# Quality Analysis of 3D Point Cloud Using Low-Cost Spherical Camera for Underpass Mapping
## 2 Materials and Methods
### Case Study and Sensor Selection  
The case study of this research was conducted in the graffiti underpass of the Theodor Heuss Bridge in Wiesbaden, Germany (Figure 1). The Theodor Heuss Bridge is a significant piece of infrastructure and connecting bridge between the Mainz and Wiesbaden cities in Germany. Beneath this bridge is an underpass with concrete walls painted with graphic drawings, and it was selected as the main structure in this research endeavor. The underpass is covered in graffiti and spans approximately 30 m in length, 4.5 m in height, and 6 m in width. It features an arched design with sloping ground and contains a single lighting resource, placed in the center of the ceiling, excepting outdoor lighting. This paper excluded professional spherical cameras such as the Teledyne FLIR Ladybug, Professional360 Panono, Insta360 Pro2, and Weiss AG Civetta [23; 24], due to their high price and more complex user interface for non-professional users, from the spherical cameras considered. Polydioptric spherical cameras are widely used due to their design principles, both for professional and low-cost sensors [24]. This is primarily due to two key factors. First, the technique provides full omnidirectional imaging, covering 360 degrees horizontally and 180 degrees vertically. The overlapping field of view facilitates image stitching to generate panorama images. Second, the imagery system has a higher resolution due to the use of stereo fisheye cameras simultaneously [23]. As a selection step, we aimed to determine the most suitable and cost-effective polydioptric spherical camera with the necessary features for data acquisition. These features include monitoring software,accessibility to raw files, high image resolution, and spatial resolution. Table 1 lists the specifications of the top four spherical camera brands. According to Table 1, the Insta360 One X2 and X3 cameras were used as the most optimized and best available spherical cameras based on their technical features in this research. The Insta360 One X3 has the best possible image resolution, but the Insta360 One X2 is more affordable and possesses optimized technical features in compare to other spherical cameras. To improve the 3D reconstruction and generation of 3D point clouds, an alternative sensor should be used alongside the spherical camera images as the base dataset. This sensor should assume the role of the most precise data collector for the graffiti underpass. Although LS surveying can be costly, it enables the precise and accurate detection of details, which is especially beneficial for reconstructing elements with complex shapes [27]. Accordingly, in this research, the Leica RTC 360 laser scanner (LS, Manufacturer: Leica Geosystems AG, Heerbrugg, Switzerland) was used to compare the quality (Figure 2). On the other hand, In order to make a comprehensive statement about the quality of the of the 3D point clouds, the data was captured by spherical cameras and other optical cameras, such as DSLR cameras, to compare the results. A well-known DSLR model (Nikon D800 with fisheye lens, 16 mm equivalent focal length, Manufacturer: Nikon Corporation, Tokyo, Japan) was used for data collection and comparison.