| dc.description.abstract | Crack development is a clear indicator to the durability of concrete bridges. Traditional bridge inspections which rely inspectors to climb on bridges with lift cars are not only unsafe for inspectors, but also time and labor consuming. While there have been some researches designing robotic arms carrying digital cameras and applying image processing for crack detection, robotic arms are expensive. Therefore, this research proposes a solution that applies unmanned aerial vehicles (UAV) and high-resolution digital cameras to measure concrete bridge cracks. To be specific, two types of images are taken, which are close-up images that can observe cracks more clearly and long-range images that covers ground control points. Afterward, we register these two types of images to establish the absolute coordinate system with ground control points and tie points through the block triangulation. This research examines three approaches of generating tie points: (1) Manually select tie points with features on the bridge such as nails and dots. (2) Randomly input tie points generated from Scale- Invariant Feature Transform (SIFT). (3) Randomly input tie points generated from SIFT as initial tie points and perform automatic tie generation with Erdas Leica Photogrammetry Suite (LPS) image matching module (Automatic Tie Generation). Afterward, close-up images are processed into ortho-rectified images with 0.1mm pixel size for crack size measurement. Crack sizes are finally determined by a manual measurement approach and an inflection point approach for comparison. An experiment was conducted on Ai He concrete bridge located in Yangmei District, Taoyuan City, Taiwan. By comparing to in-situ measurements from three surveyors, the proposed solution that manually selects tie points and measure crack sizes produces the best result with 0.18 mm root mean square difference (RMSD), 0.15 mm mean difference (MD), and 25.41% mean relative difference (MRD). Furthermore, manual crack size measurement with automatic tie point generation with SIFT and LPS achieve 0.46 mm RMSD, 0.4 mm MD, and 72.42% MRD. | en_US |