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

3.2 Distance Measurement System 39Table 3.2:Parameters of the hyperbola showing the development of the number of points belonging to the sphereaccording to the scanning mode 'superhigh'.SphereDiameter [m] Scale Factor a Power b12 cm 60,320 -1.59115 cm 90,590 -1.587Quantity is not the only aspect of importance. The quality also has a strong influence since the qualitydefines the geometry of the objects. The following Figure 3.18 shall demonstrate the importance of both thequantity and the quality of the point cloud.Depending on the handling of the point cloud, the appropriate scanning mode has to be chosen carefully.For documentation purposes the scanning modes 'middle' and possibly 'preview' provide the necessaryand desired information. In addition, if a high degree of accuracy and precision is of importance, then thescanning modes 'high' and 'superhigh' have to be used. The range for scanning objects, e.g. spheres, hasto be limited according to the accuracy. The knowledge of the parameters defining the geometry of objectsimprove the accuracy of distances significantly. Based on the desired resolution of the pointcloud and themeanrange to the object, the most appropriate scanning mode should be selected. However, more pointsand a denser point cloud do not mean a better accuracy. This is especially the case if the point spacing issmaller than the noise of the point cloud. Furthermore, other parameters influence the quality of the pointcloud considerably (cf. Section 3.4 and Section 3.5).Figure 3.18:scanning mode 'superhigh'.Points on a sphere with a diameter of lb cm in a range of 2m (left) and 19 m (right) acquired by the3.2.3 Long-Term StabilityThe distances provided by the laser scanner can vary over a long time since internal heatingeffects leadto changes of the modulation frequency, cf. Section 3.2.4, or other internal and/or external influences, e.g.atmospheric influences, cause variations in the distance.systemcan be assessed.Thus, the stability of the phase measurementFor the investigation of the long-term stability, the laser scanner and targets with different reflectivity valuesattached on the theodolite DKM1 were installed on two observation pillars.The distance between the twopillars was approximately 3 m. Three targets with reflectivity values of 90 %, 60 % and 20 % were used. Theobservation period covered 3 h bya measurement interval between two distance measurements of 1 min.