Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...

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90 5. Kinematic Laser ScanningThe accuracy of the orientation depends on the accuracy of the determination of the azimuthal direction tothe sphere and the precision of the horizontal encoder system.If the scanning profilesare not normal tothe moving direction, the misalignment can be corrected mathematically by using control pointsprofiles are shifted by a constant angle.This lever arm effect was discussed earlier.since theazimuthal direction(scanning profiles )A1S |azimuthal direction / ^^^HI(sphere centre) /oicalibration track linemoving directioninital director(azimuth),test trolley(laser scanner)Figure 5.2: Azimuthal orientation of the laser scanner. The azimuthal direction to the sphereazimuthal direction for the profiles can be determined by adding a specific angle (here: 90 °).can be derived and the5.1.2 Absolute Position and OrientationThe absolute position and orientation of an object is defined by three coordinates and three attitude angles.To measure the six degrees of freedom, different sensors and instruments can be used, e.g.acceleratorsensors, inclination sensors, tracking total station, global positioning system.Inertial navigation systems(INS) directly provide the desired six parameters.The sensors and instruments that should be used and areappropriate for the specific application depend on the desired accuracy and the environment. If the objectis in motion, these six parameters are time-dependent and define the 3D-trajectory.The kinematic application performed on the calibration track line allows for some simplifications.Thetest trolley moving along the track line is forced to run on a fixed track. The trolleydirections. The trajectory of the calibration track line is well-known and alreadyThis trajectory can be described by variations in the straightness of the alignment, bycannot run in otherdiscussed in Section 3.1.1.variations in thevertical alignment and by variations in the cant. The verification of the deviations of the different linearitiesshows only small variations. The measurement of the three attitude angles (to, ,trajectory is well-defined. For simplifications, it is also possible to approximate the trajectoryk) is not necessary since theas a constantline, which runs horizontally. The error budget of this approximation is in the same order of magnitude asthe accuracy of the point cloud acquired bythe laser scanner.The absolute position of the test trolley moving along the track line is measured bytion. The laser scanner is equipped with a prism holder to attach a prism, which is tracked byan automated total sta¬the total sta¬tion. Since in kinematic applications the synchronization between the distance and the angle measurementsystem of tracking total stations is of importance [Stempfhuber, 2004], asetup for the kinematic applicationis chosen so that the error resulting from a conceivable erroneous synchronization is minimized. The totalstation is positioned in extension from the track line (pillar number 2000, cf. Figure 3.4). The test trolleymovesaway or towards the total station, resulting mostly in varying distances rather than varying angledirections. The setup can be seen in Figure 5.1.
5.1 Test Trolley on Calibration Track Line 91Since the prism is not installed in the origin of the local scanner system, the translation vector between theprism and the origin of the local scanner system has to be verified. The translation vector consists of threecomponents in x-, y- and z-direction. The installation of the prism was chosen so that the prismaxis that is parallel to the moving direction and normal to the scanning profiles, cf. Figure 5.3.is on anmoving direction 1prismon top of laser scannerscanning profileF¥prism on top of laser scannerFigure 5.3: Installation of a prism on top of the laser scanner. The offsetsprojection (left) and in the vertical projection (right).to be calibrated can be seen in the horizontalThe translation vector can be described by• a translation normal to the scanning profiles: Ax,• a translation normal to the moving direction: Ay, and• a translation vertical to the moving direction (Az).The component Ay should be minimized, in the ideal case becomes Ay=0.The determination of theposition of the prism holder regarding the offset Ay is carried out as follows. The laser scanner is mountedon an observation pillar and a sphere is mounted on another observation pillar.The azimuthal directionto the center of the sphere is derived by scanning the sphere and calculating the center point.Based onthe coordinates of the center point, the azimuthal direction is given and the laser scanner can be positionedin such away that the scanning profile is perpendicular to the azimuthal direction of the center point ofthe sphere by rotatingthe laser scanner ±90°. Theprecision in orienting the laser scanner to a specificazimuthal direction is verified beforehand by using a total station and surveying a target attached on thelaser scanner. The precision shows a value of less than 0.0015°. The scanned sphere on the observationpillar is then replaced by a total station. The prism on top of the laser scanner is surveyeddirections are read. In the ideal case, the horizontal directions to the prism have to be equaland the horizontalif the laserscanner is rotated +90° and —90°, regarding the computed azimuthal direction to the observation pillar.In reality,there is a deviation of 0.015° between the two horizontal directions. Consideringa distance ofapproximately 5.24 m between laser scanner and total station, the deviation can be expressed by a metricvalue of 1.4 mm. Half of this value defines the offset Ay between the prismcenter and the normal line tothe scanning profile that intersects the origin of the local scanner system. Considering the accuracylaser scanner, the offset of 0.7mm is negligible.of theThe component Ax is derived by total station surveying and using the test field of observation pillars, cf.Section 3.1.3.The laser scanner is attached on a pillar and from the other pillars, a test field consisting ofthree prisms and the prism mounted on top of the laser scanner is surveyed. The local nets of the surveyingcan be transformed to the coordinate system of the observation pillars by using control points. As a result,the prism of the laser scanner has coordinates with respect to the reference system. Since the laser scanner
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DISS. ETH NO. 17036Calibration of a
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ZusammenfassungSeit einigen Jahren
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ContentsAbstractiiiZusammenfassungv
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Contentsix6.1.3 Results Ill6.2 Stat
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1Introduction1.1 Terrestrial Laser
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1.2 Motivation 3Precision [m10~1 "1
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6 1. Introduction
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8 2. Components of Terrestrial Lase
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10 2. Components of Terrestrial Las
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24 3. Calibration of Terrestrial La
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'28 3. Calibration of Terrestrial L
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"[m] of system 90%) 3540 distance b
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!36 3. Calibration of Terrestrial L
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138 3. Calibration of Terrestrial L
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Page 84 and 85: 74 4. Static Laser Scanning4.1.2 Mi
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Page 146 and 147: 136 A. Imaging System of Imager 500
Page 148 and 149: 138 B. Technical Data of Imager 500
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u140 C. Adjustment of SphereA =dfi,
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142 C. Adjustment of Sphere
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144 D. Electronic Circuit for Deter
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aMulti-SensorPlatform,PhDGeodesy Sw
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Accuracy-AeinTPS1200 AG, -Version 1
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-GenauigkeitsbetrachtungenInvestiga
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152 BibliographyWunderlich, T. A. [
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List of Figures3.22 Residuals of th
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156 List of Figures
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158 List of Tables
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200220072002Curriculum VitaePersona
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