A UAV-mounted metrology system for accurate crack measurement in bridges

Accurate crack width measurement is essential for reinforced concrete bridge condition assessment, yet most UAV-based inspections remain limited to pixel-level observations without reliable metric scaling. A lightweight UAV-mounted metrology system is developed to enable metric crack width measurement from UAV imagery. The payload integrates a focusable DOE red laser projecting a cross pattern, three TF-Luna time-of-flight distance sensors, and an ESP-WROOM-32 microcontroller with Bluetooth Low Energy data streaming and a desktop calibration tool. A calibrated relationship between standoff distance and projected cross length is used to compute an image-specific pixel-to-millimeter scale, which is then applied to pixel-level crack widths obtained from a vision-based crack segmentation pipeline. The proposed metrology system is field-deployed on the Puente Huamani Bridge in Pisco, Peru, with manual measurements conducted by six inspectors for 39 cracks classified under AASHTO MBEI condition states. Compared with manual inspection, the proposed system exhibits lower measurement variability and reduces inspection time. Cross-platform adaptability is demonstrated through additional field testing on a DJI Matrice 350 RTK at the Low Level Bridge in Edmonton, Canada. Overall, the proposed system provides a practical and scalable solution for repeatable, standards-aligned UAV-based bridge crack assessment.