Airborne laser depth sounding: improvements in position- and depth estimates by local corrections for sea surface slope

With the development of laser and data acquisition technology, the sounding density in laser depth sensing can be increased. When the distance between each laser shot is less than half the length of the sea surface wave, it is possible to estimate the local angle of the sea surface waves. The estimated slope angle is used to calculate the angular deflection of the light beam that penetrates the water surface. This allows a better estimation of the beam center position on the sea bottom than can be done by averaging of a flat sea surface. The possibility to improve the position and depth accuracy is examined with experimental data from a laser depth sounding system by comparing the repeatability of the depth estimations from different flights over the same area. The experimental data is collected from two different sites with bottom depths between 3 m and 9 m. The surface maximum wavelength and significant waveheight varies up to 8 m and 0.6 m respectively. The different measurements on a site are done using the same flight direction and a shot distance of approximately 1 m. It is seen that the combined position and depth corrections for local surface slope angles can reduce the maximum depth difference between different measurements over the same area by up to 10 cm. The use of different spatial filtering of the water surface data along with different algorithms to calibrate the position and depth corrections is demonstrated.