## 3 Methodology
### Data Acquisition  
The aerial survey was performed using a DJI Phantom 4 Pro quadcopter equipped with a mechanical shutter, 20-megapixel camera attached to a 3-axis gimbal to maintain the camera at a predefined level. The camera sensor had a 24 mm focal length (35 mm format equivalent) and a one-inch complementary metal oxide semiconductor (CMOS) sensor. The UAS was using an inertial measurement unit (IMU), a barometer, combined with a GPS/GLONASS positioning system achieving hover accuracy of \(\pm\)0.5 m vertically and \(\pm\)1.5 m horizontally. Finally, the UAS was powered using an intelligent flight battery that provided approximately 23 min of flight time under normal conditions. For each of the above scenarios, a different flight plan was designed using the Litchi Mission Hub flight planner [67] that met all the parameters (flight altitude and overlap) that emerged from the investigation into the scale issues. Initially, two flight planning parameters were investigated: the flight altitude and the overlap of the images. Overall, six flight plans were created at three different flight altitudes of 60 m, 80 m, and 100 m, and in two different overlaps, 80% front overlap with 60% side overlap and 90% front overlap with 70% side overlap (Figure 4), for the 3D mapping of the gorge. The design pattern of all flights was in grid form and the aircraft's orientation was to the next waypoint. The aircraft camera was vertical to the ground (nadir), and the flights maintained a constant altitude above the ground. The front overlap of the images was adjusted by the aircraft's speed with the assumption that the UAS would capture images every 2 sec. The characteristics of the flights performed for data collection are described in Table 2. More specifically, both Flight \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Height of Flight** & **Overlap** & **GSD** \\ \hline Scenario 1 & 60 m & 80\% and 60\% & 1.8 cm \\ \hline Scenario 2 & 60 m & 90\% and 70\% & 1.8 cm \\ \hline Scenario 3 & 80 m & 80\% and 60\% & 2.4 cm \\ \hline Scenario 4 & 80 m & 90\% and 70\% & 2.4 cm \\ \hline Scenario 5 & 100 m & 80\% and 60\% & 3 cm \\ \hline Scenario 6 & 100 m & 90\% and 70\% & 3 cm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The six scenarios’ characteristics of height of flight, overlap, and GSD. Figure 3: Graph showing flight altitude (x-axis) and the number of images (y-axis): relationship chart for the flight planning with an overlap rate of 80% forward and 60% sideways. 1 and 2 were realized at 60m height with different front and lateral overlaps; thus, 282 and 462 images were collected, respectively. Both Flight 3 and 4 were executed at 80 m having overlap values of 80% and 60% and 90% and 70%, respectively. The number of images collected from these flights were 307 and 391, respectively. As for the flights realized at 100 m altitude, 201 and 275 images were acquired having the same overlap values as at lower altitude. The data's geographical location was determined using the Phantom 4 Pro's GPS, as the exact geographical location was not required for the 3D geovisualization of the results in the virtual space. Moreover, in all acquired images, GPS coordinates were embedded in the EXIF metadata. We defined that when the 3D model was rendered through the virtual space, it was transferred to an arbitrary coordinate system. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Height of Flight** & **Overlap** & **Duration** & **Number of Images** & **Flight Speed** \\ \hline Flight 1/scenario 1 & 60 m & 80\% and 60\% & 12 min & 307 images & 20 km/h \\ \hline Flight 2/scenario 2 & 60 m & 90\% and 70\% & 20 min & 462 images & 12 km/h \\ \hline Flight 3/scenario 3 & 80 m & 80\% and 60\% & 10.5 min & 282 images & 21 km/h \\ \hline Flight 4/scenario 4 & 80 m & 90\% and 70\% & 18 min & 391 images & 14 km/h \\ \hline Flight 5/scenario 5 & 100 m & 80\% and 60\% & 8 min & 201 images & 25 km/h \\ \hline Flight 6/scenario 6 & 100 m & 90\% and 70\% & 12 min & 275 images & 22 km/h \\ \hline \hline \end{tabular}
\end{table}