A Multi-Source Non-Destructive Testing Survey for Digital Modelling and Reconstruction: The Case Study of Palazzo Ripetta in Rome

Conventional visual inspection of civil infrastructure and cultural heritage is limited by subjectivity, restricted accessibility, and safety risks. Developing an integrated methodology to accurately reconstruct and survey existing structures and infrastructures, with millimetric geometric accuracy and reliable information on material conservation conditions, is therefore essential to overcoming these constraints. With the progressive deterioration of historic structures and major cultural-heritage assets, including bridges, monuments, historic buildings, and aging roadways, the need for highly accurate measurement and documentation has become increasingly critical. To this end, the use of multi-source Non-Destructive Testing (NDT) techniques for the acquisition of fast and accurate geometric and radiometric information is a necessity. This research proposes an integrated geomatic workflow designed to digitize complex built environments through multi-sensor data integration, enabling advanced analysis within immersive virtual environments. The methodological approach relies on a robust topographic reference frame established via Global Navigation Satellite Systems (GNSS) and high-precision Total Stations, ensuring the global georeferencing required for engineering reliability. To capture the full complexity of the assets, the study employs a synergistic acquisition strategy. Terrestrial Laser Scanning (TLS) is a process of generating a high-resolution point cloud representing the geometry of the ground and other features that can be reached from ground level, while simultaneously employing Unmanned Aerial Vehicles (UAVs) to address any occluded areas caused by the ground perspective and enabling the inspection of buildings' upper levels and structural components. The UAV equipment consists of an optical-camera payload that enables millimetric-resolution acquisition for high-definition photogrammetric modelling. 
All these multi-source surveying tools were employed to reconstruct a digital model of a real architectural complex, the Bernini Hall, now incorporated into the Palazzo Ripetta ensemble in Rome, Italy. Within this context, one of the most significant historic and artistic spaces within the building preserves a refined architectural and cultural heritage of substantial value. The multi-source datasets were subsequently post-processed for georeferencing and for the registration of the different acquisitions, resolving geometric discrepancies and producing a single, multi-layered 3D point cloud. This digital model forms the basis for structural analysis, also enabling the assessment deformation reconstruction, with millimetric accuracy. The novelty of this framework lies in its shift from traditional static digital-model analysis to immersive visualization. The digital model, derived from the integration of UAV imagery through photogrammetric reconstruction and LiDAR point-cloud data by TLS, is imported into a Virtual Reality (VR) environment using Unity®, a dedicated software optimized for high-fidelity rendering, enabling immersive exploration and navigation within the model, with millimetric accuracy consistent with the NDT-based survey. The use of Head-Mounted Displays (HMDs) enables users to experience a fully immersive digital representation, navigating the space as if physically present. The reconstructed digital model improves the accuracy of inspections in critical or hard-to-access areas, opens new ways for structural-health-monitoring efficiency and broadens opportunities for the valorization and remote accessibility of the built environment.