## 4 Conclusions  
This study shows the limitations of photogrammetry in obtaining data on the Z-axis underwater. Control points above water and underwater provided information to compare with the values obtained from the surveys to determine the error of the photogrammetry of surfaces in shallow water. Environmental factors, such as turbidity and luminosity, affected the accuracy of the information obtained from the controlled site using the same body of water. The ground control points (GCPs) gave coordinates that allowed the point cloud to be georeferenced correctly. According to [75], three control points should be adequate to obtain an acceptable degree of precision. As Figure 2 shows, five control points were used in these experiments, and their corresponding coordinates are presented in Table 3.  
On flights 23 to 26, with a turbidity of 47.41 NTU, two control points located at the bottom of the pool were no longer visible. Consequently, the values of the underwater control points could not be obtained, as is shown in Table 6.  
Ref. [76] concluded that when using a UAV, there is a greater predisposition for the flight path to be influenced by the wind due to the weight of the tool and the flight height, especially when it does not have a controlled path. For this reason, the flights were carried out at heights of 30 m and 50 m over a defined section, as is shown in Table 4 and Figure 5, respectively. The wind speed remained in the range of 1 m/s to 3 m/s, with 22 of the 26 flights carried out in a wind speed of 2 m/s.  
Temperature influences the operation of the UAV, especially when it is used in high temperatures. In the present study, the temperature did not affect the operation of the UAV since the flights were carried out at temperatures below 20 \({}^{\circ}\)C (specifically, in the range of 15 \({}^{\circ}\)C to 19 \({}^{\circ}\)C, as is shown in Table 5).  
On the other hand, the results revealed that the highest luminosity generated the lowest error up to a depth of 0.97 m. In this case, two situations were observed for the turbidity variations: (1) at shallower depths (not exceeding 0.49 m), increased turbidity levels positively contributed to error reduction; and (2) at greater depths (exceeding 0.49 m), increased turbidity resulted in increased errors.  
Finally, UAV-based photogrammetry can be applied, within a known margin of error, in bathymmetric surveys of underwater surfaces in shallow waters not exceeding a depth of 1 m. In practice, this survey method could be applied to map shallow rivers, lakes, or coastal waters. It represents an affordable option for surveying any water rise changes or periodically monitoring soil erosion.  
Further research could focus on comparing the information obtained from photogrammetry with that obtained from other bathymmetric survey methods involving drones, such as those using echo sounders or integrating lidar sensors into UAV platforms [77], or even those using aerial or boat-type drones.  
Conceptualization, A.A.D.S. and A.L.T.; methodology, M.A.V.O. and A.L.T.; software, M.A.V.O. and G.T.U.I.; validation, A.A.D.S., A.N., A.L.T. and M.A.V.O.; formal analysis, M.A.V.O. and A.L.T.; investigation, S.R.L.R. and G.T.U.I.; resources, A.A.D.S.; data curation, A.A.D.S. and M.A.V.O.; writing--original draft preparation, S.R.L.R. and G.T.U.I; writing--review and editing, A.A.D.S. and A.N.; visualization, S.R.L.R., G.T.U.I. and M.A.V.O.; supervision, A.A.D.S.; project administration, A.A.D.S.; funding acquisition, A.A.D.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Scientific Research Institute (IDIC) of the Universidad de Lima under research project funding number PL56.007.2018.  
The data supporting the findings of this study are available within the article.  
The authors thank the Universidad de Lima for supporting this research.  
The authors declare no conflict of interest.