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In this paper, we present the general design of a hydrographic laser scanner (prototype instrument) manufactured by the company Riegl Laser Measurement Systems in cooperation with the University of Innsbruck, Unit of Hydraulic Engineering. The instrument utilizes a green laser source (λ=532 nm) capable of penetrating the water column. Very short laser pulses (1 ns) are emitted and the backscattered signal is digitized enabling sophisticated waveform processing online during the flight and in post processing. In combination with a traditional topographic airborne laser scanner (λ=1500 nm) mounted on the same platform a complete hydrographic and topographic survey of the riparian foreland, the water surface and river bed can be carried out in a single campaign. In contrast to existing bathymetric LiDAR systems, the presented system uses only medium pulse energy but a high pulse repetition rate of up to 250 kHz and, thus, focuses on a detailed description of shallow waters under clear water conditions. Figure 1: 3D-view of the point cloud acquired with the experimental Riegl hydrographic laser scanner VQ-820-G in July 2011 Different fields of applications are discussed in the paper and the results of a first real-world test flight are presented. It is shown that: (i) the high pulse repetition rate enables a point density on the ground of the water body of approx. 10-20 points/m, (ii) the short laser pulses together with waveform processing enable a discrimination between water and ground reflections of less than 25 cm, (iii) the combination of a topographic and bathymetric laser scanner enable, for the first time, the acquisition of the geometry data for hydraulic modeling in a single survey, thus, providing a much more homogeneous data basis, and (iv) the high point density and the ranging accuracy of less than 10 cm enable a detailed and precise description of the river bed morphology, thus, providing an excellent data source for calibrating and validating sediment transport models. With the focus on capturing shallow water bodies under clear water conditions, the instrument is not designed for mapping of broader rivers (turbid water due to suspended material). However, even for these rivers the presented technique can close the gap between the river bank (captured, e.g., by topographic LiDAR) and the main channel (e.g., by echo sounding).

Airborne Hydrographic LiDAR Mapping - Potential of a new technique for capturing shallow water bodies