## 2 Materials and Methods
### Methodology
#### 2.2.2 Data Collection  
For this investigation, a 67.5 m\({}^{2}\) swimming pool within the campus grounds of the University of Lima, located in Santiago de Surco, Lima, Peru, was set as the study area, as is shown in Figure 2. As Figure 4 shows, three types of flights were carried out: with an empty pool (flights 1 to 10), with the pool filled with clean water (flights 11 to 18), and with copper sulfate added to the pool water to simulate changes in the turbidity of the water (flights 19 to 26). Figure 3: Plan and section using measurements from the topographic equipment. Figure 2: Ground control points (GCPs) in the study area. A1, A2, B1, B2 and UL23 are the names of the control points. Copper sulfate was used to suspend particles on the water surface by dissolving the component and mixing it using a water pump. Copper sulfate (CuSO4) is an inorganic component that controls algal and plant growth in pools, lakes, and ponds [65]. However, its use in large quantities can affect the immediate turbidity of the water until it dissolves completely. It is important to note that because of the water movement and the time that elapsed during the day, different turbidity layers were present in the pool during each flight, and different tests for turbidity were performed along the pool. Water samples were taken at different pool areas for each flight to verify the turbidity levels. When recording the climatological parameters during the flights, luminosity, wind speed, and air temperature were considered [49]. These parameters were measured for every flight. The survey flight plans were developed using the Pix4 Dcapture free mobile app [64] and performed over the pool area indicated in Figure 2. Before starting these UAV flights, test flights were performed at heights of 10 m and 30 m. However, the resulting images were blurry at 10 m due to the surface proximity. In addition, the low height made recognizing the common points in the processed images difficult. Therefore, we used the following constant parameters in our flights: flight heights of 30 m and 50 m, as shown in Figure 5, 80-75% overlap, and a camera angle of 90\({}^{\circ}\). Table 4 lists the 26 flights conducted based on these parameters. Figure 4: (**A**) Empty pool. (**B**) Pool filled with clean water. (**C**) Pool filled with copper sulfate. Figure 5: \(Cont\). For the flights in Figure 4C, the turbidity level of the pool was managed throughout the day in NTU (rephelometric turbidity units). In Figure 6, the steps in the varying and measuring of the turbidity are denoted. During the day, pool water samples were taken before adding the copper sulfate compound to change the turbidity level of the water. The original turbidity level was set at 3.74 NTU (the World Health Organization (WHO) has established that water turbidity should not exceed 5 NTU for human consumption; however, less than 1 NTU is ideal (WHO, 2020)) for flights 11 through 18. For flights 19 to 22, 1.5 kg of copper sulfate and pool water were mixed in a container (Figure 6a). After the component was dissolved, it was poured into the pool (Figure 6b), and an electric pump was used to recirculate the water and obtain a uniform mixture,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Selected Flight Parameters**} \\ \hline
**Flight** & **Start Time** & **Flight Height (m)** & **GSD (cm/px)** & **Camera Angle** & **Overlap** \\ \hline
1 & 7.53 & 30 & 0.70 & 90\({}^{\circ}\) & 80-75\% \\
2 & 7.58 & 50 & 1.17 & 90\({}^{\circ}\) & 80-75\% \\
3 & 10.41 & 30 & 0.70 & 90\({}^{\  
as is shown in Figure 6c. The first turbidity test yielded an average of 20.54 NTU when recirculating the component in the pool water (Figure 6d). We then ran turbidity tests every 2 h to check whether external factors, such as light and temperature, affected the results [15; 43]. Finally, for flights 23 to 26, another 1.5 kg of copper sulfate was added (making a total of 3 kg) during these tests. The average turbidity level, in this case, was 47.41 NTU.