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

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