# GNSS-Assisted Low-Cost Vision-Based Observation System for Deformation Monitoring
## 2 General Concept and Approach  
### Design Concept of the VOS  
Regarding the idea mentioned above, it is proposed to place the sensors of the VOS along the mutually perpendicular axis and planes of the structure to be monitored. Such a configuration will allow at the stage of data analysis the further separation of the effects of torsion from deflection and roll, which is a tricky issue for high-rise buildings monitoring [1]. The VOS design concept supposes the determination of both relative and absolute displacements. An external coordinate system has to be established to monitor the absolute displacements. This coordinate system is fixed by the system of targets placed on the surrounding objects considered stable and via the GNSS observations at the building top. The coordinates of these targets are determined using total station measurements or, in some cases, GNSS measurements. The design concept of the GNSS-assisted VOS is presented in Figure 3.  
Each particular sensor of the VOS consists of a set of elements. These elements may be organized in a different manner depending on the position of the sensor. However, in the general case, the sensor contains the components depicted in Figure 4.  
In addition to standard modules, the camera must contain a module for data transmission, allowing the quick transmission of the target images from all the sensors. It is proposed to make the target a QR code with embedded LEDs. This code will enable it to include the necessary information, for example, target ID, coordinates, etc., and increase its visibility. Two sensors can be combined into a chain. There are two ways the sensors may be organized in the chain (Figure 5). Figure 4: VOS sensor and target. Figure 5: VOS single chain: (**a**) sensor to sensor; (**b**) sensor–target–sensor. Figure 3: The general concept of the VOS and its installation. Therefore, the VOS can be installed inside the building along its principal axes. There are different methods of VOS installation. Two of them are given in Figure 6. Both cases demonstrate the VOS sensors' placement in a structure's vertical plane. For a horizontal plane, the scheme will be the same; the difference is only in the directions of the coordinate axes. According to the presented schemes, each sensor-target pair is considered a chain regardless of the placement of the target (on the camera or separately). In the first scheme, the observations are performed from sensor to sensor, where each one is equipped with a QR target. This scheme is valid for structures that have relatively small sizes, otherwise a more complicated observation scheme is suggested. The sensors are interconnected with each other throughout the system of two-sided QR targets. In any case, for the first observation epoch, both the sensors and targets must be aligned in horizontal and vertical directions. The installation of the VOS is possible in different ways. VOS can be embedded inside the building communication lines or attached outside and adequately covered. It is necessary to study the technical characteristics of the optical system to determine the effect of the distance between two sensors or the sensors and the QR target. Since low-cost cameras are going to be used, the main issue is the size of the QR target in the image. The target must be recognizable in the image, which is why the resolution plays a significant role. The conventional scheme of image acquisition in a simple camera is given in Figure 7.  
Figure 6: VOS installation schemes: (**a**) the scheme with sensor-to-sensor observations; (**b**) the scheme with sensor-target–sensor. In a simplified form, the visible area of size \(a\times b\) is formed on an \(m\times n\) matrix. The size of the visible area depends on the camera field of view \(\gamma\) and the focal length \(f\) and is described by the following relationship:  
\[\gamma=2\cdot arctg\bigg{(}\frac{d(S-f)}{(2S\cdot f)}\bigg{)}. \tag{1}\]  
where \(d\) is the frame size. Using such a parameter will be incorrect for an image with a rectangular shape.