**1. Introduction**

The initial support of the tunnel refers to a form of surrounding rock support used on-site to protect the safety of the construction during the excavation of the tunnel project. The flatness refers to the degree of unevenness of the sprayed concrete lining on the initial support surface during the tunnel excavation process [1]. It is one of the important criteria for the quality acceptance of the tunnel project. The results directly affect the subsequent laying of waterproof coiled materials, the paving quality of the second-stage lining, and the safety of the tunnel project; the surface level of the initial support of the tunnel is too large, which directly leads to the decrease of the construction quality of the laying of the waterproof coiled material and the second-stage lining [2]. This indirectly affects the water seepage condition of the tunnel surface, making it more prone to water seepage and leakage in the tunnel. The traditional detection methods for the flatness of the initial tunnels are low in accuracy and efficiency, and fail to meet the expectations of future construction guidance, and the current flatness detection system of the tunnel is not optimistic. The introduction of the emerging 3DLS technology and the establishment of a method system for the flatness detection of the initial support of the tunnel based on the 3DLS technology are of grea<sup>t</sup> significance to the improvement and development of the flatness detection system of tunnel engineering [3].

**Citation:** Xiang, L.; Ding, Y.; Wei, Z.; Zhang, H.; Li, Z. Research on the Detection Method of Tunnel Surface Flatness Based on Point Cloud Data. *Symmetry* **2021**, *13*, 2239. https:// doi.org/10.3390/sym13122239

Academic Editors: Yang Yang, Ying Lei, Xiaolin Meng and Jun Li

Received: 7 September 2021 Accepted: 16 November 2021 Published: 23 November 2021

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