# Review of Photogrammetric and Lidar Applications of UAV
## 6 Discussion
### Discussion on Applications of UAVs
#### 6.2.4 Natural Hazards and Geohazards
The authors of [163] deployed UAV in the mining area. They created a digital elevation model (DEM) for the area in the northwestern part of India, in the city of Chittorgarah. Where it is not possible to obtain direct contact measurements due to terrain or safety restrictions, a UAV equipped with a LiDAR, or a digital camera, is used to check the stability of slopes. Slope stability control should serve to mitigate economic losses, property damage, environmental degradation, and severe injuries or deaths. Both methods are effective for monitoring slope stability, but the authors more often use UAV photogrammetry. The advantage of photogrammetric methods is their low purchase price and the high density of point clouds. Geology is another area where UAV technology is frequently used. The authors [164] used photogrammetric data outputs to create a 3D model of the geological structure in the Lehotka pod Brehni perlite deposit in the Ziar nad Hronom district using a Phantom 2 Vision+ UAV and 14 Megapixel FC200 camera. From a flight height of 35 m AGL, 58 images were made and georeferenced using 18 GCP. The study [165] used an AustroDrones X18 multicopter equipped with a Sony Alpha 7R camera for engineering geology applications in the Saigesbach basin, Sellrain valley, in the Alps of Austria. In total, 640 images were acquired in two flights. They deploy also Multiplex Mentor fixed-wing UAV with a Sony NEX5 camera that collected 4000 aerial images during four flights. The UAV images were processed in Agisoft Photoscan. The resulting ortho-mosaics reached a GSD of 0.02 m when imaging the valley and 0.08 when imaging the basin. For DSM, the GSD value was 0.05 m when imaging the valley and 0.20 m when imaging the basin. [PERSON] et al. [166] carried out structural geological mapping of a sedimentary outcrop in South Ayrshire (Scotland). They used a UX5 HP fixed wing and a TRIMBLE ZX5 hexacopter, which produced the resulting ortho-mosaics of the study area. High-resolution photographs that can be transformed into digital models of the outcrops [167; 168] are most suitable for geological purposes. The authors of [168] tracked outcrops within Dinosaur Provincial Park using a senseFly eBee fixed-wing UAV with an 18.2 megapixel Sony WX220 digital camera and created 3D models using Pix4 Dmapper software. Tracking dikes and outcrops making point clouds and 3D models using data collected by the UAV were also addressed in [169]. The authors of [170] analyzed the geological hazard in Stara Baska on the island of Krk, Croatia, using a DJI Phantom 4 Professional UAV. In the field of geology, the SfM method is most often used to create a 3D model of a geological structure, for engineering geological applications and geological mapping, but also for the prediction of geological hazards. The SfM method obtains high-resolution photographs, from which 3D digital models or ortho-mosaics are subsequently generated. In the field of geology, the SfM photogrammetric method is currently the most effective and popular. The use of UAVs has also opened new frontiers of observation in hydrology. The authors of [171; 172] have used UAVs to observe stream flows. They engage hydrological modeling through the DEM of stream bathymetry with water level estimation. The authors of [173; 174; 175] have mounted miniature lidar systems on UAVs to estimate water level and bathymetry. They achieved a measurement accuracy of several centimeters. In reference [176], the author used a UAV to monitor the flow restoration of the Hostivicky Brook in Prague, Czech Republic, using a DJI Inspire 1 Pro platform with a DJI Zenmuse X5 camera, which produces images with a resolution of 4608 \(\times\) 3456 pixels. The acquired data were used to generate a dense point cloud, a digital surface model (DSM), and an ortho-mosaic. In the publication [177], the authors used a UAV multi-copter carrying a Sony alpha 7R camera to monitor and reconstruct the topography of the Joyce River in Hiroshima Prefecture, Japan.