Above-Ground and Underground Architectural Heritage Documentation: A Scan-to-HBIM-to-XR Approach for Historic Centres

Abstract. This study presents a methodology integrating above-ground and underground 3D survey data within a Heritage Building Information Modeling (HBIM) framework, further developed into an eXtended Reality (XR) environment. The historic centre of Cagli (Italy), with its stratified architectural and archaeological layers, served as a pilot case. Data acquisition included Terrestrial Laser Scanning (TLS), UAV photogrammetry, and 3D Ground Penetrating Radar (GPR), producing detailed georeferenced datasets of buildings and subsurface anomalies. These were modeled as parametric volumes in an HBIM environment and enriched with structured metadata, including geometry, historical sources, and risk information. 
The HBIM model was seamlessly integrated within an XR application built in Unity and connected with Google Cloud services, enabling real-time data exchange. Users can interact with the 3D model using a Mixed Reality (MR) headset, accessing metadata and uploading new information on-site. The system supports scenarios such as post-earthquake assessment by associating building records with standardized forms. 
Results demonstrate the potential of connecting HBIM with XR not only for enhanced visualization, but also for collaborative heritage management in situ. Integration allows for intuitive, data-driven exploration of complex heritage layers, facilitating decision-making processes and opening new perspectives for monitoring, documenting, and engaging with historic urban environments. The proposed workflow is replicable and adaptable to other historic villages, which, despite their unique identities, often pose similar conservation challenges due to their layered complexity and vulnerability.