# Wikidata Support in the Creation of Rich Semantic Metadata for Historical Archives
## 6 Related Work  
**Ontology-based annotation and ontology-based data access. ** Ontologies are a formal representation of categories, properties, and relations between concepts [15]. An ontology is usually employed to extensively represent a specific domain, thus providing for it a semantic vocabulary, but it also defines a schema that can be interconnected with already developed cross-domain knowledge bases, in order to achieve interoperability [16]. One of the multiple applications of an ontology is to serve as a semantic model for document annotation in those scenarios where users are required to enrich a document by exploiting the vocabulary provided by the ontology. For example, in the annotation framework proposed by [PERSON] and colleagues [17], ontologies are considered as the reference vocabulary, and the annotation process links the document to the semantic model by associating entity mentions to categories in the ontology. The proposed approach assumes that the ontology exhaustively represents the domain, including individuals, and annotators simply have to provide a link from the document to an element already available in the ontology. Consider, for example, a document about the magistrate [PERSON], telling that [PERSON] was assassinated by [PERSON]: The aforementioned framework assumes that the ontology already contains the characterization of [PERSON] and [PERSON] (and maybe also of the assassination), and the user is called to link the document with such entities. In PRiSMHA, we do not assume that all individuals that could be mentioned in archival documents are already present in the knowledge base, and we ask users of the platform to add and characterize new entities when needed for the annotation. Since this activity is time-consuming, the hypothesis we are presenting in this paper is to exploit external resources, such as Wikidata, to get information suitable for the characterizations of the new entities, such as [PERSON] and [PERSON]. A lot of works investigated the possibility of exploiting the semantic power of ontologies to positively impact on the semantic (meta)data accessibility [5; 18; 19; 20]. In PRiSMHA, ontology-based (meta)data access turns into ontology-based (meta)data production. In this respect, one of the goals of the project is to provide users with a platform that effectively supports them in the annotation process, hiding the complexity of the ontology, while, at the same time, offering them the full expressive power of the semantic schema. This paper provides a contribution in this direction: In order to alleviate the burden of ontology-based annotation (and, in particular, of the creation and characterization of new entities), PRiSMHA provides the users of the annotation platform with automatically extracted suggestions that can be simply sifted through, thus not only saving time but also simplifying the task (see Section 5). **Using Wikidata. ** The idea of exploiting Linked Open Data [21] to get historical and cultural data is not new (see, for instance, [22]). A hub in the LOD cloud (lod-cloud.net) is Wikidata [7], born in 2012 as a central storage repository for the Wikimedia project, it represents structured data and makes them freely accessible for visualization, extraction, and modification in human- and machine-readable formats. In Wikidata, the entries represent concepts or objects. The knowledge is stored as triples, which describe entries and employ properties to characterize them. Since the beginning, Wikidata has been perceived as a linking hub for domain-specific knowledge bases [23; 24]; in particular, Wikidata is an important point of convergence for the Cultural Heritage world in canonical [25; 26] and contemporary fashion, such as the _Social Networks and Archival Context_ project (snaccooperative.org), which aims at making the relations among persons, families, and organizations explicit, also by exploiting Wikidata.