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

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