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Recent developments in sensor technology yielded a major progress in airborne laser bathymetry for capturing shallow water bodies.\udModern topo-bathymetric small foot print laser scanners do no longer use the primary near infrared (NIR) signal (λ=1064 nm) but\udonly emit and receive the frequency doubled green signal (λ= 532 nm). For calculating correct water depths accurate knowledge of the\udwater surface (air-water-interface) is mandatory for obtaining accurate spot positions and water depths. Due to the ability of the green\udsignal to penetrate water the first reflections do not exactly represent the water surface but, depending on environmental parameters like\udturbidity, a certain penetration into the water column can be observed. This raises the question if it is even feasible to determine correct\udwater level heights from the green laser echoes only.\ud  \udIn this article, therefore, the near water surface penetration properties of the green laser signal are analyzed based on a test flight of the\udRiver Pielach (Austria) carried out with Riegl's VQ-820-G (532 nm) and VQ-580 (1064 nm) scanners mounted on the same airborne\udplatform. It is shown that within the study area the mean penetration into the water column is in the range of 10–25 cm compared to the\udNIR signal as reference. However, as the upper hull of the green water surface echoes coincides with the NIR signal with cm-precision,\udit is still possible to derive water surface models from the green laser echoes only via statistical analysis of aggregated neighboring\udechoes and robustly keep the underestimation of the water level below 6 cm. This especially holds for still and stationary flowing water\udbodies

Analyzing near water surface penetration in laser bathymetry – A case study at the River Pielach