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

3.6 Precision and Accuracy of Terrestrial Laser Scanner Data 6910-1020^1"-— ^-~-""**-—:z^-^:=,>---^ ^"^X" 7^^^" C" "" Ji^ts—+-T~*'—'—•Js*ï'r^^rr^r^ Z^*»~«i=.»-*S-30re-40-5010 15range [m]—•—styrofoam (2cm)-»--styrofoam (5cm)wood—white paperI20 25 30Figure 3.43: Distance accuracy of some materials (styrofoam, wood, white paper).In Figure 3.44, another systematic effect based on the reflected intensityof the laser beam can be seen. Sincein most laser scanners the signal-to-noise ratio is not regulated and not optimised the receiving avalanchediode is over-steered and generates systematic effects, which result in distance errors. This effect becomesclear considering the distance accuracy of metal. The upper part of Figure 3.44 shows systematic deviationswithin the first 25 m of several meters. Only beyond 35 m does the intensity of the reflected laser beam allowthe avalanche diode to work properly and the distances become reasonable. However, a systematic offsetof nearly 5 mm is left. .JEE_Üre1010\ J—T\^~ -^I0 5 10 15 20 25 30 35 40 45 50—•—metalrange [m]J\ fV« -200 5 10 15 20 25 30 35 40 45 50range [m]Figure 3.44: Distance accuracy of metal. Only beyond 35 m does the distance accuracybecomes reasonable.The discussed selection of only three different materials (styrofoam, wood, metal) demonstrates that theinfluence of the surface properties of different materials is important. For achieving a high accuracy, thepropertiesof the material to be scanned have to be verified. Especially,regardingthe influence of thesurface reflectance, there are several investigations and publications for the specific laser scanner system,e.g.[Lichti and Harvey, 2002b], [Clark and Robson, 2004], and [Reshetyuk, 2006a]. Each laser scanner andits wavelengthgeneralise the findings.of the laser beam has a different behavior for different materials and makes itand other high-reflective surfaces.difficult toThere are some materials that may cause systematic effects, such as glass, metal,3.6 Precision and Accuracy of Terrestrial Laser Scanner DataThe investigation and calibration of each single component of a terrestrial laser scanner are necessary andimportant. The interesting question, which also has to be answered, is how precise and how accurate are theresults of one acquired single point and the results of the modeled objects. In the following, these questionsshall be answered.