# Using Dynamo for Automatic Reconstruction of BIM Elements from Point Clouds
## 1 Introduction  
Throughout the history of construction, various authors have delved into the survey and graphic documentation of existing buildings [1; 2; 3; 4; 5]. In recent years, new technologies, like 3D laser scanning and digital photogrammetry, have gained popularity for capturing pre-existing structures due to their efficiency and non-invasive nature. These technologies can perform surveys with high accuracy and precision, capture all the details of complex and large buildings, and are less time-consuming than traditional measurement techniques [6; 7]. The resulting outputs from these surveys vary widely. They commonly include documentation created using Computer Aided Design (CAD) software [8; 9; 10], three-dimensional models [8; 11; 12], or Building Information Modeling (BIM) models [13; 14; 15; 16; 17], which are widely used in the architecture, engineering, and construction (AEC) industry. The use of BIM, together with light detection and ranging (Lidar) and digital photogrammetry, commonly known as scan-to-BIM [18], brings many gains to the process of the digital reconstruction of pre-existing buildings, even if, in some cases, the BIM software is used only for the modeling tasks. Some of the benefits are the possibility of working on a complete 3D model of the building and not just on isolated plans, a better level of accuracy, speed in obtaining building sections and elevations, ease of detecting errors and deviations, and the possibility of collaborative work, among others [19]. Despite this, many professionals still point out some barriers that make it difficult to fully adhere to this new way of working, such as the cost of migrating to new platforms, the need for specialized professionals or training of the existing team, the lack of customer demand, and the type of project they work on not justifying the change [20; 21]. In addition, professionals who have already incorporated scan-to-BIM in their companies mention that some weak points in thisprocess make the work harder, such as the excessive time consumed in manual modeling tasks, the inaccuracy and difficulty of modeling organic elements or with unconventional shapes, and the lack of automation tools that are efficient and reliable [19]. With recent technological changes, government incentives and requirements, and new paradigms in the AEC industry [21; 22; 23], there has been an increase in requests for BIM models as the main product generated from architectural surveys, and as more companies make the transition to BIM, the trend is that this product will be increasingly requested. The BIM model must contain geometric information and reflect the complexity of the built architectural object, with its attributes, parameters, materiality, and constructive characteristics [24; 25; 26]. It should also be considered that the model is an abstraction of reality; thus, it does not and should not represent all aspects of the object but rather represent it in its simplified form. No model can faithfully reproduce all aspects of its reference, and the purpose for which it will be used must be well defined from the beginning, as this avoids the creation of an inappropriate model for the situation [27]. When dealing with graphical representations for construction, a common practice is to create the lines and geometry with an orthogonality approach, but orthogonality does not exist in the real world. Even the most perfect construction made with precast elements presents deviations, either in its manufacture or due to deformities presented after use and time. Some questions arise from these observations. How can we represent a construction that contains deviations and irregularities? What is the deviation tolerance for each situation? These questions become more critical when we use contemporary digital tools that, for the most part, encourage the element to be drawn following an orthogonal logic because this facilitates the computer's processing and avoids certain inaccuracies [28].