"High-Resolution Laser Bathymetry“

Abstract

The high-resolution acquisition of aquatic structures is becoming increasingly important in the fields of water management, flood protection, and ecological assessment. Conventional hydrographic surveying methods reach their limits, particularly in shallow, difficult-to-access, or environmentally sensitive inland waters. Consequently, topo-bathymetric laser scanning technology (laser bathymetry) has become established as an area-wide complement to traditional profile-based surveying methods.

Skyability GmbH has more than ten years of experience in contactless geospatial data acquisition and employs airborne laser bathymetry (ALB) across a wide range of water bodies. These include alpine rivers, navigable waterways, lakes, and coastal environments. Depending on the required spatial resolution and survey coverage, UAV, helicopter (Fig. 1), and fixed-wing aircraft platforms are used. The objective is to achieve continuous topo-bathymetric mapping from the riverbank to the riverbed. In river surveys, multiple parallel flight lines are commonly flown to ensure consistent datasets over extended river reaches. In deeper water or under optically challenging conditions, multibeam echo sounding (MBES) is additionally employed.

The sensor configuration consists of an integrated ALB, airborne laser scanning (ALS), and RGB imaging system synchronized through a GNSS/INS navigation unit. The ALB sensor (RIEGL VQ-860-G) operates at the green wavelength of 532 nm and additionally records full-waveform data. This enables separate extraction of information from the water surface, water column, and riverbed. Under favorable conditions, the system achieves penetration depths of up to approximately 2.5 times the Secchi depth; in practical river applications, water depths of up to approximately 17 m have already been successfully surveyed. The system is complemented by an ALS sensor (RIEGL VUX-SYS-22), characterized by a small laser footprint, high measurement accuracy, and a flexible 360° field of view. High-resolution RGB cameras can optionally be integrated into the system.

The combination of ALS and ALB provides significant advantages for comprehensive aquatic mapping (Fig. 2). ALS delivers reliable topographic information together with precise measurements of the water surface, while ALB captures the bathymetric information beneath the water surface. Owing to its wider scan angle, ALS also improves strip overlap and provides additional tie points for georeferencing. Due to its optical design, the ALB sensor is limited to approximately ±20°, whereas ALS systems typically operate with scan angles of up to ±45°. Furthermore, ALS enhances vegetation penetration in riparian zones and extends data coverage beyond the actual water corridor.

For river mapping, flight planning with at least three parallel flight lines has proven effective. Depending on river width and the desired point density, long river sections can be surveyed efficiently. Typical acquisition rates range between approximately 60 and 100 km of river length per survey day. Acquiring the entire dataset within a short time period is essential because hydrological parameters such as water level, discharge, and turbidity may change rapidly and affect data consistency. Completing the survey within a single day minimizes these temporal effects.

Ideally, the survey results in a nearly seamless, high-density point cloud stored in a compressed raw data format. Subsequent post-processing consists of several stages. First, the precise aircraft and sensor trajectory is determined to establish accurate georeferencing. The raw data are then decompressed, filtered, and the recorded echoes are classified into usable point categories. One of the key processing steps is the correction of underwater signal propagation, accounting for both refraction and the reduced speed of light within the water column.

Data quality strongly depends on environmental conditions during acquisition. Influencing factors include increased water turbidity, unfavorable optical conditions, surface disturbances caused by floating debris such as leaves or organic material, and turbulence generated by vessel traffic. In addition, geometric shadowing caused by bridges, piers, hydraulic structures, or vehicles results in data gaps. At greater water depths, the physical penetration limit of the laser system constitutes another limiting factor.

The resulting point cloud (Fig. 3), with densities exceeding 100 points/m², serves as the basis for a variety of derived products. Besides digital surface models (DSM) and digital terrain models (DTM), a digital riverbed model (DTM-W) can be generated. Additional products include water depth maps, bathymetric contour lines, and shoreline delineations (Fig. 4). Raster-based analyses typically achieve spatial resolutions of up to 0.25 m, enabling highly detailed spatial investigations.

Repeated surveys are particularly valuable for temporal analyses. In river environments, they enable the detection and quantification of morphological changes resulting from both natural processes and anthropogenic interventions. These include erosion, sediment deposition, and structural modifications. Consequently, the datasets provide a reliable basis for analyzing river dynamics and long-term morphological development.

Overall, this contribution demonstrates that the combination of ALB, ALS, RGB imagery, and complementary MBES data acquisition provides an efficient and robust methodology for the integrated mapping of inland waters. Continuous acquisition from the shoreline to the riverbed enables the generation of consistent, high-resolution models of complex aquatic environments and provides a reliable basis for applications in hydrography, water management, and environmental monitoring. The presented results highlight the considerable potential of modern multisensor remote sensing technologies for scalable and practical aquatic environment analysis.