Crack assessment of bridge piers using unmanned aerial vehicles (UAVs) eliminates unsafe factors of manual inspection and provides a potential way for the maintenance of transportation infrastructures. However, the implementation of UAV‐based crack assessment for real bridge piers is hindered by several key issues, including the following: (a) both perspective distortion and the geometry distortion by nonflat structural surfaces usually appear on crack images taken by the UAV system from the pier surface; however, these two kinds of distortions are difficult to correct at the same time; and (b) the crack image taken by a close‐range inspection flight UAV system is partially imaged, containing only a small part of the entire surface of the pier, and thereby hinders crack localization. In this paper, a new image‐based crack assessment methodology for bridge piers using UAV and three‐dimensional (3D) scene reconstruction is proposed. First, the data acquisition of UAV‐based crack assessment is discussed, and the UAV flight path and photography strategy for bridge pier assessment are proposed. Second, image‐based crack detection and 3D reconstruction are conducted to obtain crack width feature pair sequences and 3D surface models, respectively. Third, a new method of projecting cracks onto a meshed 3D surface triangular model is proposed, which can correct both the perspective distortion and geometry distortion by nonflat structural surfaces, and realize the crack localization. Field test investigations of crack assessment of a real bridge pier using a UAV are carried out for illustration, validation, and error analysis of the proposed methodology.

中文翻译：

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基于图像的无人驾驶飞机桥墩裂缝评估和三维场景重建

使用无人飞行器（UAV）对桥墩进行裂缝评估可以消除手动检查的不安全因素，并为维护运输基础设施提供了一种潜在的途径。但是，实际关键桥墩的基于UAV的裂缝评估的实施受到以下几个关键问题的阻碍：（a）非平坦结构表面的透视变形和几何变形通常都出现在UAV系统从以下位置获取的裂缝图像中：码头表面；但是，这两种失真很难同时校正。（b）通过近距离检查飞行无人机系统拍摄的裂缝图像被部分成像，仅包含码头整个表面的一小部分，从而阻碍了裂缝的定位。在本文中，提出了一种使用无人机和三维（3D）场景重建的基于图像的桥墩裂缝评估新方法。首先，讨论了基于无人机的裂纹评估的数据获取，并提出了用于桥墩评估的无人机飞行路径和摄影策略。其次，进行基于图像的裂缝检测和3D重建，分别获得裂缝宽度特征对序列和3D表面模型。第三，提出了一种将裂纹投影到网格3D表面三角形模型上的新方法，该方法可以校正非平坦结构表面的透视变形和几何变形，并实现裂纹定位。为了说明，验证和验证，我们使用无人机对实测桥墩的裂缝进行了现场测试研究，