## 6 Conclusions  
This review comprehensively examined both the hardware component of the UAV platform (drone design, sensors for navigation, and image acquisition) and the software component concerning the design and realisation phase of the flight as well as the image processing phase using SfM-MVS algorithms. Many applications of UAV photogrammetry in the field of CH have been shown; each case study shown has morphological characteristics, areas, and complexity of survey operations. The application of SfM-MVS algorithms to images acquired with UAVs is a perfect combination for the construction of accurate and detailed 3D models. Indeed, UAV photogrammetry for the representation of CH is a scientifically growing field, as shown by the increase in  
Figure 12: Two-dimensional CAD representation from orthophoto: UAS survey (**a**), 3D processing in SfM software (**b**), orthophoto (**c**), and 2D architectural drawing (**d**). manuscripts in the various databases. A bibliometric analysis carried out on the Scopus database showed that the use of the UAV platform is constantly increasing, as also shown by [PERSON] et al. [137], who highlighted the presence of more than 80,000 manuscripts since 2001. As far as the analysis of photogrammetry from UAVs is concerned, growth with a rather linear trend and the presence, since 2012, of approximately 2400 manuscripts has emerged. This trend is also reflected in the field of UAV photogrammetry for CH documentation, with a slight increase from the year 2016; this means that in recent years, UAV photogrammetry has become more and more widespread in CH applications, and this can be asserted by the quality of the 3D models and orthophotos generated by UAV images. In this paper, various 3D models and orthophotos applied to CH structure were shown; furthermore, different applications and processing were conducted from these models. For example, from point clouds, it is possible to model elements and structures belonging to the CH field in order to obtain parametric 3D models useful for subsequent analyses in the field of BIM and FEM. Following this line of research, the photogrammetric survey from UAV applied to CH plays a fundamental role in the management, valorisation, and conservative maintenance of historical-architectural heritage. From this point of view, the UAV photogrammetric approach not only reduces acquisition times but also makes it possible to map areas that are difficult to access and is more versatile in situations where the use of other sensors is strongly conditioned or inadvisable. For example, UAV photogrammetry allows acquisition at modest heights (4 to 5 m), making it a highly competitive method compared to a survey performed with TLS, which requires platforms or instruments capable of reaching the heights necessary for the purpose. For greater heights, on the other hand, the UAV platform becomes an even more competitive tool than aerial photogrammetry, with the advantage of acquiring images at relatively low altitudes and consequently with high geometric resolution. Furthermore, the possibility of equipping UAVs with a wide range of other sensors (such as ultra-high resolution digital cameras, thermal imaging cameras, multispectral sensors, and laser scanner sensors) makes this solution useful in the field of photogrammetry and remote sensing. A bibliographic analysis conducted on the documents indexed in the Scopus database, using the keywords \"UAV\", \"UAS\", \"Drones\", and \"Cultural Heritage\", was carried out in order to highlight the type of sensor (or multi-sensors), the UAV platform, and the purposes used in the photogrammetric survey of CH. Regarding the use of sensors, it was found that, in the field of CH, in 80% of the cases examined, the sensor consisted of an RGB camera, usually integrated into the platform itself; the remaining 20% concerned the equipping of the UAV platform with an RGB camera and possible hyperspectral, thermal, and LiDAR sensors.