Mar 24 2020
10:30 am - 12:00 pm
Track Names: TOPO-BATHYMETRIC LIDAR, Tuesday 10:30 - 12:00
Session Date: Mar 24 2020 10:30 am - 12:00 pm
Accuracy assessment and performance evaluation of a novel topo-bathymetric UAV laser scanner
While UAV-borne LiDAR sensors delivering 3D point clouds in survey grade quality are available for topographic applications since around 2015, RIEGL recently introduced the VQ-840-G, a compact airborne laser scanner for combined hydrographic and topographic surveying. The scanner features a selectable beam divergence of 1-6 mrad resulting in laser footprints of 7.5-45 cm when flown at an altitude of 75 m. For the 6 mrad beam divergence, the employed class 3B laser features a nominal and extended nominal ocular hazard distance of 15 m and 75 m, respectively. The pulse repetition rate is user selectable between 50-200 kHz and the point density for UAV-borne operation is approx. 100 points/m2. The conical scanning (Palmer scanner) with an off-nadir angle of 20° results in a swath width of 55 m for a typical flying altitude of 75 m AGL. This makes the instrument especially well-suited for capturing medium sized clear-water rivers and constitutes a cost-effective alternative to topo-bathymetric acquisition from a manned platform. To assess the precision, accuracy, and depth performance of the system, several ponds and a section of the Pielach River were captured with the VQ-840-G mounted on an octocopter UAV on Aug, 27th. To assess the sensor’s maximum penetration depth, four 1x1 m2 metal plates floating in a known depth of 1.5-3 m were installed in one of the ponds (Secchi depth: 1.5 m). To test the absolute accuracy of the sensor system, checkerboard targets were anchored at the river bottom in depths between 0.5-2 m and surveyed with a Leica TS16 total station. First results revealed a maximum depth of approximately 3 m, thus 2x the Secchi depth, based on data from online waveform processing. A further depth increase is expected by averaging waveforms in post processing. The results of different scanner settings and flight configurations are compared and referenced to the scan data of other sensors (e.g. VUX1, BDF-1) and terrestrial ground truth measurements.
TU Vienna, Department Of Geodesy And Geoinformation
Lidar Mapping by the Canadian Hydrographic Service (CHS) in Support of the Ocean Protection Plan (OPP).
The CHS has undertaken a major near shore mapping campaign to help improve hydrographic knowledge in poorly surveyed areas of Canada. This new data is being used to facilitate safety to navigation through the creation and updating of charts and other nautical publications. This extensive near shore mapping initiative involved collecting bathymetric lidar and acoustic multibeam data across multiple project areas across Canada including, Haida Gwaii (the Queen Charlotte Islands) in British Columbia, the Great Lakes, the Gulf of St. Lawrence in Quebec and various locations in Atlantic Canada. The program has heavily leveraged private sector involvement to conduct both lidar and multibeam bathymetry to complement on-going CHS hydrographic survey activities. This presentation will describe the lidar technology that the CHS utilized to map thousands of kilometers of the Canadian coastal zone, provide an overview of our current projects, describe our future objectives and share our experiences up until now.
Canadian Hydrographic Service
Airborne LiDAR topo-bathymetry and the potential of full waveform analysis for water bottom detection
Airborne topo-bathymetry using green wavelength has established as a state-of-the-art survey technology for shallow water areas along coasts, lakes and rivers. The penetration depth into the water depends on turbidity, waterground substrate and its color. Depths down to 10-15m are possible in clear water. The newest LiDAR sensors (VQ880G, RIEGL) do not only deliver a discrete pointcloud based on online waveform processing, they also record the full waveforms (FWF). So far, only the discrete pointcloud was processed. The FWF is mostly not considered in ALB processing, but was always seen as a data source of high potential related to penetration depth and water/soil conditions. Due to its high amount of storage volume, its handling requires new approaches for project-oriented massdata processing. AHM GmbH developed a new software solution HydroVISH to visualize the FWF without data processing. By combining the discrete pointcloud with the FWF, we envision the FWF directly on points to check, if a detailed FWF-analysis is reasonable instead of having just the discrete pointcloud. HydroVISH delivers a complete FWF analysis chain including deconvolution of the system wave with the FWF and detection algorithms of the deconvoluted signal. Promising are curve-fitting algorithms: With a single/more e-function/s a convolution with the system wave can be carried out following a curve fitting with the FWF. This is successful in the water because the damped backscatter signal follows an e-function. Another possibility is pure peak detection, with gradients of the FWF and threshold values a new pointcloud can be derived. Within a research project for the German water board authorities (WSV, BfG, BAW), FWF-analysis and its possibilities was investigated for the river Elbe. Results allow to improve the penetration depth from 1.5 to 2m and the overall ALB waterground coverage roughly up to 20-30%. The described approach is now applied within the federal survey project of 600km Elbe.
Airborne HydroMapping GmbH
Evaluation of 4X Topo-Bathymetric Lidar Point Density for Object Detection
In 2018, the Chiroptera II-4X shallow water topo-bathymetric lidar sensor was upgraded to the 4X standard, which increased the point density by a factor of 4. During the past two survey field seasons, experiments were carried out to validate and evaluate the potential benefits of the increased point density of the system. The study site has been surveyed on a regular basis with the sensor since 2014 with a maximum depth penetration of 9 m below sea level. During the 2018 survey a maximum depth of 11 m was achieved using the 4X upgrade. Solid 1 cubic meter cubes were deployed at various depths on the seabed during the survey. The lidar point density was elevated along with the measured dimensions of the cubes at different depths. Variations in the colour of the cubes (white versus green) were also examined to determine the effects of reflectivity of the bathymetric lidar returns and the metrics of the object. In all cases, the 4X application increased the number of points defining the cubes by 25% and the apparent size of the cubes increased with depth with or without the 4X upgrade. The point density of the green cube was less than that of the white cube at a given depth and showed less of an increase in point density with the 4X upgrade, confirming the importance of reflectivity of the seabed for target detection. Other targets included flat 2.25 square meter targets that rested on the seafloor and small cinder blocks with retro reflector tape which were readily identified using the bathymetric lidar reflectance signal. All deployed objects had pressure sensors to measure water level and Hobo light sensors on them. Practical applications of this enhanced point density include improved detail of the seabed morphology (charting) and the ability to map submerged aquatic vegetation (seagrass) within the point cloud directly (benthic habitat mapping).
Applied Geomatics Research Group, NSCC
Q&A and panel discussions with session presenters
There will be a 15 minute Q&A /panel discussion with the presenters of the Topo-bathymetric Lidar session.