*4.3. Feasibility Analysis*

To verify the effectiveness and feasibility of the method of detecting the surface flatness of the initial branch of the tunnel based on the 3DLS technology, this study compares and analyzes the point cloud data detected by the total station and the point cloud data detected by the 3DLS technology. The tunnel mileage K107+780, K107+785, K107+790, and K107+795 are, respectively, taken from four cross-section information, and each cross-section is taken from the left and right arch bottom, left and right arch waist, and the position of the archtop. The points were measured 4 times with a three-dimensional laser scanner at the same time interval. The thickness of the difference between the monitoring point and the design section collected by the total station is recorded as *d*1, and the thickness of the difference between the monitoring point and the design section collected by 3DLS is recorded as *d*2, and the statistical results of the test data collected by the two measuring instruments are summarized, as shown in Table 5 below.

It can be seen from the data in the tab that the detection value collected by 3DLS is roughly the same as the detection value collected by the total station. To further illustrate the accuracy and effectiveness of the flatness detection method based on the 3DLS technology, this study will the detection value of the total station is regarded as the most reliable value *x*ˆ. and the detection value of the scanner is regarded as the observation value *xi*. The detection difference Δ*d* of 4 tests can be calculated respectively, and finally the median error *σ*ˆ of 3DLS can be calculated by the detection difference Δ*d*. The error *σ*ˆ in the


calculated observation value can represent the true error of 3DLS. The Medium error resultis shown in Table 6.

**Table 5.** Section monitoring point detection value.

Note: \* indicates that the location is blocked, and the data at this point has not been measured.

> **Table 6.** The two instruments measure the difference.


Note: \* indicates that the location is blocked, and the data at this point has not been measured.

It can be seen from Table.6 that the true error of the 3DLS will increase as the distance between the section and the station increases. The cross-sectional instrument method, total station coordinate method or 3DLS method can be used, and the error in the measurement should not be greater than 25 mm [35]. Because the selection of the initial section of the tunnel in this study is a cross-sectional area of 2 m in the direction of the central axis By default, the center position of the cross-sectional area is the position of the measuring station, so the distance between the instrument and the scanned cross-section has little effect on the flatness detection results of this study. The true error of the 3DLS meets the requirements of the specification within a certain measurement range. The accuracy of the point cloud data collected by the 3DLS is improved.
