# Contour Mission Flight Planning of UAV for Photogrammetric in Hillside Areas
###### Abstract  
Unmanned Aerial Vehicle (UAV) photogrammetry is an effective method for acquiring terrain information. However, in hillside areas, the terrain is complex, and the altitude varies greatly. The mission flight is planned by using equal altitude; in the actual shooting, the geometry and resolution of the pixel within the same image or between adjacent images will be inconsistent due to the different shooting distances. The number and accuracy of point clouds are affected. We propose a contour mission flight plan method, which involves designing flight plans based on the existing digital elevation model (DEM) and the desired flight altitude. This method for aerial photography is more effective in maintaining a consistent ground shooting distance during image capture. Experiments were conducted using a simulated DEM and the undulating terrain of the Kaohsiung Liugui area in Taiwan to verify the effect of contour mission flight planning in the hillside area. The results show that, due to the significant variation of terrain in the hillside area, the use of a contour mission flight plan for aerial photography can be more consistent with the originally planned altitude but requires more planning and operating time. The minor height difference, higher overlap, and improved accuracy of the results show that contour mission planning can provide a suitable solution for UAVs in hillside areas. U 2023 13 7666 10.3390/app1313766 10.  
the distance variation to the object, which can cause problems in stereoscopic coverage or result in significant differences in ground sampling distance (GSD) [20; 21; 22]. The GSD is the relationship between camera equipment parameters (camera focal length, camera pixel) and flight altitude above ground level (AGL) [23]. It is essential to evaluate GSD size before capturing UAV images. As a result, UAV-based topographic reconstructions of mountainous environments are often considered less accurate than those carried out in other flat terrain settings. In studies related to flight planning in mountainous areas, many researchers have proposed solutions for some characteristics of mountainous terrain variations. During the mission, it is desirable to maintain a constant flight altitude relative to the ground surface to obtain a uniform ground resolution. In a study of volcano monitoring, [PERSON] et al. consider that the altitude of the UAV mission is not absolute but is always referenced to the altitude of the origin (take-off point). When the whole survey area is decomposed into sub-regions, special attention needs to be paid to referring the altitude of each mission to a specific origin [24]. [PERSON] et al. also consider the existing topography of the survey area. In areas with steep slopes, very high-resolution digital models and orthophotos are needed to analyze the geometry of the fracture surfaces in detail. The terrain is divided into blocks according to its elevation. Then the flight mission is carried out separately according to the blocks to maintain the same shooting distance to the ground [25]. [PERSON] et al. dealt with extreme topography in an almost vertical road cut-slope in Spain using a UAV. They took horizontal images in four flight lines on a vertical plane, and oblique images were taken at 45 degrees downwards in two passes [26]. [PERSON] et al. mapped landslides associated with earthquakes in Greece. The actual UAV survey covered the landslide area with vertical and oblique views [27]. [PERSON] et al. proposed a method that combines vertical and oblique images to capture steep terrain [28]. [PERSON] suggests that schemas are the combined classical block flight and corridor flight in one mission for slope and line objects [19]. The methods described above have improved the suitability of UAV surveys in steep terrain by tilting the photo or defining different heights of the flight routes. In this study, we adopt the concept of contour to plan the UAV flight mission. Free digital elevation model (DEM) data can easily generate the contour.