## 4 Discussion  
The variable density of canopies, forests, and forest infrastructure presents unique challenges for both aerial and terrestrial surveying methods, setting them apart from other types of environments. By utilizing this method for measuring the longitudinal slope of preexisting forest/skid roads, accurate slope values can be obtained while minimizing errors that may result from human operations. This approach avoids long-term terrestrial field measurements, and in conjunction with forest/skid road design software programs facilitates the production of precise slope measurements. The GNSS records locations based on the information received from satellites (independently of the location of other points). As the research area was sparsely forested land with good GNSS signal coverage and an internet signal for connecting to CROPOS, in general, we can conclude that GNSS provided precise and reasonable measurements under this forest canopy condition [19]. The use of differential correction techniques, specifically, a network of reference stations (CROPOS), was essential to achieving high levels of accuracy and precision in this research. The real-time kinematic (RTK) mode, virtual reference stations (VRS) mode, and postprocessing observations after fieldwork are all enabled by access to such a network. This has been confirmed by researchers worldwide [31; 32; 33], who have found that this technique is not only applicable to GIS or geodetic-class receivers but also to some smartphones [34]. The influence of PDOP was not checked, but points were recorded when PDOP was at its lowest value. The UAV device provides a on-demand, highly precise photo resolution for forestry research [35], but the good resolution obtained by the photogrammetric method does not mean that the data generated in the raster maps of DSM and DTM on all parts of the image are of sufficient precision for future use. This research was also carried out in good terrain conditions in order to achieve a lower value of PDOP so that the values recorded by the GNSS device could be considered as a reference. To test this method in denser stands, a new study should be conducted, and how the accuracy of the data changes under the denser stand should be observed. The influence of the number and position of ground control points (GCPs) has been widely studied. Typically, 4 to 20 GCPs are used, regardless of the size of the area being studied. To minimize GNSS errors caused by the forest canopy and postprocessing corrections, it is necessary to place GCPs in areas with a clear view of the sky [36]. Therefore, in the research, a large number of GCPs (14 positions) were placed on surfaces that are clearly visible from the air, and they were placed more densely on parts where the configuration of the terrain was more uneven and had more lateral forest vegetation. The measurement of terrain profiles often consists of 4-6 terrain breaks (perpendicular to the direction of the route), and when calculating earthworks, significantly more points are taken to define the 3D terrain. For this reason, there was a significant deviation in the results in the soil masses of the Cut, so hypothesis 2 was not proved (Figure 8). When observing the 2D profile in the longitudinal direction, minor possible errors occur, because only 54 height points in the middle of the designed forest road are observed. This alone reduces the possibility of major deviations, and even if there is a deviation in an individual profile, it does not affect the position of the level line segment itself, which uniformly passes through the same slope with respect to the length of several designed profiles. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Earthworks**} \\ \hline  & **FILL, m\({}^{3}\)** & **CUT, m\({}^{3}\)** & **Difference (CUT-FILL), m\({}^{3}\)** \\ \hline GNSS & 23.07 & 221.94 & 198.87 \\ DTM & 21.53 & 272.19 & 250.66 \\ \hline SUM difference & \(\mathbf{-1.54}\) & \(\mathbf{50.25}\) & \(\mathbf{51.79}\) \\ \hline \hline \end{tabular}
\end{table}