Investigation and calibration of pulsed time-of-flight terrestrial laser scanners
Abstract: This thesis has two aims. The first one is the investigation and analysis of the errors occurring in the measurements with pulsed time-of-flight (TOF) terrestrial laser scanners (TLS). A good understanding of the error sources and the relationships between them is necessary to secure the data accuracy. We subdivide these errors into four groups: instrumental, object-related, environmental and methodological. Based on our studies and the results obtained by other researchers, we have compiled an error model for TLS, which is used to estimate the single-point coordinate accuracy of a point in the point cloud, transformed to the specified coordinate system.The second aim is to investigate systematic instrumental errors and performance of three pulsed TOF laser scanners – Callidus 1.1, Leica HDS 3000 and Leica HDS 2500 – and to develop calibration procedures that can be applied by the users to determine and correct the systematic errors in these instruments. The investigations have been performed at the indoor 3D calibration field established at KTH and outdoors. The systematic instrumental errors, or calibration parameters, have been estimated in a self-calibration according to the parametric least-squares adjustment in MATLAB®. The initial assumption was that the scanner instrumental errors are similar to those in a total station. The results have shown that the total station error model is applicable for TLS as a first approximation, but additional errors, specific to the scanner design, may appear. For example, we revealed a significant vertical scale error in the scanner Callidus 1.1, caused by the faults of the angular position sensor. The coordinate precision and accuracy of the scanners, estimated during the self-calibration, is at the level of several millimetres for Callidus 1.1 and Leica HDS 3000, and at the submillimetre level for Leica HDS 2500.In other investigations, we revealed a range drift of up to 3 mm during the first few hours of scanning, presumably due to the changes in the temperature inside the scanners. The angular precision depends on the scanner design (“panoramic” or “camera-like”), and the angular accuracy depends on the significant calibration parameters in the scanner. Investigations of the influence of surface reflectance on the range measurements have shown that the indoor illumination and surface wetness have no tangible influence on the results. The type of the material does not affect, in general, the ranging precision for Callidus 1.1, but it affects the ranging precision and accuracy of the scanners Leica HDS 3000 and Leica HDS 2500. The reason may be different wavelength and, possibly, different design of the electronics in the laser rangefinders. Materials with high reflectance and those painted with bright “warning” colours may introduce significant offsets into the measured ranges (5 – 15 cm), when scanned from close ranges at normal incidence with the scanner Leica HDS 3000. “Mixed pixels” at the object edge may introduce a range error of several centimetres, on the average, depending on the type of the material. This phenomenon leads also to the distortions of the object size, which may be reduced by the removal of the “mixed pixels” based on their intensity. The laser beam intensity recorded by the scanner tends to decrease with an increased incidence angle, although not as assumed by the popular Lambertian reflectance model. Investigations of the scanner Leica HDS 2500 outdoors have revealed no significant influence of the “normal” atmospheric conditions on the range measurements at the ranges of up to 50 m.Finally, we have developed and tested two simple procedures for the calibration of the vertical scale (and vertical index) error and zero error in laser scanners. We have also proposed an approach for the evaluation of the coordinate precision and accuracy in TLS based on the experiences from airborne laser scanning (ALS).
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