Concept and Performance Evaluation of a Novel UAV-Borne Topo-Bathymetric LiDAR Sensor

Abstract

We present the sensor concept and first performance and accuracy assessment results of a novel lightweight topo-bathymetric laser scanner designed for integration on Unmanned Aerial Vehicles (UAVs), light aircraft, and helicopters. The instrument is particularly well suited for capturing river bathymetry in high spatial resolution as a consequence of (i) the low nominal flying altitude of 50-150 m above ground level resulting in a laser footprint diameter on the ground of typically 10-30 cm and (ii) the high pulse repetition rate of up to 200 kHz yielding a point density on the ground of approximately 20-50 points/m 2 . The instrument features online waveform processing and additionally stores the full waveform within the entire range gate for waveform analysis in post-processing. The sensor was tested in a real-world environment by acquiring data from two freshwater ponds and a 500 m section of the pre-Alpine Pielach River (Lower Austria). The captured underwater points featured a maximum penetration of two times the Secchi depth. On dry land, the 3D point clouds exhibited (i) a measurement noise in the range of 1-3 mm; (ii) a fitting precision of redundantly captured flight strips of 1 cm; and (iii) an absolute accuracy of 2-3 cm compared to terrestrially surveyed checkerboard targets. A comparison of the refraction corrected LiDAR point cloud with independent underwater checkpoints exhibited a maximum deviation of 7.8 cm and revealed a systematic depth-dependent error when using a refraction coefficient of n = 1.36 for time-of-flight correction. The bias is attributed to multi-path effects in the turbid water column (Secchi depth: 1.1 m) caused by forward scattering of the laser signal at suspended particles. Due to the high spatial resolution, good depth performance, and accuracy, the sensor shows a high potential for applications in hydrology, fluvial morphology, and hydraulic engineering, including flood simulation, sediment transport modeling, and habitat mapping.Remote Sens. 2020, 12, 986 2 of 28 UAV-based 3D data acquisition was first accomplished using light-weight camera systems, where advancements in digital photogrammetry and computer vision-enabled automatic data processing workflows for the derivation of dense 3D point clouds based on Structure-from-Motion (SfM) and Dense Image Matching (DIM). Due to advancements in UAV-platform technology and ongoing sensor miniaturization, today compact LiDAR sensors are increasingly integrated on both multi-copter and fixed-wing UAVs, enabling 3D mapping with unprecedented spatial resolution and accuracy. The tackled applications include topographic mapping, geomorphology, infrastructure inspection, environmental monitoring, forestry, and precision farming. While UAV-borne laser scanning (ULS) can already be considered state-of-the-art for mapping tasks above the water table, UAV-based bathymetric LiDAR still lacks behind, mainly due to payload restrictions.The established techniques for mapping bathymetry are single-or multi-beam echo sounding (SBES/MBES), incl...

Field evaluation of a compact, polarizing topo‐bathymetric lidar across a range of river conditions

“…A technology with potential to address issues associated with both shallow water mapping and the safety issues inherent to streamgaging is airborne lidar bathymetry (ALB). Over the past two decades, ALB systems initially designed for marine and coastal environments (Guenther, Cunningham, LaRoque, & Reid, 2000;Nayegandhi, Brock, & Wright, 2009) have been used for surveying inland waterways, specifically river channels (Hilldale & Raff, 2008;Kinzel, Legleiter, & Nelson, 2013;Kinzel, Wright, Nelson, & Burman, 2007;Legleiter et al, 2016;Mandlburger, Pfennigbauer, Schwarz, Flöry, & Nussbaumer, 2020;Mandlburger, Pfennigbauer, Wieser, Riegl, & Pfeifer, 2016;McKean et al, 2009;Saylam, Hupp, Andrews, Averett, & Knudby, 2018;Tonina et al, 2020). Manufacturers have produced bathymetric lidars that vary in system design, power, and performance, but mostly employ a 532 nm (green wavelength) waterpenetrating laser.…”

Flight Planning for LiDAR-Based UAS Mapping Applications

“…Finally, reality-like models of an urban region and a communication tower are scanned in two Similar to conventional camera-based missions, LiDAR sensors are widely used nowadays on board of UASs for different mapping applications like 3D modeling, land surveying, power line inspection, forestry, smart agriculture, mining, shallow water bathymetry, etc. [22][23][24][25]. It is worth mentioning that LiDAR sensors vary in their scanning mechanism as being spinning or solid state, multi or single beam, and in their range measurement principle, either by using time of flight TOF or the amplitude modulation of a continuous wave (AMCW) systems [26,27].…”

The optical river bathymetry toolkit

“…For smaller-scale investigations, unmanned aircraft systems (UAS) have become a viable tool for mapping depth, often using photogrammetric structure from motion algorithms (e.g., Dietrich, 2017). In addition, bathymetric light detection and ranging (lidar) has emerged as an alternative remote sensing technology for measuring bed elevations via water-penetrating, green wavelength lasers (e.g., Mandlburger, Pfennigbauer, Schwarz, Flöry, & Nussbaumer, 2020). Most recently, Niroumand-Jadidi, Bovolo, and Bruzzone (2020) built upon the spectrally based approach emphasized herein by incorporating multiple band ratio predictors that vary as a function of depth.…”