## 2 Materials and Methods
### Multi-Sensor Integrated 3D Metric Survey
#### 2.3.1 TLS and Digital Photogrammetry  
In total, the building was surveyed through the acquisition of eight scans, which were collected while paying particular attention to ensure adequate overlap between adjacent scans aimed at facilitating the subsequent co-registration operations.  
Additionally, a set of 455 was acquired with the aim of performing a photogrammetric reconstruction to provide a high-resolution texture to the TLS model. For this task, a full-frame high-resolution (Digital Single Lens Reflex (DSLR) camera was used (model: Canon EOS 5D Mark II). The average acquisition distance was 20 m, leading to an estimated ground sampling distance (GSD) of approximately 0.005 m.  
The TLS scans were registered using the software Trimble RealWorks v. 11.3, exploiting an automatic plane-based algorithm which enables the merging of the eight point clouds. The following procedure allowed for reaching an average deviation between the adjacent scans lower than 0.002 m. After the optimisation and filtering process--carried out using the open-source software CloudCompare v. 2.11 Alpha--the final point cloud was composed of 60 million points (Figure 17).  
The digital images were processed using the consolidated SfM-based software Agisoft Metashape v. 1.8.0, enabling the relative orientation and the generation of a sparse cloud of tie points (Figure 18). The absolute orientation of the photogrammetric block was performed using a set of points derived from the TLS point cloud, with the aim of co-registering the two datapoints. Natural points that could be easily and unambiguously identified were selected for this task (for example, edges, corners or points characterised by significant radiometric contrast compared to the surrounding ones due to a difference in terms of material). The Root Mean Square Error (RMSE) observed on the Ground Control Points (GCPs) and Check Points (CPs) at the end of this procedure was approximately 0.010 m.  
Exploiting the fact that, at the end of these registration procedures, the TLS scans and the photogrammetric block are both aligned in the same local reference system, the TLS point cloud was textured using the oriented images. In fact, although the laser scanner is equipped with an integrated camera that, at the end of the scan, captures a set of digital images, allowing an RGB value to be associated with each collected point during the processing phase, the radiometric resolution of the photogrammetric data acquired using the DSLR camera sensor is significantly higher. In Figure 19, it is possible to observe the difference in terms of radiometry between the two textured point clouds.  
Figure 17: Point cloud after registration of the eight TLS scans.  
Therefore, considering the accuracies achieved during the photogrammetric process (ca. 1 cm, which is consistent with the data resolution, since the GSD is approximately 5 mm) and the results obtained from the registration of the TLS point clouds, it is possible to observe that the model derived from the integration of the range-based approach and the image-based one can support architectural scale resitutions (1:50/1:100).