Underwater Heritage documentation using photogrammetry: the CRAB system

Abstract. The documentation and preservation of submerged cultural heritage present significant challenges due to the complexity of the underwater environment, including issues related to accessibility, visibility, and the lack of reliable reference data for scaling and orientation. This study introduces CRAB (Calibrated Rig for Aquatic photogrammetric Bicamera system), a stereoscopic imaging system designed to support high-resolution 3D reconstruction of underwater archaeological sites. The system integrates two synchronised, full-frame 60 MP cameras on a fixed baseline and employs dome ports with optically calibrated alignment to minimise refractive distortion. Field testing was conducted at a submerged Roman shipwreck site in the Marine Protected Area of Porto Cesareo, Italy, where five large “cipollino” marble columns were surveyed. Over 1300 stereoscopic image pairs were acquired using a structured acquisition pattern. Post-processing included camera calibration, colour correction, and 3D model reconstruction using a standardised SfM (Structure from Motion) approach. Results demonstrated that the CRAB system thus offers a reliable and scalable solution for accurate underwater photogrammetry and represents a valuable contribution to the documentation and conservation of submerged cultural assets.