Abstract
This work addresses issues related to digital 3D models of cultural heritage in the context of FAIR data. It introduces OntPreHer3D, an ontological extension of the CIDOC CRM for the comprehensive preservation of 3D models. OntPreHer3D semantically maps the intricate relationships between 3D models, their components, and their real-world equivalents. Through specialised classes and properties, it provides a robust framework for documenting diverse 3D models, including digitised objects, hypothetical reconstructions of lost heritage, and visualisations of never-built architectural projects. The paper highlights various documentation challenges and scenarios, underscoring the crucial need for data protection and the ability to re-model content in case of loss of the original 3D data. The presented ontology is rooted in the application ontology OntSciDoc3D, developed by the Institute of Architecture at Mainz University of Applied Sciences (AI MAINZ), for the scientific documentation of hypothetical 3D digital reconstructions. This further development included assessing the current limitations of the OntSciDoc3D, and official extensions of the CIDOC CRM family in the context of documenting 3D models. Crucially, given the inherent interpretative nature of hypothetical reconstructions, the presented framework integrates Inference to the Best Explanation (IBE) to transparently capture the human reasoning, choices, and uncertainties driving reconstruction decisions. It facilitates the semantic documentation of not only what was modelled, but also why and how confident we are in the interpretation, including the systematic quantification of uncertainty. Finally, the paper briefly presents results of a case study of the hypothetical reconstruction of the ruins of the Synagogue in Speyer (Germany) conducted by four different modellers equipped with the same source package, and predefined granularity and scope of the research. The study empirically examines how varying technical decisions made at the 3D software level impact the final models, demonstrating the critical role of such detailed process documentation in ensuring the long-term preservation and future reusability of 3D cultural heritage content.