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

76 4. Static Laser ScanningOptical crosstalk depends on several parameters, e.g. orientation of the deflecting unit and intensity values.Thus, compensation for internal reflections is nearly impossible.The results of the range/intensityof the laser scanner.crosstalk is consideredin Chapter 3 within the calibration procedureThe precision as well as the accuracy of distances acquired to objects with varyingreflectivity are also discussed. However, crosstalk should also be considered at regions characterized by achange in range and in reflectivity.Another affect, called temporal mixing [Hancock, 1999], can also be categorizedmixing can be noticed at transitions between darker and brighter surfaces.two different reflectivity values leads to a jump in the returned signal amplitudejump can cause large errors in phase estimation, which correlates with the range.as crosstalk. TemporalThe transition between theseof the laser beam. The4.1.4 MultipathMultipath effects are well-known, e.g. in GPS technology. Multipath is defined as the effect that signalsare reflected by more objects than desired. For example, with GPS, the signals do not travel directly fromsatellites to receivers. They are reflected on nearby and high-reflective objects. In laser scanning technology,multipath is characterised as the laser beam being reflected two times or more. The signalis not receiveddirectly after hitting the first object, rather, the signal is reflected by several objects and therefore, is nottravelling the shortest path between the laser scanner and the object.The probability of multipath is prevalent in scanning high reflective materials, scanning at close ranges andscanning corners [Runne et al., 2001]. Errors produced by multipath can be of any magnitude to unambiguity,i.e.the maximum range. Multipath results mostly in isolated pixels since the distance is highly differentto the pixels in the neighbourhood. Multipath returns are mirror-inverted, which can be seen especially byscanning windows. The detection of points affected by multipath becomes difficult if theyare surroundedby other points and if they are not clearly detectable as isolated pixels, e.g. in corners and at nearby objects.In Figure 4.3 the effect of multipath is shown. The laser scanner was set upfrom the ceiling to the floor at the right side and a mirror and window in the upperbeam is reflected twice due to multipath and the distance measurement results in pixelswhich are mirrorinverted.The inside of the room is projectedto the outside of the room.in a room which has a windowleft corner. The laserFigure 4.3: Effect of multipath. The scanned room is clearly visible in the middle of the figure. In the left partthe right part erroneous pixels are caused bymirrors and windows.and in