# Suitability of Aerial Photogrammetry for Dump Documentation and Volume Determination in Large Areas
## 4 Discussion  
The mesh surface comparison (Figure 8) was the initial analysis in this research. It shows a level of agreement of the compared surfaces obtained by different surveying methods. TLS vs. AP surface evaluation (Figure 8a,c) is almost identical at both heaps. The most significant differences between the surfaces were detected mainly on the upper part of the heap and partly on its sidewalls (red and blue colours). This can be assigned to the absence of a sufficient number of points at these locations measured by the TLS method. Despite the goal of making TLS measurements with a full coverage of heaps and with a raised instrument station position, these areas were hidden by obstacles. The TLS vs. GNSS surface comparison (Figure 8b,d) identifies significant differences of surfaces at both heaps. These are mainly found on the edges of the formed planar areas and on the circumference at the base of the heaps. This is due to the substantially lower density of points in the model created from the GNSS RTK measurements. Furthermore, all the direct methods, including GNSS, are influenced by human factors: when moving on the heap, it is not easy for the operator to select the appropriate measured points for optimal surface description. To obtain numerical values for comparison, the frequency difference histograms were created in sets with a 5 mm size for the residual values of \(-0.2\) m to \(+0.2\) m (Figure 9). TLS vs. AP surface analysis has confirmed the initial assumption about the possible correlation of surface models obtained by the employed method. TLS vs. AP surface validation shows a standard deviation \(\sigma\) = 39 mm for Heap no. 1 and \(\sigma\) = 42 mm for Heap No. 2. TLS vs. GNSS surface validation shows a standard deviation \(\sigma\) = 67 mm for Heap no. 1 and \(\sigma\) = 68 mm for Heap No. 2. When comparing the TLS vs. AP surface validation residuals, there is a mean error value of 9 mm for Heap No. 1 and 6 mm for Heap No. 2. TLS vs. GNSS surface validation residuals show a mean error value of 12 mm for Heap No. 1 and a mean error value of 15 mm for Heap No. 2. The correlation coefficient was calculated from the frequency of residual values at the selected intervals of 5 mm of the same data collection methods and processed in two separate heaps. The correlation coefficient value was 0.96 for the AP and 0.75 for the GNSS measurement. This parameter shows a high reliability, especially for the results obtained by the AP. Cross-section analysis (Figure 10) was performed on smaller datasets relative to surface analysis, which corresponds to the evaluated standard deviations according to initial assumptions. TLS vs. AP cross-sections validation has a standard deviation \(\sigma\) = 53 mm for Heap No. 1 and \(\sigma\) = 49 mm for Heap No. 2. TLS vs. GNSS surface validation shows a standard deviation \(\sigma\) = 100 mm for Heap No. 1 and \(\sigma\) = 91 mm for Heap No. 2. TLS vs. AP cross-sections validation residuals show a mean value \(\overline{z}\) = 6 mm for Heap No. 1 and \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Heap** & **Surveying Method** & **Volume (m3)** & **Volume Difference (m3) (\%)** \\ \hline \multirow{4}{*}{Heap 1} & TLS & 4584 & - & - \\ \cline{2-5}  & AP & 4588 & 4 & 0.09 \\ \cline{2-5}  & GNSS & 4430 & \(-\)154 & \(-\)3.36 \\ \hline \multirow{4}{*}{Heap 2} & TLS & - & - \\ \cline{2-5}  & AP & 14 & 0.08 \\ \cline{2-5}  & GNSS & \(-\)311 & \(-\)1.67 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Volume differences comparison. \(\overline{z}=3\) mm for Heap No. 2. TLS vs. GNSS surface validation shows a mean value \(\overline{z}=11\) mm for Heap No. 1 and \(\overline{z}=8\) mm for Heap No. 2. [PERSON] [58] compared photogrammetric DSMs obtained from different flight altitudes up to 350 m AGL using UAS. The reference model was DSM obtained by TLS. When comparing GNSS and TLS measurements by comparing the Z coordinate in the sections, he achieved a standard deviation of 73 mm.