High–resolution 3D reconstruction of sea caves in Malta through underwater photogrammetry techniques

Abstract

This thesis aims to develop a high-resolution, three-dimensional photogrammetric model of a selected sea cave in the Maltese Islands, this will allow for the monitoring of geomorphic change and the rate of coastal erosion. The resulting model will provide a spatially accurate and visually detailed baseline for scientific analysis of coastal geomorphology and long-term monitoring of erosional processes with data integrated from aerial, terrestrial, and underwater sources. This model will combine data sets from terrestrial, submerged and aerial views of the cave, something that at the time of writing has yet to be done. Data collection was accomplished via the use of two GoPro 7 Black editions for the photogrammetric model and an iPhone 15 for a LiDAR model of the terrestrial component of the cave, used by hand as a team member walked the accessible regions of the cave. A GoPro 13 black edition was carried by a second team member whilst snorkelling in grid patterns at the surface of the submerged portion. Finally, a DJI Mavic 3 multispectral drone was used for the aerial components of the site, flown from a promontory above the cave site itself. The data collected was processed through Agisoft Metashape Professional v2.2.1 (Agisoft LLC, St Petersburg, Russia) with a model being created for each component of the cave. The four models once processed were integrated to form one model with scaling accuracy confirmed by the LiDAR model. The level of accuracy in the model allowed for specific measurements to be taken such as width or height, these measurements could allow for the calculation of the mass of rock likely to fall or give bathymetric data on the current submerged section. The combination of terrestrial, underwater, UAV, and LiDAR photogrammetry proved to be a robust approach for capturing both the external and internal morphology of the cave. Each method contributed complementary datasets: UAV photogrammetry effectively mapped the promontory and entrance geometry, while underwater and terrestrial images documented the cave’s internal surfaces in high detail. The integration of LiDAR scanning from the iPhone 15 enhanced the scaling accuracy of the final model, compensating for the potential geometric distortion associated with freehand image capture. This multi-platform approach aligns with recent studies that advocate for the combination of close-range photogrammetry and LiDAR to improve the geometric precision of complex natural structures (Colica et al., 2021; Furlani et al., 2023).