Photogrammetric multisensor application for the documentation of the Underwater Heritage

Department: PhD in Architectural and Landscape Heritage
Supervisor: Prof. Filiberto Chiabrando
Candidate:  Alessio Calantropio

Underwater environments have long received the attention of the scientific community across disciplines. With more than 70% of our planet’s surface covered with water, the marine space is still largely unexplored in many respects. Underwater exploration is, essentially, interdisciplinary, and thus requires a strong collaboration between researchers in different fields (e.g geology, biology, archaeology, engineering, geomatics). Consistent with emerging documentation requirements, the study of underwater cultural heritage (including ancient or historic shipwrecks, now-submerged prehistoric coastal villages, and archaeological sites in general) or submerged infrastructures via geomatics employs Computer Vision techniques – such as Structure-from-Motion (SfM) photogrammetry – which can be used for remote or indirect study of inaccessible sites by domain experts. The growing number of applications in underwater photogrammetry in recent years allows one to virtually reconstruct the seafloor, shipwrecks, submerged structures and infrastructures, and therefore enables the study of marine environments and their submerged contents without divers or vehicles in the water (ROV).

Although underwater photogrammetry has become widely adopted, there are still significant unresolved issues – especially those related to the acquisition and the processing phases of underwater imaging – that are worthy of attention. Due to the need for generating an accurate virtual 3D replica or twin of the surveyed object or site, it is necessary to address certain issues, such as preserving consistent radiometry, avoiding blurry and low-contrast or over/under-exposed images, and the like. It is possible to identify three main topics regarding the issues connected to photogrammetric applications in underwater environments: a first point concerns the generation of the 3D models and the related metric products, which is related to best practices and standards adopted during the acquisition phase (e.g. coverage, camera calibration, radiometric correction, integration with data from different sources, etc.). A second point is related to the correct georeferentiation of the generated 3D model and the related metric products, due to the impossibility of relying on GNSS navigation systems. Last, but not least, a third topic is related to dissemination and management, as certain visualization platforms offer the possibility of interacting with the model (with features like generating geometric sections, writing dynamic annotation, etc), and sharing models in ways that facilitate communication within or between different user groups. To solve the problem of the impossibility of relying on GNSS systems and to acquire the coordinates of control points, in acquisitions with an optical sensor cases, it is possible to use an integrated underwater positioning system GNSS and Short Baseline Acoustic positioning system to obtain set-up and position of the sensor related to the product to be detected.