Lidar is a measurement technique that measures the distance from a georeferenced laser sensor to a ground target by illuminating the target with pulsed light (Figure 1). The sensor detects the reflected pulses. The time shifts in laser return and wavelengths can then be used to create digital 3-D representations of the target areas, e.g. a river section. The laser pulses can have different wavelengths, most commonly red and green.
The red laser is more common and therefore cheaper. However, other than the green NeoDyn Yag laser it cannot penetrate the water surface. Therefor the green laser (wavelength 532nm) is most preferred in scientific and especially river related measurements (Figure 2). However, a certain distance from the source of the light to the eye of any person, passing accidentally the measurement, must be guaranteed due to safety reasons. This is usually not a problem, as this application is used for large scale approaches, mainly with so called Airborne Laser Scanning (ALS) where the laser is mounted to a small plane, a helicopter or even a large (more than 15 kg load) drone.
As the laser and also the plane or helicopter are a quite expensive set of devices, the measurements are usually taken by private contractors. Most of the time the client is a municipality, the government or a hydropower operator interested in the geometry of floodplains and the bathymetry of rivers.
In Norway, most of the rivers are currently measured with a red laser (hence no underwater registrations). These are relatively easily available for research institutions through the public website hoydedata.no. Also available on this website are green laser derived bathymetry data of a selection of river reaches.
The penetration depth of the green laser through water depends highly on turbidity, flow velocity, reflections on the surface / waves and water depth. It also depends most probably on the type of suspended load. Thus, green laser measurements are in many cases supplemented with echosounding data.
Problems can also be caused by any other overlap of objects such as trees above ground or submerged vegetation above river bottoms (Figure 3). This can lead to blurred areas and various z-data (height marker) for the same x/y coordinates. The measurements generate a point cloud. Post-processing is usually done in a specific software (commercial software, but also freeware) as is the case many other applications (such as SfM). However, as it is a usual output format, it is not a specific part of the Lidar system.
Relevant mitigation measures and test cases
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