# Multi-Point Displacement Synchronous Monitoring Method for Bridges Based on Computer Vision
## 6 Conclusions  
The structural health monitoring of bridges can be carried out to obtain data only at the local measuring points, and these incomplete measuring data lead to difficulties in evaluating the structural safety state. We proposed a structural multi-point synchronous monitoring method to solve this problem. The conclusions are as follows:  
1. The Scale-invariant Feature Transform (SIFT) algorithm can extract the structural feature points. These feature points can be used as measurement points for displacement monitoring. By establishing an image coordinate system, the displacement of feature points before and after deformation can be calculated. 2. A 7 m long test beam was made in the laboratory. By drawing calibration lines on the vertical members of the test beam, the monitoring resolution of the image can be calculated. The monitoring resolution of the test beam image in this paper is 0.18 mm. 3. The structural surface's weak or repeated natural texture features can lead to the mismatches of some displacements. Hence, a displacement mismatch elimination method was proposed. This method uses the extracted deflection curve to constrain \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Load**} & \multirow{2}{*}{**Code Mark No.**} & \multicolumn{2}{c}{**Validation Value**} & \multicolumn{2}{c}{**Calculated Value**} & \multirow{2}{*}{**Error (\%)**} \\  & & **(mm)** & & **(mm)** \\ \hline \multirow{8}{*}{100 kN} & 1 & 1.00 & 1.03 & 3.26\% \\  & 2 & 4.96 & 4.75 & 4.29\% \\  & 3 & 7.82 & 8.29 & 5.95\% \\  & 4 & 9.52 & 9.84 & 3.35\% \\  & 5 & 10.89 & 11.23 & 3.05\% \\  & 6 & 11.92 & 11.13 & 6.60\% \\  & 7 & 12.14 & 12.32 & 1.42\% \\  & 8 & 12.60 & 13.11 & 4.12\% \\  & 9 & 12.26 & 12.57 & 2.49\% \\  & 10 & 11.89 & 12.22 & 2.80\% \\  & 11 & 11.11 & 11.38 & 2.51\% \\  & 12 & 8.83 & 9.28 & 5.12\% \\  & 13 & 6.18 & 6.52 & 5.38\% \\  & 14 & 3.67 & 3.99 & 8.70\% \\  & 15 & 0.92 & 0.95 & 3.52\% \\  & 16 & 0.96 & 0.91 & 4.97\% \\  & 17 & 5.00 & 4.68 & 6.38\% \\  & 18 & 7.95 & 8.26 & 3.85\% \\  & 19 & 9.32 & 9.74 & 4.42\% \\  & 20 & 10.96 & 11.19 & 2.06\% \\  & 21 & 11.90 & 11.60 & 2.57\% \\  & 22 & 12.29 & 12.78 & 4.00\% \\  & 23 & 12.40 & 12.85 & 3.65\% \\  & 24 & 12.53 & 12.15 & 2.99\% \\  & 25 & 12.50 & 12.07 & 3.48\% \\  & 26 & 11.12 & 11.65 & 4.81\% \\  & 27 & 8.31 & 8.82 & 6.08\% \\  & 28 & 6.00 & 5.50 & 8.31\% \\  & 29 & 3.82 & 3.92 & 2.70\% \\  & 30 & 0.81 & 0.84 & 3.54\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Validation of multi-point displacement synchronous monitoring results. the displacement's length and rotation angle. Hence, we achieved structural multi-point displacement mismatch elimination. * We validated the test results using a three-dimensional laser scanning method. The maximum error of the monitoring results was 8.70%, and the average error was 4.21%. The monitoring results are consistent with the actual structural deformation. * This method can expand the monitoring data. The monitoring results are similar to those of the structural full-field displacement monitoring and are expected to fundamentally solve the bridge safety evaluation problem of incomplete test data. * This study yielded good monitoring results in the laboratory. However, the test beam's image in the laboratory exhibits obvious noise, leading to the structural edge's line shape undulation. The bridge environment is more complex than that of the laboratory, and the image noise is obvious. Therefore, the noise interference problem in the application of this method to practical actual bridges should be researched in future studies. It is recommended to use higher pixel hardware devices and more accurate feature point extraction algorithms in future studies to reduce the impact of noise. * The multi-point displacement synchronous monitoring method of structures can be combined with structural damage identification.