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The Linking Open Data (LOD) Constellation

The objective of the Linking Open Data (LOD) community 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. All of the sources on these LOD diagrams are open data.

This LOD constellation diagram complements the instance-level view in the LOD cloud [1] (click here to view the LOD cloud).  The LOD cloud centrally hubs around DBpedia, the linked data structured representation of Wikipedia. The connections shown in the cloud diagram mostly reflect owl:sameAs relations, which means that the same individual things or instances are referenced and then linked between the datasets.

As a complement, the LOD constellation diagram below centrally hubs around UMBEL and displays class-level linkages. The importance of class-level linkages is described in a number of articles and often goes by the term of 'exploding the domain' [2].

The definition of class-level linkages is based on one of four possible predicates (rdfs:subClassOf, owl:equivalentClass, umbel:superClassOf or umbel:isAligned). Because of the newness of UMBEL as a vocabulary, only a few of the sources linked to UMBEL have the umbel:superClassOf relationship and none yet have isAligned.

Note that some of the sources are combined vocabularies (ontologies) and instance representations (e.g., UMBEL, GeoNames), others are strict ontologies (e.g., event, bibo), and still others are ontologies used to characterize distributed instances (e.g., foaf, sioc, doap). Other distinctions might be applied as well:

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The current 21 LOD datasets and ontologies that contribute to these class-level mappings are (with each introduced by its namespace designation):

  • bibo -- Bibilographic ontology
  • cc -- Creative Commons ontology
  • damltime -- Time Zone ontology
  • doap -- Description of a Project ontology
  • event -- Event ontology
  • foaf -- Friend-of-a-Friend ontology
  • frbr -- Functional Requirements for Bibliographic Records
  • geo -- Geo wgs84 ontology
  • geonames -- GeoNames ontology
  • mo -- Music Ontology
  • opencyc -- OpenCyc knowledge base
  • owl -- Web Ontology Language
  • pim_contact -- PIM (personal information management) Contacts ontology
  • po -- Programmes Ontology (BBC)
  • rss -- Really Simple Syndicate (1.0) ontology
  • sioc -- Socially Interlinked Online Communities ontology
  • sioc_types -- SIOC extension
  • skos -- Simple Knowledge Organization System
  • umbel -- Upper Mapping and Binding Exchange Layer ontology
  • wordnet -- WordNet lexical ontology
  • yandex_foaf -- FOAF (Friend-of-a-Friend) Yandex extension ontology

The diagram was programamtically generated as described in the announcement of the diagram [3], which also describes its source files.

Diagram Source Files

The class-level mapping file used as input to the Cytoscape program that generated the network diagram is available as:

The Cytoscape file that generates the base LOD constellation diagram is available as:

The pre-final SVG vector diagram is available as:

As described in [3], a few final steps such as shading were applied to generate the final diagram.

[1] The current version of the LOD cloud may be found at the World Wide Web Consortium's (W3C) SWEO wiki page. There is also a clickable version of the diagram that will take you to the home references for the consituent data sources in this diagram; see
[2] For further information about exploding the domain, see these postings: F. Giasson, Exploding the Domain: UMBEL Web Services (April 20, 2008), UMBEL as a Coherent Framework to Support Ontology Development (August 27, 2008), Exploding DBpedia's Domain using UMBEL (September 4, 2008); M. Bergman, ' Exploding the Domain' in Context (September 24, 2008).
[3] See the announcement in M. Bergman, A New Constellation in the Linking Open Data (LOD) Sky (October 5, 2008).