Photogrammetry: Linking the World across the Water Surface

Three-dimensional (3D) surveying and modelling of the underwater environment is challenging; however, it becomes even more arduous when the scene or asset to measure extends from above to underwater through the water surface. While this is topic of high interest for a number of different application fields (engineering, geology, archeology), few solutions are available, usually expensive and with no guarantee of obtaining homogenous accuracy and resolution in the two media. This paper focuses on a procedure to survey and link the above and the underwater worlds based on photogrammetry. The two parts of the asset, above and underwater, are separately surveyed and then linked through two possible analytical procedures: (1) independent model adjustment or (2) relative orientation constraints. In the first case, rigid pre-calibrated rods are installed across the waterline on the object to be surveyed; in the second approach, a synchronized stereo-camera rig, with a camera in water and the other above the water, is employed. The theoretical foundation for the two approaches is provided and their effectiveness is proved through two challenging case studies: (1) the 3D survey of the leak of the Costa Concordia shipwreck and (2) 3D modelling of Grotta Giusti, a complex semi-submerged cave environment in Italy.