## 3 Results and Discussion
The flight trajectory was adjusted to \"snake\" along the E-W line over the entire area of the fortification, first from north to south, and then from south to north (Figure 8). A processing analysis of the obtained material showed:  
* vertical objects, such as walls, towers, trees, etc., appeared in significantly better detail, and the texture was of acceptable quality;
* the ground under single and sporadic trees was better displayed. Nevertheless, the shots of the central part of the eastern fortification, which was overgrown with dense vegetation, did not match together and were not included in the estimation, since the pictures did not contain readable and easily recognizable objects. This was solved by including a series of shots taken from a height of 250 m in the estimation (Figure 9). Dense vegetation in the territory of the eastern fortification, together with easily readable landmarks and details, such as roads, roofs of houses, etc., in the neighboring area, interfered with the picture and they were automatically recognized and included in the construction of the final 3D model. However, this resulted in an orthophotoplan with suitable characteristics, and thus provided us with a high-quality plan of the fortress and we were able to carry out the necessary measurements (Figures 10 and 11). Figure 8: Position and tilt of cameras when photographing Pitiunt fortress, with a shooting height of 25 m.  
Figure 10: Orthophotoplan of the territory of Pitiunt fortress. Figure 9: Introducing a series of images from a height of 250 m to the estimation. Pitiunt fortress has no significant surviving structures, except for geometrically simple sections of walls and towers. When studying complex shapes, e.g., fortresses with well-preserved temples, shooting at an angle of 60\({}^{\circ}\) is not necessarily sufficient for a detailed study. Therefore, we decided to try shooting at obtuse angles when exploring Bzyb Fortress (Republic of Abkhazia, Figure 2, No. 3). The fortress stands on a spur of a mountain ridge, and the territory had practically no trees and was covered with grass. In the upper part of the fortress, there is a well-preserved cross-domed three-apse 10 th century temple, with preserved vestibule walls on the south, west, and north. For a rigorous study of all the fortress walls, towers, and the temple from the outside and inside, shooting was carried out at a 45\({}^{\circ}\) angle to the horizon to obtain both the vertical and horizontal surfaces of the object with the same quality. The flight path was chosen, as in the previous example, as several contrasting \"snaking\" paths at different azimuths in order to photograph all the details of the fortress and not to leave any blind spots (Figure 12). As a result, a good quality 3D model of the fortress was obtained without noticeable distortions, in addition to an orthophotoplan and DEM, allowing further processing, measurements, and research (Figures 13 and 14). The preliminary conclusions were as follows:  
* with a 90\({}^{\circ}\) camera angle the open objects only without significant vertical details can be photographed;
* the more complex the object is geometrically, the more vertical details it contains, and so the more difficult it is to photograph. Preferably, a series of flights with shooting angles of 90\({}^{\circ}\), 60\({}^{\circ}\), and 45\({}^{\circ}\) is required. The flight path should be characterized by a series of oncoming \"snakes\" at several azimuths, depending on the geometry of the object. * the flight height should be adjusted according to the height of the surrounding obstacles;
* the overlap of shots should be 60-80%;
* in order to avoid unusable shots due to trees, if the area is heavily forested, it is necessary to shoot the area from a higher position. Figure 11: DEM of Pitiunt fortress. Figure 12: Trajectory of shooting and tilt of the camera during photogrammetry of Bzyb fortress. Figure 13: Orthophotoplan of Bzyb fortress.