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

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80 4. Static Laser Scanningand generally fulfill the constraints of orthogonalityFurthermore, a scale factor can be introduced, whichincreases the number of unknowns up to sevenThe minimum number of corresponding points definingthe transformation between the two coordinate frames is threeGenerally, these corresponding points canbe either defined by targets, which have to be modeled using the point cloud, or by using the point clouddirectlyA distinction can be made between aregistration based on artificial or natural targets and a registrationbased on overlapping point cloudsBoth methods are introduced and discussed regarding advantages anddisadvantages Depending on the method, the registration describes onlya transformation between lo¬cal coordinate frames or defines a transformation between several local coordinate frames and one globalreference frameThe transformation into a global reference frame requires control points with known co¬ordinates4.2.1 Target-Based RegistrationRegistration using targets requires considerations regarding two different aspectsthe type of targets andthe spatial distribution of targets Targets can be both artificial and natural In either case, the target in¬formation has to be derived based on the acquired point cloud, egcenter point, radius, normal vectorGeometrical primitives seem to be well-suited for supporting this information, such as planar objects,lplanes, spheres, cylinders Generally, the best results can be obtained by specific artificial targets Mostlaser scanner manufacturers also provide targets, wich define such tie points or control points2 Often,these tie points can be surveyed by total stations or by GPS This is helpfulsince the local coordinate frameedefined by the laser scanner can be included into other local or globalreference frames Anotheraspect isthe surveying of such targets reduces the constraint that neighbouring pointby at least three tie points or objectsclouds have to be connectedArtificial targets have to be designed according to the specifications of the laser scanner The target shouldbe dimensioned such that a sufficient number of points hit the surfaceThe material and the colour haveto have the appropriate wavelength and reflectance propertiesThe use of different reflectivity values mayease the calculation of the geometrical center of the target, e g retro-reflective targets Figure 4 5 showsthree different targets used for laser scanningThe left one is a circular planar retro-reflective target witha white inner circle and a blue outer circle, designed by HDS Leica GeosystemsIn the middle, a planarblack-white coded targetof Zoller+Frohlich can be seen Theright partcontains a wooden and whitepamtedsphere mounted on a tribrach constructed byETH Zurich Allthree targets have dimensions andmassesappropriate for field work Thebalance between size and mass of the targeton one side and theease of transportation on the other side are of importanceInstead of artificial targets, natural targets can also be used Often, planar objectscan be found in theenvironment, eg walls of buildings Sometimes, also cylinders or spheres are available They are goodchoices to be included in the registration process, especially if they are located in regionsin which artificialtargets can be rarely positioned and/or are inaccessibleThe accuracy provided by natural targetsare notas high as that provided by artificial targetsHowever, the use of such targetsis recommended to stabilizethe registrationBased on registration results, targets can be assessed and deactivated when theydo notsatisfy accuracy requirementsThe algorithms for calculating the mathematical point of reference for each target is either implemented inthe software tools provided by the manufacturers or available in numerous software tools for processinglaser scanning point clouds The accuracy achievable for the point of reference is dependenton severalaspects, e ggeometry and orientation of the target, noise within the point cloud, but can be increased upto a few millimeters2 A control point is a tie point that was surveyed and can be defined by global coordinates
4.2 Registration 81Figure 4.5: Types of artificial targets used for laser scanning: HDS Leica Geosystem circular planar target (left), ZFcoded planar target (middle), ETH Zurich sphere target (right).The distribution of targets is often disregarded during laser scanningfield work. The instruments are in¬vestigated and calibrated intensively to improve the accuracy of laser scanners. But in return, the accuracyis decreased through carelessness in the field caused by awkward and poor targetdistributions. The re¬sults based on the distribution worsens the accuracy significantly. In photogrammetry and analogous interrestrial laser scanning, control points have to be chosen in such a way that they• cover the full object volume to avoid extrapolation,• do not hide important parts of the object, and• are of a reasonable number.Investigations showed that for an indoor project with a dimension of 10 x 10 x 3 m3 the influence of poordistributions of control points can reach several centimeters. A reasonable number of targetsof about 4 to6 for two different scan positions is recommended. The use of moretargets does not increase the accuracysignificantly unless there is a probability of unsuitable targets that should not be used for the registrationprocess.The coverage of the full object volume is essential since extrapolation should be avoided and mayresult inincreasing errors. High or linear objects, such as towers and buildings, require an appropriate distributionof the targets in height as well. Unfortunately, in most cases the attachment of artificial targets is impossible.Thus, other registration techniques should be used, e.g.natural targets or point cloud registration, andshould be combined with each other.4.2.2 Point Cloud RegistrationThe point cloud registration offers the possibility to transform one or more data sets to another withoutusing targets. The only constraint is the two data sets show areas of overlapping pointclouds. This meansboth data sets have to contain point clouds that describe the same objects. The advantages of this procedureis that no artificial or natural targets are required and a high redundancy is achieved. The disadvantagesare due to the principle of laser scanning:
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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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26 3. Calibration of Terrestrial La
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'28 3. Calibration of Terrestrial L
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130 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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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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