Modelling Point Clouds for Precise Structural Deformation Measurement

Terrestrial laser scanners can rapidly acquire thousands of 3D points over a structure. The individual scan points are of relatively low precision (±2mm – ±50mm) depending on the instrument type. However, combining the dense 3D point data with judicious modelling strategies can produce a very precise surface model. A surface model has advantages for structural deformation monitoring where deflections are small (<50mm) and the shape change can potentially vary across the entire structure. The notion of the research presented in this paper is to exploit the dense 3D point data (clouds) to assess the sensitivity of laser scanners for measuring structural deformation. Two experiments have been undertaken where, in each experiment, a beam was subjected to controlled loading. The first experiment involved a timber beam (5.0m x 0.2m x 0.1m) mounted on an indoor load-testing frame. The beam was subject to a maximum of 40mm of vertical deflection. The focus of the second experiment was a concrete beam (7.0m x 0.5m x 0.5m) placed on an outdoor load-testing frame where it was loaded until failure. All loading was induced by a hydraulic jack and occurred in increments permitting measurements to be made by laser scanners. A Riegl LMS-Z210 laser scanner was used for both experiments and a Cyra Cyrax 2500 was available for the first. The scanner measurements were validated using close-range photogrammetry (accuracy of 1:40,000 of object size or better). * Corresponding author.