# Geometric Accuracy Evaluation Method for Subway Stations Based on 3D Laser Scanning
## 5 Discussion  
The accuracy of the building surfaces and structural columns of the Hongqi Road subway station is evaluated in the experiments described above. The average deviation value of the east facade of the Hongqi Road subway station is \(-\)12.9 \(\pm\) 19.3 mm, showing a double peak distribution, with error values of \(-\)22.5 mm and 0.4 mm at the two peaks (Figure 16). The deviation range remains at the centimeter level. The mean deviation value of the west facade is 4.5 \(\pm\) 33.7 mm, which also shows double peak distribution, with error values of \(-\)29.8 mm and 20.2 mm at the two peaks (Figure 17), located in the W1 and W4 areas. The mean deviation values of the south and north facades are 0.00 \(\pm\) 25.8 mm and \(-\)5.4 \(\pm\) 26.4 mm, respectively (Figures 18 and 19), both of which show a peak distribution. The deviation ranges of the four facades in the east, west, north, and south are all at the centimeter level, and the deviation distributions are relatively similar, which indirectly reflects the rationality of the coarse-to-fine registration method proposed in this paper. Additionally, there are relatively large distance deviations at the edge of adjacent surfaces, which is a normal phenomenon due to the local shrinkage or expansion near the mold caused by concrete pouring [25]. The mean value of deviation of the top slab is \(-\)8.8 \(\pm\) 21.2 mm, showing an obvious peak distribution, and the peak of deviation is distributed in the T1 area. The mean value of deviation of the bottom slab is \(-\)6.9 \(\pm\) 36.6 mm. Because of the presence of ponding water in the second underground layer during the data acquisition, the high reflective property of this ponding water causes \begin{table}
\begin{tabular}{c c c} \hline \hline
**Structural Column** & **SCDM (mm)** & **ADD (\({}^{\circ}\))** \\ \hline A-2 & 80.5 & 1.55 \\ C-2 & 72.6 & 0.39 \\ B-36 & 50.9 & 0.19 \\ B-35 & 48.1 & 0.36 \\ B-34 & 38.3 & 0.64 \\ C-6 & 36.6 & 0.41 \\ A-5 & 36.6 & 0.39 \\ A-7 & 32.8 & 0.94 \\ C-13 & 32.1 & 0.65 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The _SCDM_ and _ADD_ results of structural columns that exceed the tolerances. the acquired point cloud data to be missing. The missing data area cannot meet the requirements of geometric accuracy evaluation. By evaluating 50 structural columns of the Hongqi Road subway station, the average value of their deviations is 5.5 \(\pm\) 19.9 mm (Figure 22b), and the overall deviation shows a peak distribution, with significant deviations in the structural columns on the east and west sides (Figure 22a). The dimensions of the measured point clouds of structural columns C-2 and A-2 are significantly larger than those of the structural columns in the design state (Figure 26a), which is a measurement error caused by site occlusion. A red curtain was wrapped around the perimeter of the structural columns, as shown in Figure 27. Among them, A-2 has the largest _SCDM_ of 80.5 mm; the mean deviation of structural column C-2 is 61.8 \(\pm\) 15.7 mm (Figure 23). The average value of deviation of structural column B-35 is 9.7 \(\pm\) 40.2 mm (Figure 24), the medial axis of the structural column is inclined to the north, and the deviation is mainly concentrated in the upper part of the structural column (Figure 22a). The reasons for the deviations of B-34 and B-36 are similar to those of B-35. Therefore, the projection of the point cloud of these three structural columns has a greater thickness in the north-to-south direction compared to the other structural columns (Table 2). The accuracy evaluation results of all structural columns are presented in Figure 27, most of which have an _SCDM_ within the tolerance range and meet the construction requirements, and all of which have _ADD_ values that meet the construction requirements. The experimental analyses show that the subway station is large in space and long in depth, and the construction layout is easily affected by the subjective judgment of construction personnel, which may lead to the generation of deviations.