## 3 Results  
### Accuracy Assessment of Orthophotos  
Three high resolution orthophotos were extracted from the photogrammetric processing for each case study, consisting of UAV data from the three different acquisition groups, i.e., (a) nadir-viewing imagery, (b) oblique-viewing imagery, and (c) the integration of nadir and oblique imagery. The aforementioned orthophotos, covering the area of case study 4 (Messarista), are depicted in Figure 6. As can be observed, a visual comparison between the orthophotos created by data for the different groups of geometry acquisitions is a particularly difficult task, due to apparent similarity. Figure 5: UAV acquisitions over the landslide of Messarista. (**a**) Nadir-viewing acquisition trajectory; (**b**) Oblique-viewing acquisition trajectory. In this context, the evaluation of the accuracy of the derived orthophotos was performed using comparison approaches in an ArcGIS environment. GIS applications have been utilized in respective studies for the characterization of geological features and geological mapping [41]. In this study, GCPs were utilized as precise reference positions. Then, two geodetic lines were shaped from the x-y coordinates of the GCPs of each study area. Figure 7 displays the reference geodetic lines, as formed for each case study. It is important to highlight that we tried to create the geodetic lines in different topographic reliefs, varying from steep slopes to flat regions. Subsequently, we digitized the same lines in all orthophotos, derived from the processing of the UAV imagery of all groups of acquisition geometry. The digitization of the lines was based on the visual identification of the respective GCPs, which were used to shape the reference geodetic lines. The length of each digitized and reference geodetic line were calculated and compared (Table 3). The length variations were significantly small, which proved the high accuracy of the photogrammetric products. In addition, the integration of nadir and oblique viewing acquisitions displayed the smallest length variations in eight out of ten comparisons, demonstrating the positive influence of the specific acquisition geometry on the enhancement of the accuracy of the derived orthophotos. Moreover, oblique-viewing geometry showed a better performance, compared to nadir-viewing acquisitions. In particular, the difference in the length of line 1, in comparison to the corresponding reference line in the case study of [PERSON], was zero for the orthophoto resulting from the integration of nadir and oblique imagery. In addition, the variation for the nadir-viewing and oblique-viewing orthophotos was estimated at 0.190% and 0.067%, respectively. In the case of Egnemni, the percentage variations in line 2 were calculated at about 0.034% for the orthophotos that emerged from either the synergistic use  
Figure 6: UAV orthophotos of the landslide of Messarista, emerging from the processing of (**a**) nadir-viewing images, (**b**) oblique-viewing images, and (**c**) synergistic use of nadir and oblique images. The scale of the orthophotos was set to 1:1500.  
of nadir and oblique images or the nadir-viewing imagery. In contrast, the corresponding difference for the nadir-viewing orthophoto was obviously larger (i.e., 0.228%). Figure 7: Reference geodetic lines as created in: (**a**) case study 1, (**b**) case study 2, (**c**) case study 3, (**d**) case study 4, and (**e**) case study 5. The processing of nadir-viewing images, along with oblique imagery, enhanced the overall accuracy in all the investigated areas. In more detail, for nine of ten cases (Table 3) the length difference between the GNSS measurements and the measurements on the orthophotos was less than 0.05 m (5 cm). Taking into consideration the terrain steepness and the dense vegetation, we assume that the overall accuracy is excellent.