Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...
Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ... Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...
72 3. Calibration of Terrestrial Laser Scannerthan the calculated center points based on the 'free' adjustment and is within 2 mm. Referringto the inves¬tigation of the distance measurement system in the 'scanning mode', an additive constant of 3 mm to 4 mmin the 'free' adjustment was identified. By applying the coordinate transformation, this systematicoffset istaken into account and the center points are shifted by this additive constant. Thus, the 3D accuracy of thecenter points is better than the distance accuracy, cf. Figure 3.16 and Figure 3.46.The 3D accuracy of the center points can be also derived by interpreting the distance accuracy along thecalibration track line asaccuracy in x-direction, the accuracy of the horizontal encoder transversal to thecalibration track line asaccuracy in y-direction and the accuracy of the vertical encoder as accuracyin thevertical direction of the calibration track line. The results are identical to the coordinate transformationexcept that the additive constant has to be subtracted manually with respect to the 'free' adjustment.The use of the other 'scanning modes' does not change the achievable 3D accuracy.Only the range fromwhich the accuracy is getting worse decreases, cf. the corresponding results gained by the investigation ofthe distance measurement system in the 'scanning mode', Section 3.2.2.
4Static Laser ScanningThis chapter discusses the post-processing of 3D point clouds as applied to static laser scanning. Staticlaser scanning implies that the objectsto be scanned and the laser scanner do not move relative to eachother during the data acquisition process. Post-processing of 3D point clouds requires software packagesto derive the final products, e.g. 3D models. Since the implemented algorithms in these software packagesare not fully automated, the operator has to assist the software. Thus, the process of deriving the finalproduct based on the 3D point cloud is time-consuming. The time ratio between post-processingscanning point clouds and data acquisition can be upaspects of data processing, registration, and modelingto 10 : 1or more. The followingand visualization.of lasersections cover some4.1 Data ProcessingLaser scanning is one of the fastest data acquisition techniques in geodesy and generates a hugenumber ofpoints, which describe the environment, in a short time. However, data acquisition produces blunders andnoise within the point clouds. Noise can be distinguished in white noise and coloured noise. Here, noise isassumed to be white noise and Gaussian noise. The following sections givean overview of the conceivableerrors and error sources. They result in noisy data and have to be processed by applying special filteringand noise reduction techniques.4.1.1 Blunder DetectionThe laser scannersystem acquires data within a distance and an intensity interval. Sometimes, only datawithin a specific range regarding distance and intensity values are required. The pointswhich fulfil theserequirements can easily be separated from the points that do not fulfil the requirements by introducingminimum and maximum limits for both range and intensity data, a threshold:rangemm
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72 3. <strong>Calibration</strong> <strong>of</strong> <strong>Terrestrial</strong> <strong>Laser</strong> <strong>Scanner</strong>than the calculated center points based on the 'free' adjustment <strong>and</strong> is within 2 mm. Referringto the inves¬tigation <strong>of</strong> the distance measurement system in the 'scanning mode', an additive constant <strong>of</strong> 3 mm to 4 mmin the 'free' adjustment was identified. By applying the coordinate transformation, this systematic<strong>of</strong>fset istaken into account <strong>and</strong> the center points are shifted by this additive constant. Thus, the 3D accuracy <strong>of</strong> thecenter points is better than the distance accuracy, cf. Figure 3.16 <strong>and</strong> Figure 3.46.The 3D accuracy <strong>of</strong> the center points can be also derived by interpreting the distance accuracy along thecalibration track line asaccuracy in x-direction, the accuracy <strong>of</strong> the horizontal encoder transversal to thecalibration track line asaccuracy in y-direction <strong>and</strong> the accuracy <strong>of</strong> the vertical encoder as accuracyin thevertical direction <strong>of</strong> the calibration track line. The results are identical to the coordinate transformationexcept that the additive constant has to be subtracted manually with respect to the 'free' adjustment.The use <strong>of</strong> the other 'scanning modes' does not change the achievable 3D accuracy.Only the range fromwhich the accuracy is getting worse decreases, cf. the corresponding results gained by the investigation <strong>of</strong>the distance measurement system in the 'scanning mode', Section 3.2.2.
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