Abstract
Airborne Lidar Bathymetry (ALB) provides a rapid means of data collection that provides seamless digital elevation maps across land and water. However, environmental factors such as water surface induce significant uncertainty in the ALB measurements. In this study, the effect of water surface on the ALB measurements is characterized both theoretically and empirically. Theoretical analysis includes Monte Carlo ray-tracing simulations that evaluate different environmental and hardware conditions such as wind speed, laser beam footprint diameter and off-nadir angle that are typically observed in ALB survey conditions. The empirical study includes development of an optical detector array to measure and analyze the refraction angle of the laser beam under a variety of environmental and hardware conditions. The results suggest that the refraction angle deviations ( 2 σ ) in the along-wind direction vary between 3–5° when variations in wind speed, laser beam footprint size and the laser beam incidence angle are taken into account.
Highlights
Airborne Lidar Bathymetry (ALB) is a remote sensing technology that uses pulsed green lasers to measure coastal water depths [1]
Previous studies discussed the effect of sea-state on the ALB measurements, we have identified two gaps that we attempt to address in this study: (1) The water surface has not been decoupled from other environmental contributions in order to calculate a total propagated uncertainty (TPU) error budget for the ALB; and (2) extended empirical validations, needed to validate the model results, are missing
In order to verify the NOAA forensic analysis that ALB surveys can be considered International Hydrographic Office (IHO) S-44 order 1b [11], the total horizontal uncertainty (THU) and total vertical uncertainty values (TVU) values are estimated for ALB survey conditions with a wind speed of 5.25 m/s at 20◦ incidence angle (Table 5)
Summary
Airborne Lidar Bathymetry (ALB) is a remote sensing technology that uses pulsed green lasers to measure coastal water depths [1]. The laser beams that are emitted from an aircraft interact with the water surface, water column and seafloor and return to the receiver in the aircraft. These interactions are recorded as time-series signals, that is, waveforms. Bathymetric measurements are calculated by taking into account the time-of-flight difference between the water surface (surface return) and the seafloor (bottom return). Environmental factors such as water surface, water column and seafloor induce uncertainty in the ALB measurements [3,4]. Water surface plays a primary role on the measurement uncertainty as it significantly alters the laser’s ray-path direction and geometry as the laser pulse refracts into the water column [1]
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