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

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32 3. Calibration of Terrestrial Laser Scanner3.2.1 Static ModeThe manufacturer defines for the distance measurement system two remaining errors after range calibra¬tion3: the linearity error and the range noise. The linearityerror means that all deviations between thenominal value, i.e. nominal distance, and the measured value, i.e. measured distance, lie within a limit.Because of the presence of random noise and blunders in distance measurements, the measured value isreplaced by the mean value of several repeated measurements, e.g.10,000 measurements. The result is anon-linear curve called a linearity error curve, where its magnitude should be independentThe range noise describes the measurement noise of repeated measurements againstof the distance.the mean value andcorresponds to the empirical standard deviation, i.e. precision. In contrast to the linearity error the rangenoise is dependent of both the object range and the object reflectivity. Thus, characteristic errors of a dis¬tance measurement system, the additive constant and the scale factor, should be eliminated by calibration.The calibration procedure of the distance measurement system in the 'static mode' includes several targetswhich differ in reflectivity. The selection of different targets are limited to three reflectivity values, cf. Figure3.8:• reflectivity 90% (white)• reflectivity 60% (grey)• reflectivity 20% (dark grey)Figure 3.8: Targets with varying reflectivity values used for investigating the distance measurement system. Lefttarget: reflectivity 90% (white), middle target: reflectivity 60% (grey), right target: reflectivity 20% (dark grey).The targets are painted with a white colour defining a reflectivity of « 95 %. Subsequently, the targets arecovered by transparent papers with defined reflectivity.represent a system-related reflectivity,cf. Section 3.5.1.Thus, the given values are approximations andThe targets are mounted on the telescope of a theodolite (Kern DKM1). The telescope can be aligned pre¬cisely in the vertical direction as well as in the horizontal direction. Furthermore, the telescopeoriented in user-defined directions for aligning the targets in different ways with respectrepresenting different angles of incidence, cf. Section 3.5.2. Thecan beto the laser beam,theodolite Kern DKM1 used for the align¬ment of the targets with respect to the laser beam with a target mounted on the telescopeFigure 3.9.The system consisting of the theodolite and the target is calibrated with respectand the uprightness between the top of the target planecan be seen into an additive constantand the center of the theodolite. The system isrecognized as being constructed accurately without a significant additive constant. Furthermore, the planesused astargets were proven as being flat to a maximum deviation of 0 5 mm.3Range calibration is defined by applying an error correction function or a look up table derived bydistances and measured distances, l e mean values, as defmed by several calibration procedures [Mettenleiter, 2004]residuals between nominal
3.2 Distance Measurement System 33Figure 3.9: Kern theodolite DKM1 used as a target holder for investigating the distance measurement system of thelaser scanner. The telescope is turned upright and the target planescan be attached.The calibration of the distance measurement system in the 'static mode' was performedon the calibrationtrack line. The laser scanner was set up on the observation pillar located in extension with the track line(pillar number 2000, cf. Figure 3.4). The different planar targets were mounted on the test trolley by usingthe theodolite DKM1, cf. Figure 3.9, including centering and levelling. The targets were alignedin suchway that the laser beam was horizontal and hit the target plane at a normal angle. Therefore, the theodolitecan be rotated and the target can be shifted in height. Based on this alignment, the targetswere orientedwith respect to the laser beam using the theodolite. This is of importance with respect to the influence ofthe angle of incidence on the distance measurement system, cf. Section 3.5.2. Thecalibration procedure isshown schematically in Figure 3.10.mode' were performed with the following settings4:All investigations of the distance measurement systemin the 'staticrange mode : farfilter frequency : 125,000 points/secpoints per line : 10,000The chosen range mode and filter frequency correspond to the common settings applied duringthe 'scan¬ning mode'5. The number of points per line influences the mean value and the empirical standard deviation,i.e. precision, since these parameters are based on the number of acquired measurements.RepeatabilityThe repeatability of the distance measurement system allows for the evaluation of systematiceffects con¬cerning the additive constant and the scale factor. The determination of such parameters are only meaning¬ful if they are repeatable. Therefore, the same measurement procedure is repeatedwithin a short time andis based on the identical measurement setup. The target that was used is white in colour and is defined bya reflectivity value of 90%. The laser beam hits the target orthogonally.4It is of importance to refer to the settings made for the distance measurement. This is especially the case for the rangemode andthe filter frequency as they mayinfluence the distances to be measured.5The filter frequency depends on the selected scanning modes. The frequency goes from 125,000 points/sec for the scanning modes'preview' and 'middle' to over 250,000 points/sec in the mode 'high' and up to 500,000 points/sec for the scanning mode 'superhigh'
Page 1: DISS. ETH NO. 17036Calibration of a
Page 5 and 6: ZusammenfassungSeit einigen Jahren
Page 7 and 8: ContentsAbstractiiiZusammenfassungv
Page 9 and 10: Contentsix6.1.3 Results Ill6.2 Stat
Page 11 and 12: 1Introduction1.1 Terrestrial Laser
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Page 18 and 19: 8 2. Components of Terrestrial Lase
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112 6. Applications of Terrestrial
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132 7. Summaryrange, achievable by
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134 7. Summary
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136 A. Imaging System of Imager 500
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138 B. Technical Data of Imager 500
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142 C. Adjustment of Sphere
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144 D. Electronic Circuit for Deter
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152 BibliographyWunderlich, T. A. [
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List of Figures3.22 Residuals of th
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200220072002Curriculum VitaePersona
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