Browsing by Author "Jahraus, Adam"

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    Explanation for the seam line discontinuity in terrestrial laser scanner point clouds
    (Elsevier, 2019-08) Lichti, Derek D.; Glennie, Craig L.; Al-Durgham, Kaleel; Jahraus, Adam; Steward, Jeremy
    The so‐called seam line discontinuity is a phenomenon that can be observed in point clouds captured with some panoramic terrestrial laser scanners. It is an angular discontinuity that is most apparent where the lower limit of the instrument’s angular field‐of‐view intersects the ground. It appears as step discontinuities at the start (0° horizontal direction) and end (180°) of scanning. To the authors’ best knowledge, its cause and its impact, if any, on point cloud accuracy have not yet been reported. This paper presents the results of a rigorous investigation into several hypothesized causes of this phenomenon: differences between the lower and upper elevation angle scanning limits; the presence of a vertical circle index error; and changes in levelling during scanning. New models for the angular observations have been developed and simulations were performed to independently study the impact of each hypothesized cause and to guide the analyses of real datasets. In order to scrutinize each of the hypothesized causes, experiments were conducted with seven real datasets captured with six different instruments: one hybrid‐architecture scanner and five panoramic scanners, one of which was also operated as a hybrid instrument. This study concludes that the difference between the elevation angle scanning limits is the source of the seam line discontinuity phenomenon. Accuracy assessment experiments over real data captured in an indoor test facility demonstrate that the seam line discontinuity has no metric impact on the point clouds.
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    New approach for low-cost TLS target measurement
    (ASCE, 2019-08) Lichti, Derek D.; Glennie, Craig L.; Jahraus, Adam; Hartzell, Preston J.
    The registration and calibration of data captured with terrestrial laser scanner instruments can be effectively achieved using signalized targets comprising components of both high and low reflectivity, so-called contrast targets. For projects requiring tens or even hundreds of such targets, the cost of manufacturer-constructed targets can be prohibitive. Moreover, the details of proprietary target center co-ordinate measurement algorithms are often not available to users. This paper reports on the design of a low-cost contrast target using readily-available materials and an accompanying center measurement algorithm. Their compatibility with real terrestrial laser scanner data was extensively tested on six different instruments: two Faro Focus 3D scanners; a Leica HDS6100; a Leica P40; a Riegl VZ-400; and a Zoller+Fröhlich Imager 5010. Repeatability was examined as a function of range, incidence angle, sampling resolution, target intensity and target contrast. Performance in system self-calibration and from independent accuracy assessment is also reported. The results demonstrate compatibility for all five scanners. However, all datasets except the Faro Focus 3D require exclusion of observations made at high incidence angles in order to prevent range biases. Results also demonstrate that the spectral reflectivity of the target components is critical to ensure high contrast between target components and, therefore, high-quality target center co-ordinate measurements.
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    The Use of Three-Dimensional Documentation Technologies in Archaeological Applications
    (2017) Jahraus, Adam; Lichti, Derek; Dawson, Peter; Levy, Richard; Shahbazi, Mozhdeh
    In archaeology, it is useful to document the shape of features of interest. There are many three-dimensional measurement technologies available that can help accomplish this task. An error model for a handheld 3D scanner called the DPI-7 was created. This error model reduced the errors in the in-plane directions by up to 59%. The levels of precision in two technologies, terrestrial laser scanning and computer vision assisted photogrammetry, were determined through the simulation of observations in a virtual environment. It was found that terrestrial laser scanning point observations had a standard deviation (in the direction of least precision) of 6mm, while photogrammetry could achieve a value of 10mm. The point cloud data from the scans of an excavation in the Canadian arctic were used to create a detailed and coloured visual model of the site, and was subsequently used in a virtual reality visualization of the site in question.