# An Ontology-Based Representation of Valued System for HBIM
## 4 Overall Methodology  
Definitions of ontologies are necessary to set up the organization of building information [43]. As discussed in the previous section, many initiatives are ongoing to define a standard ontology for BIM. However, the case of HBIM presents some specific challenges since the building itself is a cultural heritage artefact. For this reason, HBIM is on the edge between different disciplines and the definition of a reference ontology is even more a crucial issue. In such a multi-domain environment, different methods can be used to defining a shared ontology:  
1. Develop a unique ontology that covers the different domains involved. 2. Each domain develops its own ontology in an independent way and in a second step the ontology is aligned in terms of vocabulary and relations for information exchange. 3. A \"foundation\" ontology is defined sharing the basic concept and relationship for each domain and starting from this each domain develop specific ontologies. Each solution presents pros and cons. The development of a unique ontology requires a significant standardization and a general agreement among different actors. For that reason, this solution is quite difficult to achieve. In addition, the resulting ontology would have thousands of concepts and relations that would be difficult to understand and maintain. Finally, this solution will lead to a quite rigid structure of the ontology. Indeed, the addition of a new actor would determine the redesign of the ontology and of the general agreement. A more flexible approach would be the definition of completely independent ontologies for the different actors working in the HBIM domain. For example, specific ontologies for as-built survey, restoration, scheduling, cost estimating etc. The main advantage of this approach is the fact that new actors can contribute in the overall project with a new ontology without the need of modifying existing ontologies defined in another domain. However, the lack of shared vocabularies results in difficulties in the integration of completely independent ontologies. In order to align them a manual and time consuming \"mediation\" phase is necessary in order to provide concept mapping, i.e., finding correspondences between concepts in two different ontology [44]. The definition of a \"foundation\" ontology can partially cope with the shortcomings of the previous two ontology definition methods. The main aim of the \"foundation\" ontology is to provide a common vocabulary among the concepts used by different domain ontologies. In this case, a loose interaction among the different actors is needed to define a relatively general framework. Starting form this shared view each domain can develop its own specific domain ontology by extending the \"foundation\" ontology. In this architecture, Semantic Web Rule Language (SWRL) rules or SPARQL queries are used to integrate data from different domains. The feasibility of this approach in the case of BIM was proved in [PERSON] et al., 2017 [19] to integrate design, cost, and schedule domain ontologies. The aim of this paper is to extend this concept by enlarging the \"foundation\" ontology and including specific aspects that are relevant in the case of HBIM. For example, HBIM is generally an as-built BIM and the survey concept has to be added in the general framework. The addition, the survey concept is fundamental to report the accuracy and reliability of the HBIM model. In addition, it can provide the link to other survey products such as orthoimages and/or discrepancy maps among the original data and the HBIM model. We have to also underline that this approach is not in contrast with the standardization approaches existing in both BIM and cultural heritage communities. Indeed, for the \"foundation\" ontology we modelled building elements at a high abstraction level making direct reference to standard ontologies and schemas (e.g., ifcOWL), thus enabling the extension to other domain as well as integration with existing cultural heritage ontologies (e.g., CIDOC-CRM).