## 4 Discussion  
Our methodology represents a significant advancement in integrating land and underwater photogrammetry, offering a comprehensive and unified approach to survey coastal environments. This innovative technique addresses several challenges in underwater photogrammetry, including camera calibration, georeferencing issues, and the limitations of underwater positioning systems, by combining UAVs with a snorkeler equipped with underwater camera. The UAV ability to detect the snorkeler in multiple images using a target system allows us to achieve positioning accuracy in the order of 8 cm for the snorkeler. This level of accuracy is a promising starting point, considering the challenging nature of positioning a dynamic GCP from orthorectified images. For instance, the dynamic GCP,  
Figure 14: Virtual model showing the underwater seagrass.  
Figure 13: Virtual model of Silence Bay in Sestri Levante (Italy).  
moving over the water surface, can lead to inaccuracies in the orthophoto reconstruction due to water movement and surface undulations. Additionally, the target attached to the snorkeler is vulnerable to minor movements caused by waves. Despite these challenges, the methodology demonstrates significant potential for improving the integration of terrestrial and underwater photogrammetric data. Furthermore, this approach effectively reconstructs both underwater and emerged environments within a single 3D model, accurately positioning underwater habitats and features in the same reference frame as terrestrial features. This holistic approach provides a comprehensive understanding of the coastal environment, highlighting the spatial relations between underwater and terrestrial features. Moreover, the methodology is cost-effective, as it avoids the need for expensive underwater positioning systems by leveraging existing technologies in innovative ways.  
However, despite the advantages, the methodology presents challenges and limitations. The reliance on visual targets for positioning, coupled with the effect of water clarity, lighting conditions and depth, can influence the quality of underwater images. Furthermore, the accuracy of the unified model depends on precise synchronization between the UAV and the snorkeler camera, and on the correct calibration of the underwater camera. Additionally, image distortions correction and accurate georeferencing in the orthophoto stage are crucial elements to achieve appreciable results.  
Future research should focus on enhancing the integration of topographic and underwater photogrammetry by incorporating additional technologies. A promising direction is the integration of underwater acoustic sensors, such as Multi-Beam Echo Sounders (MBES), for bathymetric data collection. By combining underwater photogrammetry with MBES data, more detailed and accurate 3D models of underwater environments can be achieved. Finally, replacing snorkeler with USV (Unmanned Surface Vehicle) for underwater photogrammetry can be beneficial: equipping USV with GNSS technology, the positioning accuracy from orthorectified images could be directly assessed. This comparison would facilitate the validation and potential improvement of the dynamic GCPs accuracy.