-status=dead}}</ref> the AKN4EU project for machine-readable legislative data,<ref>{{Citation |last1=Flatt |first1=Amelie |title=Model-Driven Development of AKN Application Profiles: Background and Requirements |date=2022 |url=https://link.springer.com/10.1007/978-3-031-14132-4_2 |work=Model-Driven Development of Akoma Ntoso Application Profiles |pages=5–12 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-031-14132-4_2 |isbn=978-3-031-14131-7 |access-date=2023-01-07 |last2=Langner |first2=Arne |last3=Leps |first3=Olof|url-access=subscription }}</ref> the PlanetData project,<ref>{{cite web|url=http://planet-data.eu/|title=Welcome to PlanetData! - PlanetData|website=planet-data.eu|access-date=22 March 2018|archive-date=21 April 2021|archive-url=https://web.archive.org/web/20210421082019/http://www.planet-data.eu/|url-status=dead}}</ref> the DaPaaS (Data-and-Platform-as-a-Service) project,<ref>{{cite web|url=http://project.dapaas.eu/|title=DaPaaS|website=project.dapaas.eu|access-date=22 March 2018|archive-date=18 December 2020|archive-url=https://web.archive.org/web/20201218070059/http://project.dapaas.eu/|url-status=dead}}</ref> and the Linked Open Data 2 (LOD2) project.<ref>[https://web.archive.org/web/20180929075540/http://lod2.eu/ Linking Open Data 2 (LOD2)]</ref><ref>{{cite web |url=http://cordis.europa.eu/fetch?CALLER=PROJ_ICT&ACTION=D&CAT=PROJ&RCN=95562 |publisher=European Commission |title=CORDIS FP7 ICT Projects – LOD2 |date=2010-04-20}}</ref><ref>{{cite web|url=http://static.lod2.eu/Deliverables/LOD2_D12.5.1_Project_Fact_Sheet_Version.pdf |title=LOD2 Project Fact Sheet – Project Summary |date=2010-09-01 |access-date=2010-12-18 |url-status=dead |archive-url=https://web.archive.org/web/20110720164405/http://static.lod2.eu/Deliverables/LOD2_D12.5.1_Project_Fact_Sheet_Version.pdf |archive-date=2011-07-20 }}</ref> 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.

===Ontologies===
[[Ontology (information science)|Ontologies]] are formal descriptions of data structures. Some of the better known ontologies are:

* [[FOAF (ontology)|FOAF]] – an ontology describing persons, their properties and relationships
* [[UMBEL]] – a lightweight reference structure of {{formatnum:20000}} 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 (ontology)|YAGO]]

===Datasets===
* [[DBpedia]] – a dataset containing extracted data from Wikipedia; it contains about 3.4 million concepts described by 1 billion [[Semantic triple|triples]], including abstracts in 11 different languages
* [[GeoNames]] – provides RDF descriptions of more than {{formatnum:7500000}} geographical features worldwide
* [[Wikidata]] – a collaboratively-created linked dataset that acts as central storage for the structured data of its [[Wikimedia Foundation]] sibling projects
* [[Global Research Identifier Database]] (''GRID'') – an international database of {{formatnum:89506}} institutions engaged in academic research, with {{formatnum:14401}} relationships. GRID models two types of relationships: a parent-child relationship that defines a subordinate association, and a related relationship that describes other associations<ref name="gridStats">{{Cite web|url=https://grid.ac/stats|title=GRID Statistics|website=grid.ac/stats|language=en-GB|access-date=2018-10-26}}</ref><ref name="gridPol">{{Cite web|url=https://grid.ac/pages/policies|title=GRID Policies|website=grid.ac|language=en-GB|access-date=2018-10-26}}</ref>
* KnowWhereGraph<ref name="kwg">{{Cite web|url=https://www.knowwheregraph.org|title=KnowWhereGraph|website=knowwheregraph.org|language=en-US|access-date=2022-05-16}}</ref> – an integrated 12 billion triples strong [[knowledge graph]] of 30 data layers at the intersection between humans and their environment using Semantic Web and Linked Data technologies.<ref name="kwgaimag">{{cite journal | title= Know, Know Where, Knowwheregraph: A Densely Connected, Cross-Domain Knowledge Graph and Geo-Enrichment Service Stack for Applications in Environmental Intelligence| year= 2022| doi= 10.1609/aimag.v43i1.19120| author1= Krzysztof Janowicz| author2= [[Pascal Hitzler]]| author3= Wenwen Li| author4= Dean Rehberger| author5= Mark Schildhauer| author6= Rui Zhu| author7= Cogan Shimizu| author8= Colby K. Fisher| author9= Ling Cai| author10= Gengchen Mai| author11= Joseph Zalewski| author12= Lu Zhou| author13= Shirly Stephen| author14= Seila Gonzalez Estrecha| author15= Bryce D. Mecum| author16= Anna Lopez-Carr| author17= Andrew Schroeder| author18= Dave Smith| author19= Dawn J. Wright| author20= Sizhe Wang| author21= Yuanyuan Tian| author22= Zilong Liu| author23= Meilin Shi| author24= Anthony D'Onofrio| author25= Zhining G| author26= Kitty Currier |journal= AI Magazine| volume= 43| issue= 1| pages= 30–39| doi-access= free| hdl= 1983/be176aba-9dec-456c-9615-01a0e8556b7b| hdl-access= free}}</ref>
* {{ill|Open ICEcat|uk||nl}} - a [[multilingual]] open catalogue containing product [[datasheets]], related [[digital assets]] and usage [[statistics]].

===Dataset instance and class relationships===
Clickable diagrams that show the individual datasets and their relationships within the DBpedia-spawned LOD cloud (as by the figures to the right) are available.<ref>{{cite web|url=http://www4.wiwiss.fu-berlin.de/bizer/pub/lod-datasets_2009-07-14.html|title=Instance relationships amongst datasets|website=fu-berlin.de|access-date=22 March 2018|archive-date=2012-10-17|archive-url=https://web.archive.org/web/20121017231016/http://www4.wiwiss.fu-berlin.de/bizer/pub/lod-datasets_2009-07-14.html|url-status=dead}}</ref><ref>{{cite web|url=http://umbel.org/sites/umbel.org/lod/lod_constellation.html|archive-url=https://web.archive.org/web/20110828103804/http://umbel.org/sites/umbel.org/lod/lod_constellation.html|url-status=dead|archive-date=28 August 2011|title=Class relationships amongst datasets|access-date=22 March 2018}}</ref>

==See also==
* [[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
* [[data.gov.uk]]
* [[Hyperdata]]
* [[Network model]] – an older type of database management system
* [[Schema.org]]
* [[VoID]] – Vocabulary of Interlinked Datasets
* [[Web Ontology Language]]
* [[List of datasets for machine-learning research]]

==References==
{{reflist|30em}}

==Further reading==
{{ref begin|2}}
* Ahmet Soylu, Felix Mödritscher, and Patrick De Causmaecker. 2012. [http://www.ahmetsoylu.com/wp-content/uploads/2013/10/soylu_ICAE2012.pdf "Ubiquitous Web Navigation through Harvesting Embedded Semantic Data: A Mobile Scenario."] {{Webarchive|url=https://web.archive.org/web/20180919092519/http://www.ahmetsoylu.com/wp-content/uploads/2013/10/soylu_ICAE2012.pdf |date=2018-09-19 }} Integrated Computer-Aided Engineering 19 (1): 93–109.
* ''[http://linkeddatabook.com/book 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 <!-- note this resources supersedes the tutorial [http://www4.wiwiss.fu-berlin.de/bizer/pub/LinkedDataTutorial/ How to publish Linked Data on the Web] by Bizer, Cyganiak, and Heath -->
* [http://wifo5-03.informatik.uni-mannheim.de/bizer/pub/LinkedDataTutorial/ How to Publish Linked Data on the Web] {{Webarchive|url=https://web.archive.org/web/20210419093458/http://wifo5-03.informatik.uni-mannheim.de/bizer/pub/LinkedDataTutorial/ |date=2021-04-19 }}, by Chris Bizer, Richard Cyganiak and Tom Heath, Linked Data Tutorial at Freie Universität Berlin, Germany, 27 July 2007.
* [http://www.scientificamerican.com/article.cfm?id=berners-lee-linked-data The Web Turns 20: Linked Data Gives People Power], part 1 of 4, by Mark Fischetti, ''[[Scientific American]]'' 2010 October 23
* [https://web.archive.org/web/20120809091450/http://knoesis.wright.edu/library/publications/linkedai2010_submission_13.pdf 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.&nbsp;82–86.
* [https://web.archive.org/web/20180726135146/http://knoesis.org/library/resource.php?id=1718 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.
* Freitas, André, Edward Curry, João Gabriel Oliveira, and Sean O’Riain. 2012. [http://www.edwardcurry.org/publications/freitas_IC_12.pdf "Querying Heterogeneous Datasets on the Linked Data Web: Challenges, Approaches, and Trends."] IEEE Internet Computing 16 (1): 24–33.
* [https://web.archive.org/web/20070626121000/http://sites.wiwiss.fu-berlin.de/suhl/bizer/pub/LinkingOpenData.pdf Interlinking Open Data on the Web] – Chris Bizer, Tom Heath, Danny Ayers, Yves Raimond. In Proceedings Poster Track, ESWC2007, Innsbruck, Austria
* [https://web.archive.org/web/20180726135332/http://knoesis.wright.edu/library/publications/iswc10_paper218.pdf Ontology Alignment for Linked Open Data] – Prateek Jain, [[Pascal Hitzler]], Amit Sheth, Kunal Verma, Peter Z. Yeh. In proceedings of the 9th International Semantic Web Conference, ISWC 2010, Shanghai, China
* [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3121711/ Linked open drug data for pharmaceutical research and development] - J Cheminform. 2011; 3: 19. Samwald, Jentzsch, Bouton, Kallesøe, Willighagen, Hajagos, Marshall, Prud'hommeaux, Hassenzadeh, Pichler, and Stephens (May 2011)
* [http://www.community-of-knowledge.de/beitrag/the-hype-the-hope-and-the-lod2-soeren-auer-engaged-in-the-next-generation-lod/ Interview with Sören Auer, head of the LOD2 project about the continuation of LOD2 in 2011], June 2011
* [http://www.semantic-web.at/LOD-TheEssentials.pdf Linked Open Data: The Essentials] - Florian Bauer and Martin Kaltenböck (January 2012)
* [https://web.archive.org/web/20150110000748/http://semanticweb.com/the-flap-of-a-butterfly-wing_b26808 The Flap of a Butterfly Wing] - semanticweb.com Richard Wallis (February 2012)
{{ref end}}

==External links==
{{Scholia|topic}}
* [http://www.w3.org/wiki/LinkedData LinkedData] at the W3C Wiki
* [https://web.archive.org/web/20200106144712/http://linkeddata.org/ LinkedData.org]
* [https://web.archive.org/web/20120722075239/http://virtuoso.openlinksw.com/white-papers/ OpenLink Software white papers]

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