# Contour Mission Flight Planning of UAV for Photogrammetric in Hillside Areas
## 6 Conclusions  
In an era of rapid development of information, how to quickly obtain three-dimensional spatial data has become the major point of research; the UAV aerial photography system, as a new aerial remote sensing tool, has unique advantages: high spatial resolution, flexibility, and mobile, high efficiency, low cost, fast map. In addition, it is also widely used for rapid, detailed investigation of land changes, disaster monitoring, and other aerial  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & \multicolumn{2}{c}{**Control Points (cm)**} & \multicolumn{3}{c}{**Check Points (cm)**} \\ \hline  & **Horizontal** & **Elevation** & **Total RMSE** & **Horizontal** & **Elevation** & **Total RMSE** \\ \hline Grid & 6.4 & 8.8 & 10.9 & 8.9 & 13.4 & 16.1 \\ \hline
**Contour** & 5.1 & 7.3 & 8.9 & 6.5 & 9.8 & 11.8 \\ \hline \end{tabular}
\end{table}
Table 2: The RMSE of control points and check points of two types of flight planning.  
Figure 15: The distribution of point clouds for two types of routes. (**a**) Grid-type flying mission, (**b**) contour-type flying mission. The point clouds of the grid-type are sparse, especially in the valleys (red circles) with holes in the point clouds. In contrast, the contour-type has a more complete representation.  
photography operations in high-risk areas. In recent years, carrier and camera technology has constantly evolved, allowing for more convenient flight, longer aerial photography, and higher resolution cameras to quickly reconstruct UAV aerial images into a more detailed three-dimensional picture.  
Nowadays, UAVs and cameras are rapidly developing and maturing, becoming new tools for acquiring geographic information. However, in the face of mountainous areas and complex terrain, if UAVs take pictures in the same traditional way, the images obtained will have different slope distances, inconsistent image proportions, and inconsistent image resolution. Therefore, excellent and effective flight planning is needed to ensure the accuracy of the subsequent 3D reconstruction. This study proposes a contour-based aerial photography method to maintain the same height as the original plan for areas with significant terrain changes.  
To verify the differences between the different flight methods, we used the existing DEM to simulate the terrain and select a field with complex terrain changes for testing. The study demonstrates that the contour-based route planning method is effective in non-uniform terrain and produces an average AGL height of 244 m with a standard deviation of 22 m. The image coverage range is unaffected by uneven terrain, maintaining over 80% overlap. The reconstruction surface results show an RMSE of 11.8 cm and a point cloud density of 48 points/m\({}^{2}\). The assessed image quality indicates that the contour-based route planning method is better than the generally used grid route planning methods.  
The contour method is superior to the commonly used grid method in both comparisons. Because the contour method can avoid height differences due to slope fluctuations, it can maintain a consistent flight height and has good results due to compatible image resolution, providing a good solution for mountain monitoring by UAV.  
Conceptualization, funding acquisition, methodology, writing--original draft preparation, C.-S.H., D.-Y.L. and D.-H.H.; data curation, software (data experiments), writing--review and editing, visualization, C.-S.H., D.-Y.L. and D.-H.H.; formal analysis, investigation (physical experiments), validation, C.-S.H. and D.-Y.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
All data are available on request.  
All the works were performed in NKUST. They are appreciated.  
The authors declare no conflict of interest.