## 4 Ontology Development  
The present section is dedicated to defining an ontology structure following the conceptualisations and definitions carried out in the methodological workflow. The ontology has been designed in Protege (an open-source ontology editor and framework for building intelligent systems) [35]. Here, different classes (concepts with definitions) with their relations (object properties), characteristics and values (data properties and datatypes) have been added. Moreover, thanks to the implemented tools in Protege, it was possible to integrate and merge existing ontologies of IFC and CityGML. To enrich the first level (Figure 1, concepts and relation), with information relevant to topology and other useful elements for generating the second level (the domain knowledge), we also considered conceptualisations from CIDOC CRM (the core ontology for cultural heritage, by the International Committee for Documentation), CIDOC CRMba (for the documentation of archaeological buildings) and CIDOC CRMgeo (for spatiotemporal properties of temporal entities and persistent items) ontologies. Figure 6 shows the hierarchy classes with their properties. Various classes derive from different standards, ontologies and reasonings about HBH elements. A particular focus has been paid to AbstractBuilding and building classes (building element and building part) definitions from IFC and CityGML, defining meromorphic (has-part) and hierarchical relations. Moreover, classes from the cultural heritage domain, such as the architectural element, castle and fortified structure have been added. Every concept in the developed ontology has URI (Uniform Resource Identifier) and a description derived by standards analyses (above reported) or CIDOC-CRM ontologies documentation. Table 3 presents an example of definitions from CIDOC. The ontologies' properties have been implemented in Protege by using object properties and data properties (Figure 7) They correspond to the relationships between concepts (entities/classes defined) and their attributes or values. Some of them are ad-hoc created for the domain ontology of historical built heritage; others are integrated and imported from existing ontologies (such as CIDOC-CRM). Figure 8 shows entities and relations in the WebVOWL application, and Tables 4 and 5 show the reported list of properties with their domain and range entities and datatypes. Figure 5: Castellina angular tower taller than the other part of the building. Central courtyard and angular towers. \begin{table}
\begin{tabular}{p{113.8 pt} p{284.5 pt}} \hline \hline
**CIDOC Concepts** & **Definitions** \\ \hline Built Work & Man-made things such as freestanding buildings, components of buildings, and complexes of buildings. \\ \hline Morphological Building Section & Man-made things that are considered functional units for the whole building (e.g., rooms, foundations, roof, and so forth). \\ \hline Filled Morphological Building Section & Man-made things that are completely filled with matter. Instances of filled morphological building section are components of the functional units of a built work. These are elements like walls, floors and ceilings \\ \hline Physical Man-Made Thing & This class comprises all persistent physical items that are purposely created by human activity. This class comprises man-made objects, such as a sword, and man-made features, such as rock art. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: CIDOC concepts definitions. Figure 6: Ontology concepts and their relations.