Jul 2026· The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences· Vol XLIX-B1-2026, pp. 175-183· 0 citations· 3 references
Abstract
Abstract. Green-wavelength LiDAR systems enable high-resolution 3D sensing in underwater environments, but the geometric evaluation of measurements acquired across the waterline remains challenging. This is mainly because traceable reference instruments typically operate only in air, while refraction at the waterline systematically affects both the 3D point cloud and the geometry of partially submerged objects. This study presents a controlled experimental framework for assessing waterline-induced effects in an Underwater LiDAR (ULi) system, using a terrestrial laser scanner (TLS), the Z+F IMAGER 5016A (IMAGER), as an above-water reference. A rigid reference frame (RRF) spanning the waterline was deployed in a swimming pool. First, the RRF was scanned by the IMAGER under in-air conditions to establish its reference geometry. Subsequently, in the waterline configuration, the ULi system measured the complete RRF, while the IMAGER captured only its above-water part. The analysis investigated refraction- and interface-related effects on the 3D point cloud and geometry in the above-water, cross-waterline, and underwater parts of the RRF. For a physically meaningful assessment, the evaluation considered overall geometric deviations and rigid-body-invariant internal quantities, including pairwise distances, which are independent of the overall pose of the RRF. Refractive-index sensitivity was analyzed by perturbing the refractive index and quantifying the resulting changes in the derived geometric quantities. The proposed workflow provides a practical and traceable basis for isolating waterline-related refraction effects, evaluating their impact on 3D point cloud geometry, and assessing refractive-index sensitivity.
Abstract. Next to the known photogrammetric or acoustic measurement techniques, nowadays LiDAR is a promising option for high-resolution surveys or monitoring of underwater structures in low turbid and shallow waters. The objective of this investigation is to determine the performance of an underwater laser scanner and an air-borne bathymetric laser scanner for mobile data acquisitions. For this purpose, the underwater LiDAR (ULi) from Fraunhofer IPM is installed on a vessel. To enable the registration and georeferencing of the scan data, ULi is added by an Inertial Navigation System (INS) and two GNSS antennas. On the other hand, the air-borne bathymetric laser scanner (ABS) is integrated into an unmanned aerial system (UAS). Both systems are used to survey in the same measurement area to acquire comparable data sets. With the selected measurement settings, both systems can penetrate through water for more than 10 m and are able to resolve small underwater structures. Caused by shorter measurement ranges, ULi offers a point density which is approximately four times higher than the ABS and is able to resolve vertical underwater structures. The advantage of the UAS mounted ABS is that it can survey in shallow areas which cannot be accessed by vessels.
Annette Scheider, Sethmiya Herath Mudiyanselage, Christoph S. Werner et al.· The International Archives o...· 0 citations
Abstract. This study empirically evaluates the geometric accuracy of point cloud data acquired using an underwater lidar (ULi) system in a tropical shallow water environment. The field test was conducted in the tropical waters of the Seribu Islands, Indonesia, characterised by relatively low turbidity. Terrestrial laser scanning (TLS) and close-range photogrammetry were employed as independent reference datasets. Geometric discrepancies between datasets were quantified using the multiscale model-to-model cloud comparison (M3C2) algorithm, and errors were statistically characterised using the median and median absolute deviation (MAD) to ensure robustness under non-normal distributions. The results indicate that the error of ULi relative to TLS is 0.008 ± 0.012 m, while the error relative to photogrammetry is 0.006 ± 0.013 m. In comparison, the discrepancy between photogrammetry and TLS is smaller, at 0.002 ± 0.004 m. Dimensional analysis of an acoustic Doppler current profiler (ADCP) frame further shows that ULi agrees with TLS and photogrammetry within the millimetre to centimetre range (0.000–0.015 m). Larger deviations in specific segments are attributed to local effects, including edge-related artefacts. Overall, the results demonstrate that ULi provides reliable geometric measurements in shallow-water conditions with low turbidity. Despite slightly lower accuracy compared to terrestrial methods, the system shows potential for underwater mapping applications, particularly in shallow water environments.
Mentari K. Azzahra, F. Muhammad, Arnadi Murtiyoso et al.· The International Archives o...· 0 citations
Abstract. Airborne laser bathymetry (ALB) is an efficient and accurate tool for mapping submerged environments, particularly shallow water bodies that are difficult to access with surface vessels. Modern ALB systems can achieve accuracies comparable to SONAR. However, multiple factors, including geo-referencing, water surface modelling, and range measurements, influence the resulting point cloud, making analytical error propagation challenging. Empirical evaluation against reference data is therefore essential, but difficult: ALB accuracy is typically in the low centimetre range, requiring reference data of equal or higher accuracy. Robotic total stations enable acquisition of underwater reference data for shallow water depths, e.g., up to 4.5m, with expected accuracies between 3mm to 10mm, depending on water depth, which approaches the inherent accuracy of ALB and limits the evaluation significance. In this study, we assess a UAS-based ALB data set from a mountain lake in Austria using reference planes and points acquired by robotic total stations. We separate the accuracy analysis into trueness and precision to isolate the effects of geo-referencing and water surface modelling from the intrinsic uncertainty of the LiDAR sensor. The results show that geo-referencing introduces the largest systematic bias, while the precision of the ALB data remains approximately 1 cm to 2 cm, even for submerged measurements. These findings demonstrate the high accuracy of state-of-the-art ALB systems and provide a framework for rigorous accuracy assessment in shallow aquatic environments.
Lucas Dammert, Jan Rhomberg-Kauert, P. Amon et al.· The International Archives o...· 0 citations
Lidar is a key technique for 3-D ocean observation, and improving its penetration depth has long been a central objective in system design and optimization. However, multiple scattering induces lateral photon redistribution, causing time-of-flight-based penetration-depth estimates to be overestimated due to misinterpretation of delayed photons as deeper signals. Therefore, the actual contribution of improvements in key lidar system parameters—including laser pulse energy, receiver aperture ( $D$ ), receiver field of view (FOV), and transmitted wavelength—to penetration-depth enhancement should be critically reassessed. Here, a semianalytical Monte Carlo (MC) model is developed to simulate lidar backscattering signals in typical Case-1 waters, assuming a vertically homogeneous water column and neglecting surface wave effects. Simulations are performed across a broad range of chlorophyll-a concentrations (Chl- $a$ ) and for multiple observational configurations, including spaceborne, airborne, shipborne, and underwater platforms. Photon step lengths and corresponding physical depths are simultaneously tracked to quantify effective penetration depth. The results demonstrate that system lidar parameter optimization markedly improves effective penetration depth under weak multiple-scattering conditions, whereas its impact becomes marginal when multiple scattering is strong. Notably, in coastal waters, the conventionally assumed benefits of enlarging the FOV or shifting the transmitted wavelength toward the green band are generally ineffective in enhancing effective penetration depth. Sensitivity analyses using different scattering phase functions (SPFs) yield consistent conclusions. This study provides a quantitative reassessment of lidar system parameter optimization under multiple-scattering conditions, refines the understanding of penetration-depth enhancement in optically complex waters, and offers theoretical guidance for the design and performance evaluation of oceanic lidar systems.
Yi-Rui Guo, Ming-Jia Shangguan, Zhong-Ping Lee et al.· IEEE Transactions on Geoscie...· 0 citations
Abstract. Mapping at the air–water interface in shallow coastal environments remains challenging due to the need to integrate heterogeneous datasets acquired under different geometric and operational conditions. This study presents a modular uncrewed surface vehicle (USV)-based system for simultaneous above- and underwater photogrammetric surveying supported by differential GNSS positioning. The system integrates a rigid multi-camera configuration, GNSS time synchronization, and a direct georeferencing workflow based on trajectory interpolation and lever-arm calibration. Experimental results from a rocky coastal site in Sardinia (Italy) show that underwater photogrammetry can achieve centimetric absolute accuracy (2–4 cm horizontally and ~8 cm vertically) without underwater ground control points. The USV enables controlled and repeatable acquisition in very shallow environments, while UAV photogrammetry complements the reconstruction of the emerged area. Limitations related to image quality and refraction effects are discussed. The system represents a flexible and scalable solution for integrated coastal mapping and monitoring.
Sergey Khokhlov, F. Menna, E. Nocerino· The International Archives o...· 0 citations
Abstract. Bathymetric Laser Scanning (BLS) enables high-resolution mapping of underwater topography using green-wavelength laser pulses that penetrate the water column. However, precise georeferencing of the BLS data is affected by refraction at the air–water interface, which displaces submerged features and affects conventional strip adjustment methods. This paper introduces an integrated refraction-aware georeferencing workflow that combines refraction correction with trajectory and boresight optimization within a unified adjustment framework. Implemented using the scientific OPALS laser scanning software, the workflow starts with direct georeferencing of uncorrected laser returns, derives a water surface model, applies Snell’s law-based refraction correction, and performs iterative strip adjustment until convergence. The approach was validated using UAV-borne topo-bathymetric LiDAR data from Lake Alm (Almsee) in Upper Austria, captured with a RIEGL VQ-840-GE sensor system. Comparative analysis across multiple processing scenarios demonstrates that the proposed integrated method significantly improves internal consistency between overlapping flight strips. The residual height discrepancies, quantified by the median absolute deviation (σMAD), were reduced from 4.5 cm using standard processing workflows to 2.1 cm with the integrated approach — an improvement exceeding 50%. A single processing pass was sufficient for the relatively calm conditions of the test site, though iterative refinement may benefit more dynamic water surfaces. The presented methodology is generic and can be embedded in any laser scanning framework supporting modular georeferencing and refraction correction.
Gottfried Mandlburger, Lucas Dammert, Jan Rhomberg-Kauert et al.· The International Archives o...· 0 citations
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