## 3 Materials and Methods
### Flight Planning and Control  
To collect high-quality data, it is necessary to carefully plan the UAV flight according to the technical limitations and attributes of the platform, its equipment, and sensors. To create the plan, it is first necessary to conduct visits to the site or gain access to detailed maps. A plan can then be created in stages:  
1. A photogrammetric block is designed;
2. The flight path of the strips, along with their forward and side overlaps, are established;
3. The theoretical scale is determined using ground sample distance (GSD) calculations. The photogrammetric block is determined by the size and location of the archaeological site and comprises take-off and landing points, along with the initial flight direction. From a theoretical  
Figure 3: DJI Inspire 1 Pro and Inspire 1 RAW platforms designed to assist in aerial imaging. Figure 2: Pagodas at Wat Maha That. (**a**) Prang structure. (**b**) Chedi structure. perspective, the plan includes external orientation parameters (spatial and angular positions: X\({}_{0}\), Y\({}_{0}\), Z\({}_{0}\), \(\omega\), \(\varphi\), and \(\kappa\)) and internal camera parameters (principal point and principal distance: X\({}_{0}\), Y\({}_{0}\), and \(c\)). The latter are set by the manufacturer of the camera. Once these parameters have been determined, it is possible to calculate the scale, flight altitude, side and forward image overlaps, number of strips, and number of images to be collected for each strip. If the platform does not feature gyro-stabilization, incorrect positioning will likely occur, necessitating the use of GNSS observations, i.e., GCPs (Figure 4). The final step is to ensure that the flight adheres to the original flight plan. This is accomplished using geometric controls, which ensure that the thresholds do not exceed certain criteria:  
1. The vertical deviation of the image must be controlled by monitoring the angle between the optical axis and the nadir direction at the projection center. 2. Changes in direction and the drift effect must be controlled by monitoring the difference in the image coordinate system between the flight axis and the \(x\)-axis directions. In a planned flight, the theoretical value is 0\({}^{\circ}\) because the \(x\)-axis and the flight axis are in alignment. 3. The scale must be controlled. During digital photogrammetric flights, the definition of theoretical scale depends on the pixel size that is projected to the ground. Maintaining a constant GSD requires the ground to be a plane, but this is rarely the case in practice. The GSD is therefore dependent on the flight altitude and ground elevation, and can be calculated at any given point using a digital elevation model. The main aim is therefore to obtain an estimate for the GSD for each image and for each strip, which will eventually provide the mean GSD for the entire photogrammetric block upon completion of the flight. 4. The extent of the overlap must also be controlled. After calculating the scale and GSD, it is necessary to verify the forward and side overlaps that rise between images and between strips (Figure 5). This is essential, owing to the high degree of correlation between the altitude, GSD, and overlap values. The control of overlap depends on the verification of the side overlap between images and strips, and the forward overlap found between sequential images and strips. As this particular platform type has no gyro-stabilizers and the GNSS is not highly accurate, it is necessary to use relatively wide limits to control the capture of images geometrically. Forward and side overlap limits are set at a minimum of 80% and 40%, respectively, while the error of the camera position is 3 m, and errors for the angular deviations are 0.2\({}^{\circ}\) (\(\varphi\)), 2\({}^{\circ}\) (\(\varphi\)), and 0.2\({}^{\circ}\) (\(\kappa\)). The greatest variation is 10% over the mean GSD size. Flight directional changes are restricted to a maximum of 2\({}^{\circ}\).