Metadata about documents, artifacts, and other objects is traditionally created by filling in metadata fields (e.g., dc:subject) with values taken from controlled vocabularies, such as keyword thesauri and classifications (e.g., LCSH). When this practice is applied to Linked Data a new problem is encountered: Linked data typically comes from different organizations and domains where mutually incompatible thesauri and classifications are used in annotations. This breaks links between the annotations, which creates data silos in linked data clouds. This paper argues that to solve the problem it is not enough to link data using primarily owl:sameAs mappings, as is customary today in, e.g., the Linked Open Data cloud, but one has to link annotation vocabularies, i.e., ontologies, into a Linked Open Ontology cloud. Since class hierarchies (using, e.g., rdfs:subClassOf) form the backbone of annotation ontologies, a key problem here is how to create and maintain a system of interlinked hierarchical ontologies so that the transitive subclass relations are not broken, when reasoning across ontologies in fundamental tasks such as query expansion and property inheritance. As a solution, we present the steps necessary for transforming thesauri into a cloud of ontologies and maintaining the system when ontologies are updated. Our approach has been used and evaluated in practice in building a cloud called KOKO of sixteen ontologies, with a total of 47,000 concepts, forming a basis for the Finnish national Linked Data architecture. KOKO has been published as an ontology service and is in use in, e.g., collection managing systems for both data indexing and semantic search.
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