## 2 Materials and Methods
### Multi-Sensor Integrated 3D Metric Survey  
In recent years, the role of geomatics has been increasingly consolidated with regard to the significant contribution it provides to researchers working in the field of built heritage. In particular, a clear trend has emerged involving the execution of multi-scale and multi-sensor 3D metric surveys, with the goal of providing the most complete and reliable documentation possible of historical buildings that are part of our legacy, with a specific focus on the sustainability of these documentation activities [32; 33]. The integration of different 3D sensing techniques, particularly range-based and image-based techniques, has now become  
Figure 15: Plan of the first level of the framework state prior to the intervention, 1995. Architect [PERSON]. A. I. A. V.  
an established practice that allows for the generation of dense and accurate 3D models [34], successfully supporting historical analyses and reconstructive hypotheses at various levels, as in the case of this paper. A significant aspect that should be emphasised, contributing to the sustainability of these metric survey operations, is the fact that these techniques (LiDAR, digital photogrammetry, etc.) are contactless and non-destructive approaches and therefore represent preferred techniques for the documentation of heritage assets which are intrinsically fragile and require tailored methods for being properly recorded.  
Staying on the topic of non-destructive and contactless techniques, thermography has for many years been an effective and efficient solution for monitoring historic buildings, providing a powerful tool for those involved in non-destructive diagnostics [35]. The usefulness of this technique is undeniable, and it represents an established and respected approach within the scientific community focused on monitoring and diagnostics.  
However, as previously underlined, thermal imagery has traditionally been used for visual inspections and through two-dimensional visualisations. To make the radiometric information derived from TIR (Thermal Infrared) images metrically measurable and referable to 3D models generated by the aforementioned 3D sensing techniques--specifically, TLS and digital photogrammetry--numerous research groups have developed and are developing new methods for integrating thermal images and high-resolution 3D models, primarily using photogrammetric approaches, achieving remarkable results [36; 37; 38].  
Also, in the case of the presented research, a similar approach has been followed, with the aim of providing a base to support the historical analyses. For this reason, a 3D metric survey campaign was carried out in 2022 using an integrated multi-sensor approach, as extensively described in [PERSON] et al. 2022 [20]. Namely, a TLS acquisition was performed, and the point clouds captured the geometries of the building of interest in its current state. Subsequently, a block of stereoscopic and convergent images (455) was acquired and co-registered with the TLS point cloud with the aim of providing high-resolution radiometry of the previously acquired TLS model. Finally, 308 thermal images were acquired to identify any thermal anomalies related to the different construction phases of the building in order to support the subsequent interpretive phase aimed at analysing historical maps and archival materials.  
The acquisition of the primary data and the associated processing operations related to the spatial co-registration are described in the following sections. Figure 16 presents the schematic workflow adopted in this research to establish a spatial connection between the 3D geometry of the investigated object and the detected thermal anomalies in order to enhance their interpretation.  
Figure 16: Flowchart of the proposed workflow.