Vision-based systems for structural deformation measurement: case studies

Vision-based systems offer a promising way to measure displacement and are receiving increased attention in civil structural monitoring. However, the working performance of vision-based systems, especially their measurement accuracy and robustness to different field conditions, is not fully understood. This study reports three cases studies of vision-based monitoring tests including one in a laboratory, one on a short-span bridge and one on a long-span bridge. The tracking accuracy is quantified in laboratory conditions in the range from 0.02 to 0.20 pixel, depending on the target patterns as well as the tracking method selected. The measurement performance under several field challenges is investigated, including long-range measurement (e.g. camera-to-target distance of 710 m), low-contrast target patterns, changes of target patterns and changes in lighting conditions. Three representative tracking methods for the video processing, namely, correlation-based template matching, Lucas-Kanade optical flow estimation and scale-invariant feature transform were used for the analysis, indicating their advantages and shortcomings for field measurement. One of the main observations in field application is that changes in lighting conditions might cause some low-frequency measurement errors that could be misunderstood without prior knowledge regarding structural loading conditions.