Conference Agenda

The presence of refraction-induced systematic errors has always been a cause of concern in the field of underwater photogrammetry.

This work extends previous studies from the authors with new simulations specifically aimed at practical applications underwater using popular sensor devices and configurations, such as GoPro action cameras fitted with standard flat port housing that are very common among marine ecologists and archaeologists.

We aim at investigating whether approaches used by regular photogrammetry above water can be applied underwater without a significant accuracy loss for the application of interest.

Due to the complexity of collecting ground truth data, simulations are used. We utilize the POSER framework (https://github.com/GEOSS-UNISS/POSER) developed within the 2024 ISPRS Education and Capacity Building Initiatives (ECBI).

We investigate the benefits and cons of the refractive vs the implicit modelling approaches with respect to estimability of camera calibration (refractive) parameters, need for pre-calibration setups with approaches from literature, availability of ground control points, and assessing the accuracy of both approaches against ground truth simulated data.

The accuracy is reported as discrepancies in the reconstructed 3D models, exterior orientation and camera calibration parameters.

Reconstructing the water surface in refractive domains such as rivers and lakes is challenging, since light bending at the air-water interface alters the apparent geometry and breaks the straight-ray assumption of conventional image-based 3D reconstruction. Accurate water surface models are therefore a key prerequisite for many refraction-aware applications. This contribution investigates the potential of three passive image-based methods, Structure from Motion (SfM), Multi-View Stereo (MVS), and Monocular Depth Estimation (MDE), to derive a geometrically consistent water surface model from UAV imagery of the Pielach River study site in Austria. The dataset represents a demanding scenario with clear, fast-flowing water and low texture, which causes strong refraction and poor feature stability. Quantitative comparisons against LiDAR-derived reference surfaces show that SfM yields sparse and inconsistent points, MVS reconstructs the riverbed instead of the water surface, and MDE exhibits scale and offset inconsistencies despite explicit calibration using SfM reprojections. Completeness remains below 45 % for all methods with mean vertical deviations in the decimetre-to-metre range. The results indicate that current image-based approaches are insufficient for reliable water-surface reconstruction in such settings, reinforcing the need for an explicitly derived surface model as input to refraction-aware modeling, for example in bathymetric reconstruction and future refractive neural rendering methods, rather than relying on implicitly learned water surfaces.

This article studies the complementary usage of RTI and SfM-MVS visual inspection (visual testing) of welds under water. Two compact low-cost lighting domes of different designs were developed and deployed with a monochromatic camera at close range.

The lighting domes generate homogenous and direct illumination, respectively, tailoring the requirements of SfM and RTI. The camera is housed in a cylindrical tube, equipped with a dome port interface.

The 3D reconstruction in combination with RTI models could augment existing testing strategies and provide digital, gapless documentation. Experiments were conducted in laboratory in air, clear and turbid water questioning were the capabilities and limits are for given setup with respect to visual testing of welds.

Under the correct lightning, in air the techniques perform on a high accuracy level and are well suited for inspecting welds digital and interactively. Underwater the results differ in dependence of the degree of turbidity and prove to be sensitive for configurational parameters leaving space for improvements of acquisition and processing workflows. However, even in turbid water the 3D reconstructions and RTI models could be calculated enabling novel possibilities for weld inspection.