The semantic back-end features the integration of ontologies. Thus the WissKI system is able to load nearly any ontology. However as the system is a communication platform for curated knowledge, a common top level ontology is needed for all WissKI Systems. The chosen top level ontology has been developed for more than ten years by a group of knowledge experts from museums, archives and libraries in conjunction with philosophers and computer scientists. This group is known as the CIDOC Conceptual Reference Model Special Interest Group (CRM SIG) which forms a Working Group of the International Committee for Documentation (CIDOC) as part of the International Council of Museums (ICOM). The ontology created by this group is called the CIDOC Conceptual Reference Model (CRM). The CRM claims to be a "formal ontology intended to facilitate the integration, mediation and interchange of heterogeneous cultural heritage information". Therefore the CRM is strongly recommended as a top level ontology for all WissKI systems to benefit from all features of WissKI.
A special feature of the CRM is its event-centric approach. Every aspect of a real world item is connected to an event. For instance a work of art is not simply "made" by an artist but it is the outcome of a so called "production event". The artist is a person in a specific role who participated in this event. Additional aspects that are normally assigned to the object itself, such as the creation date or the place of origin are connected with this event too. This approach offers the possibility to create a time-line of events (production, modification, destruction etc.) in the lifetime of an object and to attach dates and places to them. It additionally allows new connections between objects via special context information (weather, other people of importance, travelling information etc.) connected with their "production event". In 2006 Version 3.4.9 of the CRM has been accepted as ISO-standard 21127. Nowadays, it is often taken into account for developing information management systems in the cultural heritage domain. The current version 5.01 defines 86 entities (classes, concepts) and 137 properties (relations, roles), each of them explained by a scope note and illustrated with examples. The CRM SIG provides the ontology only as a paper document. In the run-up to the WissKI project the AI Department developed with its partners an OWL DL implementation of the CIDOC CRM called Erlangen CRM (ECRM). The ECRM incorporates all entities, properties, scope notes and examples of the CRM. In addition it defines cardinalities and constraints that are not explicitly defined in the CRM but mentioned in the scope notes.
Since the CRM/ECRM is an upper ontology, most concepts have an abstract character (e.g. E22 Man-Made-Object). To refine and specialise the ECRM concepts and relations, the project partners developed the System Ontology. The System Ontology extends the ECRM and provides more specialised concepts where the CRM is not specific enough for an implementation.
The System Ontology serves as an upper ontology for specific application ontologies that can be generated and extended by the users of WissKI. Therefore the WissKI system allows the users to add their project specific application ontology. The only precondition for the application ontologies to be used in the WissKI system is that they have to import and use the System Ontology as upper ontology. Several examples for domain specific ontologies will be given from the ZFMK and the GNM as use cases and as basis for the development of initial application ontologies.