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

2Componentsof Terrestrial Laser ScannerA terrestrial laser scanner defines an instrument for generating 3D coordinates Thecore components fordescribing each acquired point by three coordinates and an intensitydistance and reflectance measurement system and the angle measurement systemcomponents are briefly discussed with respect to understandingvalue as a fourth dimension are thelaser scannersIn the following, theseSince the laser scannersareoperating withm a field of view, the laser beam has to be deflected in two directions, horizontally andvertically, respectively Thus, the deflection system is the second important componentof a laser scanner2.1 Distance and Reflectance Measurement SystemThe corecomponent of a terrestrial laser scanner is the distance measurement systemThe distance mea¬surement system defines not only the depth resolution, 1 e the range, but depending on the technique used,additional propertiescan also be defined• the range• the precision of distance measurements• the performance of repeateddistance measurementsOptical distance measurement methods used in terrestrial laser scanners can be technicallydivided intothree categories [Bosch and Lescure, 1995] mterferometry time-of-flight methods and triangulation, cfFigure 2 1 Since mterferometry and triangulation are not widely used in terrestrial laser scanners1 and aremore related to remote sensing and close-range applications, the focus is on the time-of-flight methodsThe time-of-flight methods are based on measuring the time it takes for a pulse or pattern of energy emittedby the laser scanner to travel and strike a surface and to return Theby [Rueger, 1996]srange to be measured can be identifiedwhere c is the speed of light and At the round-trip time of the laser energy The speed of lightis a naturalconstant and well-defined for a vacuum2 If the travel time is also known, the rangecan be calculated Thedetermination of the time can be achieved either by a direct time measurement, e g direct time-of-flight,or by an indirect time measurement, eg amplitude-modulated continuous wave, frequency-modulated1 Close range scanners based on triangulationare also terrestrial laser scanners, but due to the limited working range of up to onlya few meters, here they are not categorized m the same groupof terrestrial laser scanners2Due to atmospheric conditions and turbulence, the speed of light varies and thus, is not a constantbe estimated by mathematical modelsHowever, the variations can