Image-based methods for metric surveys of buildings using modern optical sensors and tools: From 2D approach to 3D and vice versa

Nowadays, the advent of digital images and the diffusion of new tools for the transformation and the creation of 3D metric models has led to a higher quality and accuracy in the architectural survey of the buildings. Furthermore, the recent development of new sensors, such as smartphone or commercial Digital Single-Lens Reflex (DSLR) cameras or cameras incorporated in drones, the design of new algorithm and tools based on Image-Based Methods (IBM) has allowed an increasingly faithful representation of buildings. Of particular interest in the Close Range Photogrammetry (CRP), it was the introduction of the Structure from Motion (SfM) approach, which allowed to survey even buildings with complex architecture. However, the processing of construction of the 3D model building using this latter method requires High Performance Computation (HPC) and, sometimes, long processing time in order to build 3D models. Therefore, this approach is not always the better method in the construction of 3D models. In this paper, it was shown by diverse interesting case studies and wherever possible, if the building has a plane surface or it can be simplified as a plane surface, to create 3D models starting from 2D approach. In addition, a case study concerning the survey of a building with many glazed surfaces, such as skyscrapers, is discussed. This is due to the high reflectivity of glazed surfaces that do not allow (or make very complicated) an automatic alignment of multiple images using traditional software for the realization of both 2D and 3D models. Consequently, depending on the position of the operator and the position of the sensor, the image reflected in the glass changes. Therefore, a specific and suitable method should be used depending on the type of building. However, if the architecture of the building is complex, it is necessary to use a 3D approach, such as Structure from Motion. In this way, it was possible to build the 3D model and, subsequently, to export to single façade (2D) according to a traditional approach. Indeed, the use of the 2D orthophoto (or 2D CAD representation), thanks to its ease of management, is still a favourite method for many users, such as architects, engineers, restorers or maintenance technicians in many building activities.