## 3 Results
### Flight Proximity and Nap-of-the-Object Photogrammetry Accuracy
#### 3.1.3 DSM Accuracy Analysis  
Four sets of image data acquired at different flight proximities were processed to generate DSMs and analyzed with the DSM data generated for the control group (10 m) to explore the relationship between flight proximity and DSM resolution. Figure 10 shows that the DSM resolution decreases gradually as flight proximity increases. There is a close linear correlation between the DSM resolution value and flight proximity (R\({}^{2}\) = 0.914).  
Figure 8: (**a**) Before the point cloud is cropped. (**b**) After the point cloud is cropped.  
Figure 9: Mean and variance results of the distances between the four groups of point cloud data and the control group of point cloud data.  
Table 4 shows the DSM elevation errors for four sets of flight proximity schemes and the control group at nine GCPs. The highest DSM elevation error value, 0.7 cm, occurs at GCP No. 1 at a flight proximity of 30 m. Among the mean absolute errors of elevation for the nine GCPs, No. 1 has the highest, 0.5 cm; No. 8 has the lowest, only 0.1 cm. The mean absolute elevation error is lower when the flight proximity is 20 m, 0.1, and higher when the flight proximity is 25 m, 0.2 cm. The overall impact of flight proximity on the elevation of the DSM is approximately 0.2 cm.  
Concerning the resolution of the DSM, the value for the model generated by the control group (10 m) is 1.1 cm. In contrast, the resolution of the DSMs generated for the four different sets of flight proximities shows a minimum value of 1.3 when the flight proximity is 15 m. Therefore, theoretically, the DSM errors between the four sets of data and the control group at the nine GCPs could exceed 1.3 cm. However, as indicated in Table 4, the maximum error is only 0.7 cm.  
The DSM data generated from the four sets of data at varying flight proximities were compared against the DSM data from the control group, revealing volume changes among them. Increases would indicate precipitation, and decreases would indicate erosion. Only decreasing volume changes were observed in this case (Figure 11). The volume differences of the four set of data are all below 0.03 m\({}^{3}\). With flight proximities between 20 m and 25 m, volume changes remain relatively stable at around 0.025 m\({}^{3}\), suggesting that the DSM data are similar between these two flight proximities. Therefore, combining the positioning accuracy and point cloud data analysis, the flight proximity of 20 m emerges as the optimal scheme for nap-of-the-object photogrammetry in monitoring the Benggang study area.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Control Point** & **15 m** & **20 m** & **25 m** & **30 m** & **Mean Absolute** \\
**Number** & & & & & **Error** \\ \hline
1 & 0.3 & 0.3 & 0.6 & 0.7 & 0.5 \\
2 & 0.1 & 0.3 & 0.6 & 0.5 & 0.4 \\
3 & 0.4 & 0 & 0 & 0.1 & 0.1 \\
4 & 0.4 & 0.2 & 0 & 0.2 & 0.2 \\
5 & 0.3 & 0.1 & 0 & 0.1 & 0.1 \\
6 & 0 & 0.2 & 0.2 & 0.1 & 0.1 \\
7 & 0.1 & 0.2 & 0.5 & 0.2 & 0.3 \\
8 & 0.1 & 0 & 0 & 0.1 & 0.1 \\
9 & 0 & 0.1 & 0.3 & 0.1 & 0.1 \\ Mean absolute error & 0.2 & 0.1 & 0.2 & 0.2 & 0.2 \\ \hline \end{tabular}
\end{table}
Table 4: Elevation errors of DSMs for experimental groups (cm).  
Figure 10: Variations in DSM resolution with UAV flight proximity.