# Technological Development and Application of Photo and Video Theodolites
## 4 Application of the State-of-the-Art Video Theodolites--IATS in Monitoring
### Case Study
Subsequently, to reduce computation time, the region around the photo mark was defined. Then, a conversion from an RGB image to a binary black-and-white image was performed, facilitating the detection of circle boundaries and circle centers. According to the pixel coordinates of circle centers, the joint center of all four circles was calculated. The differences between joint centers of each image for every test in relation to the first reference image before the train passed could then be calculated. Accordingly, we could calculate the dynamic vertical displacements in pixels. As a function of the known spacing between the circle centers, the scale factor was calculated and used to convert pixel displacements into millimeter displacements. Lastly, using fast Fourier transformation, the time domain was converted into the frequency domain, and the natural frequencies of the bridge were determined. Figure 14 (left) shows the computed first modal shape of the bridge at 5.35 Hz frequency, and Figure 15 (left) shows the second modal shape at 7.77 Hz frequency using the finite element method (FEM). In Figures 14 and 15 (right), the first and second modal shapes and frequencies obtained from ambient vibration measurements using accelerometers are shown. Differences between the FEM and ambient vibration measurements occurred regularly because the FEM model was made using design parameters which could have differed from the constructed bridge. The aim of the testing was to determine the natural frequencies of the bridge during vibration monitoring using the IATS prototype. To assess the accuracy of our IATS prototype, we used ambient vibration measurements by accelerometers as a reference. The first natural frequency determined by the IATS prototype measurements matched the first natural frequency of the bridge measured by accelerometers, while the second natural frequency determined by IATS slightly differed from that determined by accelerometers  
Figure 14: First modal shape and frequency computed by FEM (**left**) and determined by accelerometers (**right**). Figure 15: Second modal shape and frequency computed by FEM (**left**) and determined by accelerometers (**right**). for both tests (Figures 16 and 17). At 40 km/h, the first frequency, F1 = 5.49 Hz, detected by the accelerometer was equally detected by the IATS prototype, F1 = 5.49 Hz. At 40 km/h, the second frequency, F2 = 6.85 Hz, detected by the accelerometer was detected by the IATS prototype as F2 = 6.78 Hz. At 60 km/h, the first frequency, F1 = 5.49 Hz, detected by the accelerometer was equally detected by the IATS prototype, F1 = 5.49 Hz. At 60 km/h, the second frequency, F2 = 6.85 Hz, detected by the accelerometer was detected by the IATS prototype as F2 = 6.82 Hz. The conducted study showed that IATS can be used for the determination of dominant vertical natural frequencies, confirming the previous studies performed by [7; 7; 80]. In our future work, we will present all conducted tests performed during this load testing.