New Tools for Terrestrial Laser Scanning Applied for Monitoring Rails and Buildings
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SUMMARY A main advantage of terrestrial laser scanning is the possibility to nearly simultaneously acquire the geometrical positions of millions of object points in 3D. However this is also the main bottle neck of terrestrial laser scanning when it comes to data processing. In this paper a new strategy will be proposed, presented and applied in order to show how to overcome the discrepancies. The modelling and processing will be done based on surface parameters which will be efficiently derived from point cloud measurements. It will be shown, that there is no need for a triangularisation of the points prior to a derivation of significant surface parameters. In such a way enormous data reduction will be achieved. It becomes feasible to combine adjacent scans completely based on identical surface parameters derived from natural objects detected automatically in both scans. Natural objects may be used for geo-referencing as well. Characteristic lines can be derived from the intersection of surfaces. Due to the high point density of the scan, an increased improvement of the accuracy of the surface parameters will be achieved even for small extensions of the detected surfaces. Then even deformation analysis of walls of buildings can be based on surface parameters. From the surface parameters and a defined boundary line and boundary plane also the volume under the surface can be easily determined. Examples from rail monitoring, building monitoring and the automatic determination of the volume of a pile of coal are presented in order to demonstrate the efficiency of the concept.Keywords:
Laser Scanning
Data Processing
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Deformation monitoring
Laser Scanning
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Abstract When using laser scanning for deformation analysis of a given object, there are no pre-signalled points or identical points to compare between two epochs, so we can judge for whether an object deforms only by modelling the surface of the object. There are a number of challenges in this regard presented in the paper. The first challenge is that for the deformation analysis the configuration of the measurement parameters is no longer set by the surveyor-engineer by the number and position of the observed points, but from the laser scanner. Only the location of the scanner and the scanning density could be controlled. The second challenge is to expand the error model, as the surface modelling is included in the deformation process. This unifies both the metrological and the model errors that arise from the insufficient knowledge of the object and the simplification of its surface. All these challenges are oriented to the created 3D surface model. In addition, the metrological aspects of the use of laser scanners should be considered, especially for applications where the determination of the deformation requires high accuracy. A number of conclusions and recommendations are formulated. The influence of the factors influencing the accuracy of ground laser scanning in the deformation study is summarised, and the influence of the external orientation ang georefering of the point cloud on the accuracy of the digital model is established. Preliminary assessment of the accuracy of the TLS results are given. Justification of the reliability of the determined deformations through TLS is presented. Technological scheme and recommendations for determination of deformations with TLS is shown.
Laser Scanning
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Terrestrial laser scanners based on the time-of-flight measurement principle or the phase comparison method are used for cultural heritage documentation. Only if the scanned surfaces consist of extended, only slightly curved structures, the noise of the point cloud becomes nearly irrelevant as soon as the point cloud is replaced by a parametric or analytic description of the surface. In this paper, the usage of a phase based scanner for capturing small-structured free form surfaces shall be investigated, whereas two procedures are used to reduce the noise of the point cloud: repeated scanning and calculation of average, then smoothing the point cloud with an edge-preserving filter, based on anisotropic diffusion. The physical background of the anisotropic diffusion filter is given, the work-flow is described and the gained results are discussed. It will be seen, that via repeated scanning and usage of an edge-preserving filter good results also in close-range can be achieved with a terrestrial laser scanner based on the phase comparison method.
Laser Scanning
Smoothing
Edge-preserving smoothing
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Collecting data on different building structures using Terrestrial Laser Scanning (TLS) has become in recent years a very popular due to minimize the time required to complete the task as compared to traditional methods. Technical parameters of 3D scanning devices (digitizers) are increasingly being improved, and the accuracy of the data collected allows you to play not only the geometry of an existing object in a digital image, but also enables the assessment of his condition. This is possible thanks to the digitalization of existing objects e.g., a 3D laser scanner, with which is obtained a digital data base is presented in the form of a cloud of points and by using reverse engineering. Measurements using laser scanners depends to a large extent, on the quality of the returning beam reflected from the target surface, towards the receiver. High impact on the strength and quality of the beam returning to the geometric features of the object. These properties may contribute to the emergence of some, sometimes even serious errors during scanning of various shapes. The study defined the effect of the laser beam distortion during the measurement objects with the same material but with different geometrical features on their three-dimensional imaging obtained from measurements made using TLS. We present the problem of data quality, dependent on the deflection of the beam intensity and shape of the object selected examples. The knowledge of these problems allows to obtain valuable data necessary for the implementation of digitization and the visualization of virtually any building structure made of any materials. The studies has been proven that the increase in the density of scanning does not affect the values of mean square error. The increase in the angle of incidence of the beam onto a flat surface, however, causes a decrease in the intensity of scattered radiation that reaches the receiver. The article presents an analysis of the laser beam reflected from broken at different angles surface. Scan quality was assessed using check the density of the number of points on the test object's surface.
Digitization
Laser Scanning
Structured Light
Reverse engineering
Distortion (music)
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Over the past few decades, Terrestrial Laser Scanners are increasingly being used in a broad spectrum of applications, from surveying to civil engineering, medical modeling and forensics. Especially surveying applications require on one hand a quickly obtainable, high resolution point cloud but also need observations with a well described quality, from which it is possible to reliably derive the quality of the end-product. As any measurement, TLS scans are subject to measurement noise. Currently, the manufacturers provide documentation containing only global technical specifications including precision of measurements performed on reference surfaces under laboratory conditions. After brief introduction of the principal of Laser Scanning, in this thesis an overview of the major quality influencing factors is provided, grouped in four main categories: (i.) scanner mechanism, (ii.) atmospheric conditions and environment, (iii.) object properties and (iv.) scanning geometry. In many cases, the user has limited control on the scanner mechanism, the atmospheric conditions or the object properties. The only factor on which the user has control on is the scanning geometry, as the user determines the scan location and thereby the view-point of a point cloud. This dissertation presents the research on the influence of scanning geometry on the point cloud quality. This thesis proposes a theoretical study of the scanning geometry effects on individual point quality, as well as practical assessments. The impact of scanning geometry on individual point quality is analyzed, based on local planar features. The quality investigated in this thesis relates to the random errors or precision of individual points and does not deal with systematic errors or biases. Different planar fitting techniques are presented and compared. The quality of each local fit is described using a Least Squares estimation. The main quality describers used in this work are presented for each method. By using these quality describers, the influence of the scanning geometry on the point quality is characterized both quantitatively and qualitatively. The scanning geometry is defined using two parameters: the incidence angle and the range. The incidence angle is defined as the angle between one laser beam vector and the normal vector to the surface. The range is defined as the distance between the scanner and the surface. It is shown that and how the received signal strength of the measurements decreases with increasing incidence angle and range. The presented approach allows the quantification of the contribution of noise induced by the scanning geometry, based solely on point cloud data. No additional or external measurements are needed. The contribution of the two scanning geometry parameters on the point quality has been quantified using contribution coefficients. The effect of scanning geometry on the point quality is quantified and tested on a reference test board and two point clouds sampling a standard room. It is shown that the theoretical models developed are consistent with this experimental assessment. It is shown that it is possible to reduce the total error of the measurements by placing the scanner at another position in the room, which is not necessarily an obvious position. Inspired by these results, a new method that determines near optimal view-points in a scene based on terrestrial laser scanner capabilities is presented. Using a simple approach, an improvement of the measurement set-up can be easily achieved using a small amount of computation, memory and time.
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Abstract The measurement of the particle size distribution plays an important role in mineral processing. Due to the high costs and time-consumption of the screening process, modern machine vision methods based on the acquisition and analysis of recorded photographic images. But the image analysis methods used so far, do not provide information on the three-dimensional shape of the grain. In the coal industry, the application scope of these methods is substantially limited by the low reflectivity of the black coal particle surface. These circumstances hinder proper segmentation of coal stream surface image. The limited information contained in two-dimensional image of the raw mineral stream surface, makes it difficult to identify proper size of grains partially overlapped by other particles and skewed particles. Particle height estimation based on the shadow length measurement becomes very difficult in industrial environment because of the fast movement of the conveyor belt and because of spatial arrangement of these particles, usually touching and overlapping. Method of laser triangulation connected with the movement of the conveyor belt makes it possible to create three-dimensional depth maps. Application of passive triangulation methods (e.g. stereovision) can be impeded because of the low contrast of the black coal on the black conveyor belt. This forces the use of active triangulation methods, directly identifying position of the analyzed image pixel. High contrast of the image can be obtained by a direct pointwise laser lighting. For the simultaneous identification of the entire section of the raw material stream it is useful to apply a linear laser (a planar sheet of the laser light). There have been presented basic formulas for conversion of pixel position on the camera CCD matrix to the real-word coordinates. A laboratory stand has been described. This stand includes a linear laser, two high-definition (2Mpix) cameras and stepper motor driver. The triangulation head moves on the rails along the belt conveyor section. There have been compared acquired depth maps and photographic images. Depth maps much better describe spatial arrangement of coal particles, and have a much lower noise level resulting from the specular light reflections from the shiny fragments of the particle surface. This makes possible an identification of the coal particles partially overlapped by other particles and obliquely arranged particles. It enables a partial elimination or compensation of image disturbances affecting the final result of the estimated particle size distribution. Because of the possibility of the reflected laser beam overriding by other particles it is advantageous to use a system of two cameras. Results of the experimental research confirmed the usefulness of the described method in spite of low reflectance factor of coal surface. The fast detection of changes in particle size distribution makes possible an on-line optimization of complex technological systems - especially those involving coal cleaning in jigs - thus leading to better stabilization of quality parameters of the enrichment output products. An additional application of the described method can be achieved by measuring the total volume of the stream of the transported materials. Together with the measurement signal from the belt conveyor weight it makes possible to estimate the bulk density of the raw mineral stream. The low complexity of the signal processing in the laser triangulation method is associated with the acquisition of high contrast images and analysis based on simple trigonometric dependencies.
Conveyor belt
Tracking (education)
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The observation and detection of movements of man-made structures is a noble task in engineering survey, for it is geared towards preservation of life and property. Many different methods exist for detection and monitoring deformations. These methods have served humanity very well over the years. However, most of these methods are point based. Terrestrial laser scanning allows for the monitoring of the whole surface of a structure. All these methods require data to be compared between two or more campaigns. For the data set to be comparable, it needs to be transformed not only into the same coordinate system, but also into the same computational base. An analysis of the stability of reference points by global congruency testing, coupled with the S-transformation enable this to be achieved. In this thesis, the Total station and terrestrial laser scanner were used to detect deformation of a building. The global congruency test was used to detect deformations between two epochs, followed by a single-point analysis which is used in the localization of deformations. After determination of stable scan stations, point cloud data from two epochs was transformed into the same computational base. This enabled point to surface and surface to surface deformations analysis to be undertaken
Deformation monitoring
Laser Scanning
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This paper describes the interim results of a study to characterize discontinuous rock masses using 3D laser scanning data. One of the main advantages of this method is that now an unbiased, rapid and accurate discontinuity analysis can be done. With 3D laser scanning it is now also possible to measure rock faces whose access is restricted or rock slopes along highways or railway lines where working conditions are hazardous. It is also shown that the proposed method will also be cheaper than traditional manual survey and analysis methods. Laser scanning is a relative new surveying technique, which yields a so-called ‘point cloud’ set of data, where every single point represents a point in 3D space of the scanned rock surface. Since the density of the point cloud can be high (in the order of 5 mm to 1 cm), it allows for an accurate re-construction of the original rock surface in the form of a 3D interpolated and meshed surface, using different interpolation techniques. Through geometric analysis of this 3D mesh and plotting of the facet orientations in a polar plot, it is possible to observe clusters, which represent different rock mass discontinuity sets. With fuzzy k-means clustering algorithms individual discontinuity sets can be outlined automatically and the mean orientations of these identified sets can be computed. Assuming a Fisher’s distribution it is subsequently demonstrated that the facet outliers can be removed. Finally, it is shown that discontinuity set spacings can be calculated as well.
Discontinuity (linguistics)
Laser Scanning
Interpolation
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Laser Scanning
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Building constructions are exposed to various forces and natural phenomena. Some of them are sudden and violent, e.g., an earthquake or heavy rains, causing a displacement of the ground. Other phenomena affect objects on a longer-term, e.g., vibrations caused by daily road traffic. Sometimes, building structures may have defects due to incorrect construction. In any case, if an engineering object shows changes in the relation to its correct geometry or position, deformation and displacement measurements are required. Engineering objects are also monitored during their construction. Nowadays, it is important to perform measurements quickly and with high accuracy. The use of a Terrestrial Laser Scanning (TLS) allows for the required measurement speed and accuracy. This measurement technology allows a large dataset, which can be arbitrarily elaborated, to be obtained. The structure of building objects can include vertices, lines, planes, and other shapes and can be described using mathematical functions. This allows data processing to be automated. In this article, we present the Msplit method as an effective approach to the processing of data obtained as a result of TLS measurements. The proposed approach is new because until now, the Msplit estimation method has not been used to detect adjacent planes in one-point cloud obtained from TLS. The Msplit estimation method allows a functional model to be split into two or more competitive models and thus two or more entities in a point cloud to be estimated simultaneously. Four different objects measured using TLS are presented: two objects representing vertical displacements and two objects representing horizontal displacements. The test results and analysis confirm that the Msplit estimation method can be successfully applied in the detection of adjacent planes.
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Dimensioning
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