# Detecting Underground Geospatial Features for Protection of Rights in 3D Space: Korean Cases
## 3 Results
### Direct Survey Method  
The method for acquiring the 3D data of an underground shopping center can be mainly divided into surveying control points and 3D laser scanning. To acquire the absolute coordinates of the underground shopping center, the cadastral supplementary control point near the Gwangbok Underground Shopping Center was used, and then 3D laser scanning was carried out by scanning the inside of the shopping center using the LiDAR equipment. In the process of verifying the supplementary control point using the total station, we found that there are four first-grade traverse lines with good control points at the start and end of the Gwangbok Underground Shopping Center. Using this as a starting point and an occluded point, a supplementary control survey was conducted using a closed or fixed traverse. Global navigation satellite system (GNSS) surveying equipment was used to obtain World Geodetic System-based results; however, direct observation was difficult due to interference from adjacent high-rise buildings. Therefore, the World Geodetic System (WGS) coordinates were calculated by installing an arbitrary point and connecting it to the supplementary control point used in the survey. The supplementary control points used in the survey are presented in Table 3. In Table 3, Bessel is Datum: Tokyo, Ellipsoid: Bessel (1841), coordinate system: TM, and WGS is Datum: ITRF2000, Ellipsoid: GRS80, coordinate system: TM. Ten supplementary control points, connecting the ground and underground, were established to acquire 3D data of the underground shopping center, and the elevation value for each supplementary control point was calculated. The Gwangbok Underground Shopping Center is located in an area where the average horizontal distance from the sea level is approximately 150-200 m. Therefore, the elevation value of the supplementary control point in the underground shopping center, excluding the ground, was measured as a negative value (\(-\)), as presented in Table 4 [18]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Supplementary Control Points Used** & \multicolumn{2}{c}{**Bessel Result**} & \multicolumn{2}{c}{**World Geodetic System Result**} & \multirow{2}{*}{**Elevation**} & \multirow{2}{*}{**Remarks**} \\ \cline{2-2} \cline{6-6}
1048 & 177,746.24 & 203,190.68 & 278,056.11 & 203,249.15 & 2.477 m & 1 st supplementary control \\  & & & & & (re-established in 2008) & \\
903 & 177,723.78 & 203,066.15 & 278,033.60 & 203,124.64 & 1.979 m & 1 st supplementary control \\  & & & & & (re-established in 2008) & \\
129 & 178,099.64 & 203,280.58 & 278,409.46 & 203,338.97 & 1.599 m & 1 st supplementary control \\  & & & & & (re-established in 2008) & \\
130 & 178,058.39 & 203,254.60 & 278,368.23 & 203,312.98 & 1.492 m & 1 st supplementary control \\  & & & & & (re-established in 2008) & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Supplementary control points used in the survey. 3D laser scanning was carried out to acquire the 3D coordinates of selected points of the shopping center based on the relative coordinates using the LiDAR equipment. Therefore, prior to laser scanning, 53 control points were observed near the point where three sides that can be recognized as points in a 3D space meet for conversion to absolute coordinates. After surveying and observing the control points, laser scanning was performed to acquire 3D spatial data inside the underground shopping center using the LiDAR equipment. Because of the nature of the equipment, there can be a shaded area due to the presence of pedestrians, therefore data acquisition was performed late at night and early morning when pedestrian traffic was relatively low. The total length of the Gwangbok Underground Shopping Center is 411 m, and stores are located on the left, right, and center of the shopping center, and pedestrian passages were separated into two. Therefore, the total distance was approximately 820 m back and forth.