# Geometric Accuracy Evaluation Method for Subway Stations Based on 3D Laser Scanning
## 1 Introduction  
In recent years, the rapid development of underground rail systems around the world has greatly facilitated people's daily travel and promoted economic development [1]. The construction of subway stations, as links to subways, is the key to underground rail systems. However, the construction of subway stations is difficult and technically demanding [2]. Ensuring geometric accuracy is important in urban subway construction. Subway station construction includes a series of dynamic processes from planning, to design, to construction, and to maintenance; the building information modeling (BIM) created in the design stage is often divorced from reality in the construction phase. The efficient construction monitoring and accuracy evaluation of subway stations are important to ensure the quality of subway construction and reduce construction problems [3]. Traditional construction monitoring of subway stations relies on point-to-point measurements by construction personnel using instruments such as steel rulers or total stations. However, the construction site environment is complex, and the target points to be measured are distributed in regular or irregular surface patterns, which makes it difficult for traditional measurement methods to achieve complete data acquisition on the surface of the buildingstructure [4, 5]. Three-dimensional (3D) laser scanning allows panoramic, high precision, and fast acquisition of object surface information and is widely used in various applications of civil engineering, such as automated modeling, construction progress tracking, construction safety management, and automated construction [6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. In the field of project quality inspection, some scholars [16, 17] have used laser scanning techniques for dimensional quality inspection to compare information about the size [18], shape [19], and location [20] of construction project infrastructure with design models to identify potential quality problems. In building construction, the surface flatness of concrete slabs is usually measured using the industry-specified F-number method [21, 22]. The F-number method samples concrete slabs at 300 mm intervals and statistically processes them to generate F values that reflect the surface condition. However, due to the sparseness of the measurements, it is not only impossible to pinpoint the precise site of the deviation but also fails to adequately reflect the geometrical properties of the entire surface. At the same time, for the geometric accuracy evaluation of large sites (e.g., subway stations), this method can only be applied to the bottom slab, which lacks flexibility. Some scholars have tried to reduce potential problems during the construction of subway stations by developing prefabricated component technologies [23] or controlling the assembly accuracy [24]. However, these methods concentrate on the accuracy control of specific regions and do not involve a part to whole assessment of subway stations. The key to geometric accuracy evaluation is the comparison of differences, and current problems include the following two aspects [25]: (1) The inconsistency of coordinate systems. In the design stage, if the survey coordinate system is predefined for the BIM, and the laser scanning point cloud uses the same survey coordinate system as BIM, then the coordinate system is consistent between BIM and the point cloud. The object has the same position coordinates in both data, which can satisfy the difference comparison between BIM and the point cloud; if a survey coordinate system different from the predefined BIM is used in the process of obtaining the actual point cloud, then the two coordinate systems are inconsistent, which will limit the comparison, and the point cloud needs to be registered with BIM. (2) Evaluation index and evaluation system.