## 3 Results
### Accuracy Assessment of Orthophotos
It is worth mentioning that even in the fifth test area (Patras Port), the combined processing of nadir and oblique images gave results that were more accurate than the single processing of nadir or oblique data. In detail, the length difference of the geodetic lines for the common data set was only 0.012 m (centimeter accuracy), while the respective accuracies for the nadir or oblique data set ranged between 0.12 m and 0.31 m (Table 3). Another method, utilized for the evaluation of the accuracy of orthophotos resulted from the photogrammetric processing of each acquisition geometry, was based on the comparison of the mean center of the reference geodetic lines and the respective digitized ones. The mean center constitutes the geographical center of a set of features resulting from average x and y coordinates [42; 43], which is calculated as:  
\[\overline{X}=\frac{\sum_{i=1}^{n}x_{i}}{n}\text{ and }\overline{Y}=\frac{\sum_{i=1}^{ n}y_{i}}{n} \tag{1}\]  
where \(x_{i}\) and \(y_{i}\) are the coordinates for a feature \(i\) and \(n\) is the total number of features. This approach is widely used for the identification of distribution changes or for the comparison of the distributions of features. The mean centers of the reference geodetic lines of each case study are depicted in Figure 7. The Near tool [44] was applied for the determination of the distance between the reference mean centers and the respective mean center of the digitized lines. Distance variations are displayed in Table 4. As can be noted, the multi-acquisition geometry (nadir and oblique imagery) presented the shortest distances from the reference mean centers in 50% of the UAV campaigns. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Case Study** & **Acquisition** & **Length Line** & **Difference** & **Difference** & **Length Line** & **Difference** & **Difference** \\
**ID** & **Geometry** & **1 (m)** & **(m)** & **\%** & **2 (m)** & **(m)** & **\%** \\ \hline \multicolumn{2}{c}{Reference line} & 226.676 & \multicolumn{4}{c}{129.881} \\ \hline \multirow{4}{*}{Moira} & Nadir & 226.933 & \(-\)0.257 & \(-\)0.113\% & 129.550 & 0.331 & 0.254\% \\  & Oblique & 226.745 & \(-\)0.069 & \(-\)0.030\% & 129.937 & \(-\)0.056 & \(-\)0.043\% \\  & Synergistic use & 226.734 & \(-\)0.058 & \(-\)0.029\% & 129.725 & 0.156 & 0.120\% \\ \hline \multicolumn{2}{c}{Reference line} & 32.898 & \multicolumn{4}{c}{23.221} \\ \hline \multirow{3}{*}{Egremni} & Nadir & 32.861 & 0.038 & 0.112\% & 23.274 & \(-\)0.054 & \(-\)0.228\% \\  & Oblique & 32.906 & \(-\)0.007 & \(-\)0.024\% & 23.229 & \(-\)0.008 & \(-\)0.034\% \\  & Synergistic use & 32.868 & 0.030 & 0.091\% & 23.229 & \(-\)0.008 & \(-\)0.034\% \\ \hline \multicolumn{2}{c}{Reference line} & 8.938 & \multicolumn{4}{c}{16.152} \\ \hline \multirow{3}{*}{Zachlorou} & Nadir & 8.921 & 0.017 & 0.190\% & 16.179 & \(-\)0.027 & \(-\)0.167\% \\  & Oblique & 8.944 & \(-\)0.006 & \(-\)0.067\% & 16.179 & \(-\)0.027 & \(-\)0.167\% \\  & Synergistic use & 8.938 & 0.000 & 0.000\% & 16.177 & \(-\)0.025 & \(-\)0.154\% \\ \hline \multicolumn{2}{c}{Reference line} & 60.524 & \multicolumn{4}{c}{23.064} \\ \hline \multirow{3}{*}{Messarista} & Nadir & 60.577 & \(-\)0.053 & \(-\)0.088\% & 23.149 & \(-\)0.085 & \(-\)0.368\% \\  & Oblique & 61.002 & \(-\)0.478 & \(-\)0.790\% & 23.299 & \(-\)0.235 & \(-\)1.018\% \\  & Synergistic use & 60.539 & \(-\)0.015 & \(-\)0.014\% & 23.097 & \(-\)0.033 & \(-\)0.143\% \\ \hline \multicolumn{2}{c}{Reference line} & 84.240 & \multicolumn{4}{c}{74.220} \\ \hline \multirow{3}{*}{Patras Port} & Nadir & 84.559 & \(-\)0.319 & \(-\)0.379\% & 74.492 & \(-\)0.272 & \(-\)0.366\% \\  & Oblique & 84.117 & 0.123 & 0.146\% & 73.988 & 0.232 & 0.312\% \\  & Synergistic use & 84.228 & 0.012 & 0.012\% & 74.208 & 0.012 & 0.016\% \\ \hline \end{tabular}
\end{table} 
Table 3: Comparison of the length of the geodetic lines of each case study in accordance with the respective reference length.