## 3 Results
### Data Acquisition
#### 3.1.2 Thermal Imaging  
Thermal imaging is a widely used non-invasive method to map the infrared radiation that a surface emits [29; 30]. It is mainly used in search-and-rescue missions with UAS systems and for buildings' energy efficiency evaluations in real estate and civil engineering. In the built cultural heritage field, thermal imaging is relevant for the assessment of the state of conservation. One of the main features of this method is being able to view, in real time, the different behaviours of construction materials, water infiltration [31; 32; 33], cracks [34], or detachments in built structures [35]. Thermal imaging can refer to two types of recordings: qualitative (distribution of the estimated radiation intensity over the surface) and quantitative (distribution of the real temperature of the surface with compensation for other factors that affect the measurements). For the accurate measurement of the temperature of the surface, apparent temperature, emissivity, ambient temperature, relative humidity, and surface features must be taken into account. The sensor plane of the camera must be parallel with the recorded surface at a known distance. An important aspect of outdoor quantitative measurements carried out with thermal cameras is that they must be carried out before sunrise or after sunset because the Sun's radiation and sky reflections greatly impact the accuracy of the measurements. Other reported systems, like that in [36], are able to perform both 3D and IR scanning regardless of the lighting conditions. Two IR imaging systems were employed. The UAS DJI M30T used for aerial photogrammetry already has its own integrated thermal camera that records thermograms at the same time as photo images. The thermal camera has an equivalent focal length of 40 mm, 640 \(\times\) 512 pixels, and a measurement accuracy of \(\pm\)2 \({}^{\circ}\)C. The images were saved and then processed directly in Agisoft Metashape PRO v2.0, an approach similar to that in [37]. This image set was processed directly without any prior adjustments in other software. The purpose in this case was not to obtain a usable 3D model but to generate textures that can later be baked for the accurate 3D model generated from the photo image set. Even so, with the 3D mesh generated with the medium-quality factor in Metashape, the result was better than expected (Figure 5, right, middle section), considering that thermal images retain far fewer details about the surface than photographic images. Figure 5: Aerial thermal data acquisition and processing. **Left**: The DJI RC Plus smart controller for DJI M30T during data acquisition. **Right**: Collage of screen captures within Metashape with textured model left, solid view (centre), and the sparse point cloud (right) during the photogrammetric processing. The second IR imaging system employed was a state-of-the-art FLIR T1020 camera, produced by Teledyne FLIR LLC, USA. It features a 120\({}^{\circ}\) FOV lens and 1024 \(\times\) 768 pixel resolution, with very good image stabilization. The thermal sensitivity of this system is less than 20 mK at 30 \({}^{\circ}\)C.  
With this system, two image sets were recorded: one after sunset and another in the morning. The set acquired in the morning was after sunrise, so the data are not quite reliable considering the errors induced by direct Sun exposure and other environmental reflections. These data were not included in the project. All images were recorded at the same distance from the church exterior walls, which was 5 m (see Figure 6). The high image stabilization of the camera allowed the images to be recorded handheld. For each position, the images were shot from two height levels: one for the lower part of the wall, below the horizontal decorative band that splits two vertical registers, and the other one above it to maintain a parallel orientation of the camera sensor to the wall.