# Experimental Research of the Structure Condition Using Geodetic Methods and Crackmeter
## 3 Materials and Methods  
Periodic measurements of displacements were carried out in various weather conditions (season, thermal conditions) at the points of the measurement and control network installed inside the above-ground passage that serves as a communication link between the buildings of the Bydgoszcz University of Technology. The measurement dates and the averaged thermal conditions during the measurements are given in Table 1.  
The control points and reference points have been marked inside the passage and on the structure of neighboring buildings in the form of pads glued to the floor (Figure 2). The diagram of the location of the measurement and control network points on the monitored object is presented in Figure 3. \begin{table}
\begin{tabular}{c c c} \hline
**Meas. No.** & **Date** & **Construction Temperature** \\ \hline
0 & 2020.07.28 & 31 \({}^{\circ}\)C \\
1 & 2020.08.07 & 35 \({}^{\circ}\)C \\
2 & 2020.11.04 & 8 \({}^{\circ}\)C \\
3 & 2020.12.02 & \(-3 ^{\circ}\)C \\ \hline \end{tabular}
\end{table} 
Table 1: Geodetic measurements conditions. Figure 1: Location of the research structure [own elaboration]. Figure 3: The scheme of the network points distribution [own analysis]. Figure 2: Leica TDRA6000 measuring device and method of stabilizing network points [own analysis]. The displacement of the controlled points network was measured with the TDRA6000 laser station (Figure 2). The station is equipped with direct drive technology, which allows obtaining the accuracy of 3D point measurement at the level of 0.25 mm and allows for the observation of horizontal and vertical directions with an average error of 0.0013 g and a distance of 0.2 mm [58; 59]. Due to the possible measurement accuracy, the device with appropriate software is an excellent tool for monitoring the condition of engineering structures. Measurement of the measurement and control network with a laser station consisted of recording the 3D coordinates of the control points at each of the seven free positions. Only the closest control points were observed from each position: the extreme positions--5 points, and the remaining positions--4. Measurements on each position were carried out twice. The calculations were carried out in two stages. In the first stage, the differences in height between the network points were calculated, and then its strict alignment was performed [60; 61; 62]. The second stage included determining the assumed network's horizontal displacements. The calculations were made using the plane coordinates (\(X\), \(Y\)) of the controlled points at this stage. The calculations were carried out using the isometric transformation integrating the free positions and the Helmer transformation on six reference points (R1-R6), presenting the coordinates of the controlled points from individual measurement series in a uniform local coordinate system. The horizontal displacements of the points were determined by calculating the coordinate differences (\(X\), \(Y\)) between the measurement series. At the same time, displacements were also tested with a crackmeter. The SHM-X crackmeter is a simple tool for measuring changes in crack and expansion joint widths. Four pins (measurement points) are installed at the measuring point, allowing for precise crack control. In expert measurement, displacements in two directions (perpendicular and parallel to the crack) are determined, and a crack or dilatation separates the angle of rotation between the parts of the structure. For a complete analysis, five measurements \(P1\)-\(P5\) of the distance between the selected pins are required [63]. A diagram of the location of measuring points A, B, and C is shown in Figure 4.  
The essence of the method is to determine the Cartesian coordinates \(X\), \(Y\), and measurement points 1 and 2 concerning points 3 and 4. It was assumed that points 3 and 4 have a fixed position and determine the coordinate system. The origin of the coordinate system is at point 3.