Bridging Disciplines with Photogrammetry: A Coastal Exploration Approach for 3D Mapping and Underwater Positioning  
[PERSON]  
1 Goomics Laboratory, Department of Civil, Chemical and Environmental Engineering (DICCA), University of Genoa, 16145 Genoa, Italy; [EMAIL_ADDRESS] (I.F. ); [EMAIL_ADDRESS] (B.F.); [EMAIL_ADDRESS] (D.S.) 1 Goomics Laboratory, Department of Civil, Chemical and Environmental Engineering (DICCA), University of Genoa, 16145 Genoa, Italy; [EMAIL_ADDRESS] (I.F. ); [EMAIL_ADDRESS] (B.F.); [EMAIL_ADDRESS] (D.S.) 1 Goomics Laboratory, Department of Civil, Chemical and Environmental Engineering (DICCA), University of Genoa, 16145 Genoa, Italy; [EMAIL_ADDRESS] (I.F. ); [EMAIL_ADDRESS] (B.F.); [EMAIL_ADDRESS] (D.S. from terrestrial and marine environments into a common reference frame to effectively represent coastal environment [14; 38; 39; 40; 41; 42] and to provide a better understanding of the interaction between human behavior and environmental processes [41; 42; 43; 44; 45]. The question of how to build a methodology that integrates the wide range of existing survey techniques that express both topographic and bathymetric attributes [11; 35; 46; 47; 48; 49; 50; 51] arises. Data collected from emerged and submerged environments rely on distinct equipment and techniques [2; 24; 25; 35; 51; 52; 53], each with certain accuracy and spatial resolution [32; 46; 54; 55; 56; 57; 58; 59], as well as its own type of output data. For instance, an accurate bathymetric survey can be achieved using echosounders, resulting in a precise bathymetric mapping of the seabed [10; 33; 53], but such data alone, although essential, may not be enough to provide a comprehensive assessment of ecological habitats [26; 32; 56; 60; 61]. In this context, photogrammetry represents a promising interdisciplinary solution [11; 35; 62; 63; 64; 65]. Photogrammetry encompasses terrestrial and underwater domains, each with unique characteristics and applications [23; 47; 66]. Terrestrial photogrammetry, often facilitated by Uncrewed Aerial Vehicles (UAVs), offers high-resolution data for mapping land surface [31; 34; 56; 58], while underwater photogrammetry relies on submerged cameras to explore hidden depths of aquatic environment [6; 67]. The integration of these two domains presents a unique opportunity to generate a unified geospatial and hydro-spatial dataset for a comprehensive analysis in coastal areas [37]. In recent years, photogrammetry has advanced significantly thanks to the development of techniques such as SLAM (Simultaneous Localization and Mapping) and SFM (Structure from Motion) [68; 69; 70; 71; 72]. However, the most critical aspect of underwater photogrammetry is the accurate positioning of the underwater model. The main technique involves scaling the model together with camera calibration to ensure accurate measurements [3; 47; 56; 60; 73; 74; 75; 76; 77; 78; 79; 80], thereby facilitating the monitoring of temporal changes [26; 32; 34; 35; 54; 57; 59; 61]. Although the widely recognized value of this scaling approach, a major challenge persists: the absence of georeferencing in the collected underwater data. Indeed, such data are often expressed in a separate local system because of the high costs of underwater positioning systems. The use of underwater targets measured by the GNSS (Global Navigation Satellite System) receiver extended from seabed above the water surface is a commonly adopted approach for underwater positioning [3; 32; 52; 81]. The applicability of this approach strongly depends on the water depth and sea current situation. The lack of georeferencing causes the spatial disconnection between terrestrial and aquatic environments [80]. To address this challenge, we present a methodology to seamlessly integrate photogrammetry in emerged and submerged areas, bridging the gap between these domains. This is achieved thanks to a collaborative data acquisition between a UAV performing the photogrammetric survey of the emerged area and a snorkeler equipped with an underwater camera moving on the water surface.