Monitoring the dynamics of coastal cliffs is fundamental for the safety of communities, buildings, utilities, and infrastructures located near the coastline. Structure-from-Motion and Multi View Stereo (SfM-MVS) photogrammetry based on Unmanned Aerial Systems (UAS) is a flexible and cost-effective surveying technique for generating a dense 3D point cloud of the whole cliff face (from bottom to top), with high spatial and temporal resolution. In this paper, in order to generate a reproducible, reliable, precise, accurate, and dense point cloud of the cliff face, a comprehensive analysis of the SfM-MVS processing parameters, image redundancy and acquisition geometry was performed. Using two different UAS, a fixed-wing and a multi-rotor, two flight missions were executed with the aim of reconstructing the geometry of an almost vertical cliff located at the central Portuguese coast. The results indicated that optimizing the processing parameters of Agisoft Metashape can improve the 3D accuracy of the point cloud up to 2 cm. Regarding the image acquisition geometry, the high off-nadir (90°) dataset taken by the multi-rotor generated a denser and more accurate point cloud, with lesser data gaps, than that generated by the low off-nadir dataset (3°) taken by the fixed wing. Yet, it was found that reducing properly the high overlap of the image dataset acquired by the multi-rotor drone permits to get an optimal image dataset, allowing to speed up the processing time without compromising the accuracy and density of the generated point cloud. The analysis and results presented in this paper improve the knowledge required for the 3D reconstruction of coastal cliffs by UAS, providing new insights into the technical aspects needed for optimizing the monitoring surveys.

The dolmen of Guadalperal (Spain) became well known in 2019 when the waters of the reservoir in which it had long been submerged became so depleted as to leave it above water and highly visible. This gave rise to great media and social polemic. In this study, we deal with the ‘recovery’ of the dolmen using digital techniques, including a strategy of geometrical documentation of long, medium and short-range through the use of terrestrial laser scanning (TLS) and photogrammetry. The result is a set of products that trace the changes that have taken place in the monument since its excavation in 1925, the identification of conditions affecting it and the acquisition of new information on the decorated supports that formed part of the megalithic architecture. To do so, the time during which it was accessible (i.e., not underwater) was used to acquire the only heritage information currently available on the monument. This new information offers a complete assessment of a megalithic monument using a protocol that is exportable to other sites submerged in lakes or reservoirs.

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