Optimization of UAV Flight Missions in Steep Terrain

Abstract

Unmanned aerial vehicle (UAV) photogrammetry is one of the most effective methods for capturing a terrain in smaller areas. Capturing a steep terrain is more complex than capturing a flat terrain. To fly a mission in steep rugged terrain, a ground control station with a terrain following mode is required, and a quality digital elevation model (DEM) of the terrain is needed. The methods and results of capturing such terrain were analyzed as part of the Belca rockfall surveys. In addition to the national digital terrain model (NDTM), two customized DEMs were developed to optimize the photogrammetric survey of the steep terrain with oblique images. Flight heights and slant distances between camera projection centers and terrain are analyzed in the article. Some issues were identified and discussed, namely the vertical images in steep slopes and the steady decrease of UAV heights above ground level (AGL) with the increase of height above take-off (ATO) at 6%-8% rate. To compensate for the latter issue, the custom DEMs and NDTM were tilted. Based on our experience, the proposed optimal method for capturing the steep terrain is a combination of vertical and oblique UAV images.

3D Reconstruction of Coastal Cliffs from Fixed-Wing and Multi-Rotor UAS: Impact of SfM-MVS Processing Parameters, Image Redundancy and Acquisition Geometry

“…The acquisition geometry had also an impact on the processing time, probably caused by the difficulties found by the dense matching algorithm to correlate the large differences in scale present in each single image. As observed by [20], the time spent by each image for generating the dense point cloud was less in the high oblique Phantom images than in the low oblique Ebee image (29.22 s vs. 43.26 s).…”

“…First, a high-density point cloud will be generated using a conventional flight mission/planning software adequate for vertical façade inspection. Second, by detecting the location and extension of the data gaps (or holes), a new optimized aerial coverage could be planned, using for example an extended 3D version of the approach presented in [20], in order to imaging the areas where the gaps are presented. In addition, the 3D reconstruction of coastal cliffs by combining UAS-based surveys and SfM-MVS photogrammetry should also be documented in sufficient detail to be reproducible in time and space.…”

“…In fact, the acquisition geometry of aerial coverages from UAS can be a key factor in the geometric accuracy of the Bundle Block Adjustment (BBA), the density of points in the cloud and the reduction of the occlusions, that is, in reducing the gaps (holes) in point clouds [19]. Combining nadiral and oblique (off-nadir) images optimizes the photogrammetric surveys in stepped areas [20], improves the accuracy of the generated 3D models, but also increases the processing time [21,22]. By placing the 90 • angle off nadir for vertical cliffs, a high density of points in the reconstruction is commonly obtained [7].…”

Adaptable Flight Line Planning for Airborne Photogrammetry Using DEM

“…Determining an appropriate setting for flight is essential, as different combinations of flight parameters influence the flying workload, the quality and usability of final products [7]- [10]. The planning quality and flying quality will impact the subsequent mapping task directly [11], [12]. Flight line planning has two important planning quality parameters, the ground sampling distance (GSD) and the overlap which consists of the endlap and sidelap for frame cameras.…”

Improvement of Workflow for Topographic Surveys in Long Highwalls of Open Pit Mines with an Unmanned Aerial Vehicle and Structure from Motion

“…AMSL surveys are commonly used in geomorphic studies of mining areas [13]. Improving the final accuracy can be achieved by following three different strategies: (i) a combination of several missions at different altitudes; (ii) conducting manual flights and changing the altitude with a camera time-lapse function [14]; or, (iii) using a specific computer-based mission planning software, which allows the user to program the drone to automatically keep a constant AGL, thereby the drone continuously varies its flight altitude to adapt to changes in relief [15]. The latter is an example of how this type of software becomes operational in steep terrains.…”