## 6 Conclusions
Bibliographic analysis of the type of UAV has shown that multicopters (in most cases quadricopters) represent the most widely used platform. In fact, as highlighted in the pie chart (Figure 13a), only a rather modest percentage (2%) employs hybrid UAV platforms (generally self-built and equipped with multi-sensor systems), and approximately 7% use fixed-wing drones. The latter type is widely used in contexts where the areas to be surveyed for mapping and 3D reconstruction, especially in the archaeological field, are extremely extensive. The bibliographic analysis also showed that approximately 50% of the research analysed focuses on 3D reconstruction using the photogrammetric technique from UAVs as they recognise this approach as automatic, versatile, and economical. The remaining part, on the other hand, develops its scientific research starting from the photogrammetric 3D model to obtain parametric models (HBIM, FEM, etc. ), mapping and remote sensing applications, or is useful for the creation of virtual tours and augmented reality. Figure 13b shows the purposes of photogrammetric surveying applied to CH found in the analysed manuscripts. This trend in the use of UAVs for academic purposes is also reflected in the UAV market; indeed, the global UAV drones market size was USD 19,784.8 million in 2021 and is predicted to grow with a compound annual growth rate (CAGR) of 19.6%, by generating a revenue of USD 102,466.7 million by 2030 [138]. This growing market and interest in UAV photogrammetry are due to the ability to perform three-dimensional and multi-temporal surveying in a rapid and easy way; this is also made possible by the small size of UAVs and the possibility of equipping them with different types of sensors. A very important aspect related to the technological development of such platforms is certainly that of making them perform even better in conditions where power, size, and mass can negatively influence the final result (emergency conditions, reduced survey spaces, adverse weather conditions). In this manuscript, a valid methodological approach was described for UAV surveying aimed at the reconstruction of 3D models in the field of CH and highlighting any limitations or criticalities in the proposed methods, as well as to contribute the various actors operating in these fields, in order to obtain high-quality, realistic 3D models. Furthermore, the manuscript aims to provide techniques and methods for efficient 3D acquisition and reconstruction of cultural heritage assets. Therefore, this review paper can contribute to future research, markets, and applications of UAVs for photogrammetric purposes in the field of cultural heritage. In the future, it is hoped that technological innovation will contribute to the creation of platforms where passive sensors (high-resolution cameras) and active sensors (LiDAR) can be easily integrated with thermal sensors in order to obtain 3D models not only with realistic content but also that can easily meet the current requirements for energy efficiency, which is of particular importance today in the field of cultural heritage. Conceptualisation, M.P., V.S.A., and D.C.; methodology, M.P., V.S.A., and D.C.; software, M.P., V.S.A., and D.C.; validation, M.P., V.S.A., and D.C.; formal analysis, M.P., V.S.A., and D.C.; data curation, M.P., V.S.A., and D.C.; writing--review and editing, M.P., V.S.A., and D.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable. Figure 13: Representation by means of a pie chart of literature review on platform type (**a**) and purposes of UAV photogrammetry in CH (**b**).