# Wall Structure Geometry Verification Using TLS Data and BIM Model
## 1 Introduction  
Building information modeling (BIM) enables digitalization of the whole life cycle of a building, from the design, through construction to operation. It is a process that uses a consistent and continuously updated model [1]. The use of BIM brings more simple and effective access to information, generates benefits for all partners (investor, designer, and constructor), and leads to possible reductions in the investment process [2, 3, 4, 5]. Using a common data environment for information sharing improves the onsite collaboration and communication with such efficiency, which cannot be compared to drawing sets in a form of separate files. BIM enables model-based cost estimation in the planning stage of the construction, clash detection, and very effective coordination and collaboration with contractors. 5D planning enables not only construction scheduling, but also financial planning of the construction. Finally, well-coordinated and verified models lead to increasing the quality of the building. Several carried out analyses prove that most of the companies that have adopted BIM achieved an overall reduction of costs. Therefore, there is significant attention put into BIM worldwide, focusing on model creation and management of investment processes in the construction industry [6, 7, 8]. A BIM model is an object-oriented virtual model of the building, which consists of three parts in terms of data involved. The first part is graphic data defining the geometry and, in most cases, it is a 3D model. The second and third parts are non-graphic data, representing information and parameters, and documentation respectively. The detail of a BIM model is defined by thestandard _ISO 19650-1:2018 Organization and digitization of information about buildings and civil engineering works, including building information modelling (BIM)--Information management using building information modelling--Part 1: Concepts and principles._  
Early identification of changes (deviation between the designed and built structures) in combination with high interoperability of partners using BIM enables to achieve savings during the construction project, thanks to good coordination of decisions about changes [2; 9]. Therefore, the building industry needs timely and accurate information about these changes, which means measurement of the as-built geometry of the building. The verification often consists of a set of manual operations which require deep knowledge of the project, are time-consuming, and are costly. Graphic data of the BIM model are the source of information about the designed geometry of a building's structure, and it is applicable for as-built verification of chosen parts or structural elements. Currently, the most effective methods of spatial data collection are TLS and close-range photogrammetry resulting in point clouds. The use of information contained in detailed point clouds and information derived from a BIM model is a suitable tool for quality check of a given structure, often in real time. Combining the data from BIM with the data from TLS, the entire verification process, too, can be automated. TLS is available for a wide community of surveyors, structural engineers, and civil engineers thanks to the affordability and the fact that manufacturers of instruments make them easy to use with a high level of automation. This means, that during the scanning process, only some basic parameters (field of view and scan density) are needed to set up and the whole measurement is performed in an automated way. Therefore, TLS is used for different surveying jobs, e.g., [10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. Its main benefit is the efficiency of the 3D data collection (up to 2 million points per second), meaning less time spent on the construction site (needed for measurement) and often a huge amount of data defining the geometry of the surface of the measured structure [20].