## 4 Discussion  
The capability of performing a high accuracy field campaign with excellent repeatability is the basic demand [45, 46, 47] for landslide mapping and monitoring for assessing any deformation, usually within a steep and possibly dangerous environment. Sub-centimeter accuracy is a prerequisite in both horizontal and vertical axes in order to assess the activity of a landslide. Aiming to develop a guideline for accurate landslide mapping, we conducted four repeated tests within four active landslides with diverse characteristics. Both the horizontal and the vertical accuracy of the orthophotos and the DSMs were examined. It was proved that the combination of nadir and oblique imagery produced more accurate results in all the studied cases. This output was in full accordance with the results of a previous study [28] that mentioned that overall accuracy is increased by 50% when nadir and oblique imagery are combined. The statistical comparison of the length of the digitized geodetic lines demonstrated that the integration of nadir and oblique images provides orthophotos with higher accuracy. Oblique imagery seems to provide more accurate results than nadir imagery. The only exception was the Messarista case study, where the processing of the oblique images yielded the worst results, compared to the respective result from the nadir images. This can be justified, as Messarista is a densely built-up village on a steep hill with very narrow backstreets. These narrow side streets cannot be clearly mapped in oblique imagery. Furthermore, the houses are built side-by-side and the front buildings hide the back ones. As a result, nadir imagery provides useful information that cannot be derived from oblique imagery alone. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Case Study ID** & **Acquisition Geometry** & **RMSE (m)** \\ \hline \multirow{3}{*}{Moira} & Nadir & 0.380 \\  & Oblique & 0.260 \\  & Synergistic use & 0.140 \\ \hline \multirow{3}{*}{Egkremni} & Nadir & 0.092 \\  & Oblique & 0.085 \\  & Synergistic use & 0.083 \\ \hline \multirow{3}{*}{Zachlorou} & Nadir & 0.506 \\  & Oblique & 0.498 \\  & Synergistic use & 0.475 \\ \hline \multirow{3}{*}{Messarista} & Nadir & 0.162 \\  & Oblique & 0.357 \\  & Synergistic use & 0.090 \\ \hline \multirow{3}{*}{Patras Port} & Nadir & 0.159 \\  & Oblique & 0.236 \\ \cline{1-1}  & Synergistic use & 0.047 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: RMSE values emerging from the generated DSMs. Similar results were extracted from the measurements of the near distances of the mean centers that were calculated in the ArcMap environment. In almost all the cases, the simultaneous processing of nadir and oblique imagery provided more accurate results. The same combination produced more accurate DSMs in all the study areas. As presented in Table 4, in very steep environments (the four landslide areas) and in a flat area (Patras Port), the vertical accuracy of the oblique-nadir DSM was higher in comparison to the accuracies of either nadir or oblique DSMs. Summarizing the overall assessment of the results, based on the application of simple statistical calculations, it was proved that the synergistic use of nadir and oblique imagery was considered to be the most appropriate geometry at 80% (i.e., in eight cases), according to the comparison of the length of the two lines of the five study areas (i.e., 10 cases) (Figure 8a). Moreover, comparing only nadir-viewing and oblique-viewing geometry, oblique acquisitions provided more accurate results by 70% (Figure  
Our findings are in accordance with those of previous studies [20; 21; 24; 25; 26]. Specifically, nadir and oblique images were compared with TLS data for the mapping of a fault plane [20]. The tests proved that both precision and accuracy have been increased when the two sets of images were combined. It was marked [21] that the combined use of nadir and oblique imagery offers a valued methodology for open-pit operations and for the continuous monitoring of the exploitation by managers.