HYDRO 2016 Paper 7A2
Patrick Westfeld, Katja Richter, Hans-Gerd Maas, Robert Weiß
Airborne LiDAR bathymetry (ALB) is a technique to derive the underwater topography by airborne laser scanning, provided shallow water areas and sufficient water transparency. It is recently gaining much attention both due to new sensor developments allowing for a much higher spatial resolution in scanning riverbeds and due to EU regulations requiring hydrographic measurements in water bodies at regular time intervals.
Geometric modelling in ALB data processing is more complex than in conventional laser scanning. Refraction effects of the laser pulse passing the air/water and water/air interfaces have to be taken into consideration. The simplest method is assuming a horizontal and planar water surface at which the laser beam is refracted on the basis of Snell's Law. Even small deviations from the planarity, already caused by moderate swell, can lead to significant measurements errors. Strictly speaking, the local wave-induced water surface inclination needs to be known for every single laser beam. Otherwise wave movements lead to a geometric displacement of the point hit at the bottom of the water body. This effect can take on significant dimensions in the meter range, depending on water depth and wave heights.
This contribution investigates the effect of wave patterns on refraction and subsequently on coordinate accuracy in ALB. For that purpose, typical wave patterns were simulated and their impact on the 3D coordinates at the bottom of the water body were analysed. It has been shown that, depending on water depth and wave heights, the effect on lateral bottom point displacement can take on significant dimensions in the range of several decimetres, in some cases even several meters. Furthermore, local depth errors in decimetre range have to be taken into consideration. The simplified assumption of averaging wave effects often made in many (large footprint) ALB applications is not fulfilled. The effect scales up for modern LiDAR bathymetry systems, which came with much smaller footprints and cannot be neglected in most situations.
There are basically two methods for reducing these coordinate errors: A strict procedure would require modelling the instantaneous water surface in order to perform a strict differential ray tracing for each laser pulse. This will often not be possible due to insufficient information for instantaneous water surface modelling. As an alternative, correction terms may be applied for typical wave patterns, which may be derived from the simulations shown in the paper.