## 4 Method
### 3D Survey: Aerial and Terrestrial Photogrammetry of the Church of San Valentino in Siusi, Italy  
The first step of the workflow concerns planning of image acquisition. In order to have a complete survey of the area, two different platforms were used: (i) a UAV platform (DJI Mavic Mini) to cover the roof and the bell tower, and (ii) terrestrial acquisition with SM-G975F camera. To guarantee a connection among the terrestrial and the UAV acquisition, oblique images were acquired covering the external facades of the church. A summary with the cameras and acquisition angles is presented in Table 1. The image acquired with the drone was designed to create photogrammetric strips along with the church, guaranteeing a minimum overlap of 70% among the different strips. Terrestrial images were designed as a single strip along the perimeter of the church. The external porch was complemented with additional two strips to cover the timber roof structure. Image orientation was carried out combined with both terrestrial and drone images by using the software Agisoft Metashape. Due to the main purposes of the work (i.e., generation of a geometric model to be used for VR and XR application) less concern was paid to the metric accuracy of the results. For this reason, no ground control points were \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Drone Platform** & **Terrestrial Platform 1** \\ \hline Camera & DJI FC7203 & SM-G975F \\  & & (Samsung Galaxy S10+) \\ \hline Focal & 4.49 mm & 4.32 mm \\ \hline Sensor size & 6.475 mm \(\times\) 4.856 mm & 5.751 mm \(\times\) 4.313 mm \\ \hline Average ground sampling distance & 0.005 m & 0.002 m \\ \hline Acquisition mode & Nadir and oblique & - \\ \hline Acquisition angle & Nadir \(-90^{\circ}\) & \(0^{\circ}/10^{\circ}\) \\  & Oblique \(-40^{\circ}/-10^{\circ}\) & \\ \hline Number of acquired images & 197 & 589 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Church exterior acquisition parameters for the drone and the terrestrial platform. Figure 4: Frescoes and paintings of the Church. Source: [PERSON]. used and check points were used to georeference and scale the church. Instead, some manual measurements taken in the church (i.e., main dimensions of the exterior walls, openings, etc.) were used as a scalebar to set up the main dimension of the model properly. Once the images were oriented, the dense matching of the church was carried out by using Agisoft Metashape, subdividing the reconstruction into separate tiles. This choice was adopted to speed up the reconstruction (Figure 5). Indeed, the estimated processing time (process run on an Intel i5 Core with graphic card NVIDIA Quadro T1000) for the whole project was more than 3 days. The subdivision into tiles allowed to process the church in less than 4 h. The indoor of the church was acquired with a standard photogrammetric procedure. Images were acquired in a photogrammetric block using a Sony A7 III (Table 2). The processing was carried out in Agisoft Metashape and also in this case the scaling of the model was carried out by using manual measurements. No connection exists between indoor and outdoor blocks. Due to the aim of the work the positioning of the models was carried out manually using as a reference an existing plan of the church. \begin{table}
\begin{tabular}{c c} \hline \hline  & **Indoor Acquisition** \\ \hline Camera & Sony A7 III \\ Focal & 12 mm \\ Sensor size & 35.6 mm \(\times\) 23.8 mm \\ Average ground sampling distance & 0.002 mm \\ Acquisition mode & \\ Acquisition angle & 0\({}^{\circ}\)/50\({}^{\circ}\) \\ Number of acquired images & 52 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Church indoor acquisition parameters. Figure 5: The mesh textured model and orthophotos obtained using Agisoft Metashape. The Paradigm of Complexity in Built Heritage and Architecture Representations: From Simple Points to HBIM Objects  
Digital 3D survey techniques quickly highlight the shape and dimensions from a morphological and typological point of view.