Linked data

5-star linked open data

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:^[9]

• 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 W3C standards, like using RDF and employing 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^[10] was created.^[11] The mailing list was initially hosted by the SIMILE project^[12] 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.^[14][15] 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.^[16]

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,^[17] the AKN4EU project for machine-readable legislative data,^[18] the PlanetData project,^[19] the DaPaaS (Data-and-Platform-as-a-Service) project,^[20] and the Linked Open Data 2 (LOD2) project.^[21][22][23] 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

Ontologies are formal descriptions of data structures. Some of the better known ontologies are:

• FOAF – an ontology describing persons, their properties and relationships
• 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

Datasets

• 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
• GeoNames – provides RDF descriptions of more than 7,500,000 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 89,506 institutions engaged in academic research, with 14,401 relationships. GRID models two types of relationships: a parent-child relationship that defines a subordinate association, and a related relationship that describes other associations^[24][25]
• KnowWhereGraph^[26] – 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.^[27]
• 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.^[28][29]