## 3 Materials and Methods
### Flight Planning and Data Acquisition  
For any photogrammetric work with UAVs or even with conventional technology, flight planning must first be performed. This step requires some activities, such as obtaining a flight permit, selecting appropriate software, studying the characteristics of the project area, the specifications of the onboard digital camera, and the ground sample distance (GSD) value [12]. Also, the selection of weather conditions suitable for the flight process  
Figure 1: Workflow diagram of the methodology adopted in this paper.  
Figure 2: (a) DJI Mavic Pro 2 at the study area; (b) the study area.  
is one of the most important factors contributing to the success of the study. In this study, the field survey was conducted in a rural region away from vital places between ten to twelve o'clock on 18 November 2022. Shooting in the morning was selected so that the sun's rays could shine without much misdirection, and the wind speed was very low so as to not affect the drone's course.  
Three flights were conducted over the study area to verify and evaluate the positional accuracy of the UAV photogrammetry products and the influencing factors. Depending on the flights performed, multiple datasets were created, differing in acquisition angle, flight direction, and the georeferencing method. To compare the positional accuracies, eleven groups of dense point clouds, DEMs, and orthoimages with different parameters were created in this study (see Table 1).  
Pix4 Dcapture software (Ver. 4.11.0) was used to create the flight plan in this study, as shown in Figure 3. The flight altitude was set at 60 m with 75% longitudinal overlap and 70% transversal overlap for all UAV photogrammetry tasks. Taking into account the specific imaging time of the UAV used, three tasks were performed in this study.  
Figure 3: Pix4D capture software: (**a**) the main page, (**b**) first mission, (**c**) second mission, (**d**) third mission.  
The first task was to select the GRID MISSION option in the Pix4 Dcapture software, set the acquisition angle to 90\({}^{\circ}\) (vertical), and align the flight lines along the length of the study area (Figure 3b). These parameters resulted in a GSD value of 1.41 cm/pixel. In this task, 113 images were acquired.  
The second task was also selected as a GRID MISSION option in the Pix4 Dcapture software, and the acquisition angle was also set to 90\({}^{\circ}\) (vertical), but the direction of the flight lines was set in the direction of the width of the study area (Figure 3c). With these parameters, the GSD value was 1.41 cm/pixel, and the number of images in this task was 120 images.  
In the third task, the DOUBLE GRID MISSION option of the Pix4 Dcapture software was selected and the oblique acquisition angle was set to 70\({}^{\circ}\). With these parameters, the GSD value was 1.50 cm/pixel, and the number of images in this task was 241. The images in this task were divided into two groups: one group with flight lines along the length of the study area, which contained 126 images, and the other group with flight lines along the width of the study area, which contained 115 images. All missions were planned as autonomous flights. Figure 4 shows examples of four consecutive images acquired during this study.