## 3 Methodology  
In this context, the coastal morphology evolution, including short-term onshore changes and processes, has been examined through several remote sensing techniques based on data acquisition with the use of state-of-the-art equipment. The swift development in sensor technology and geomatics during the last decade has improved the acquisition and processing of remotely sensed information, especially regarding the monitoring of infrastructure as well as medium-sized territorial zones due to geological risks [89]. Rather prominent techniques include the generation of point clouds of the areas of interest, which can be composed either directly by using surveying equipment such as Terrestrial Laser Scanners (TLSs) or indirectly by processing numerous photographs taken by Unmanned Aerial Systems (UASs). Both Light Detection and Range (LiDAR) and Structure from Motion (SfM) processing techniques can provide large amounts of digital data of very high resolution from which point clouds of similar quality can be generated. High-resolution 3D point clouds and 3D meshes are the fundamental types of data for any quantitative structural analysis afterwards.  
Laser scanning is an optimized technique for obtaining 3D surveying data. Usually, TLS has a spatial resolution ranging from millimeters to centimeters. Regarding this, the widespread use of UASs, together with SfM and Multi-View Stereo (MVS) technologies, has improved the flexibility of topographic surveys and structural analysis. In fact, nowadays, due to swift technological development, it can be used rather easily under adverse conditions, as nadiral, oblique, or frontal images can be acquired with minimal effort. Therefore, cliffs, steep slopes, hanging rocks, and morphological discontinuities in general, are either being scanned or photographed for the generation of a basement dataset on which further processing will be applied. The mapping of structural failures and rock discontinuity density at coastal cliffs proves to be very critical concerning the safety regime in the beaches beneath them, especially the popular ones [90].  
During the last few years, there has been an ongoing debate as to whether the direct TLS point clouds are more accurate and precise than the indirect data constructed after photogrammetric processing [91; 92; 93]. In this context, more attention has been given to the latter, which can also generate high-resolution 3D models of slopes, focusing on the advantage of low costs, flexible oblique view sensing and photo-realistic vision information when compared with terrestrial LiDAR point clouds [94], which are produced by rather expensive equipment. On the other hand, the accuracy of these is much more adequate, not to mention the data density, which is also difficult to be obtained with UAS flights at a reasonable elevation (e.g., 75 m).  
Furthermore, the use of UASs is a practical way of mapping areas larger than TLS can provide, and it has been proved to be optimal for landslides and rockfalls that often cover areas that range from less than one square kilometer up to a few square kilometers. Additionally, the limitations of beach width where in situ observations or the use of other techniques such as TLS are unattainable can be overcome via UAS and restrictions associated with LiDAR instrumentation (e.g., high weight and occlusion areas), since the final product is very similar, as mentioned above.  
In this context, we chose to apply the two different approaches at the two case studies of Egremnoi and Porto Katsiki, taking into consideration the most appropriate technique for each case. A point cloud derived from SfM processing was utilized for creating the basement datasets (Digital Surface Model and ortho-photograph) for the Egremnoi coastal zone, whilst LiDAR scanning was used for direct point cloud acquisition at the popular Porto Katsiki beach. The flowchart below illustrates the main steps of the methodology along with the software solutions used for each one of them (Figure 4).