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 ...
'28 3. Calibration of Terrestrial Laser ScannerThe origin isdefined by the observation pillar on the track line (pillar number 2000) and the orientationof the x-axis is along the track line towards pillar number 1000. The coordinates of all control points weresurveyed with total station from different observation pillars to achieve both high redundancy and highaccuracy.All points show an accuracy in the horizontal position of less than 0.7mm and in the verticalposition of less than 0.2 mm. « • @ « «200 201 202 203 204Si B a H m il m100 110 120 130 140 146 150—«*x Calibration Track Une Bt300$301 302 303•304m® control pointHobservation pillarFigure 3.4: Configuration of the control points. The control points are aligned alongsides and at different heights. The track line definesthe x-axis.the calibration track line on both3.1.3 Test Field of Observation PillarsThe test field of observation pillars consists of 3 x 3 pillars. They span an area of approximately 11 m x5 m. The pillars can be equipped with tribrachs and allows for mounting several kinds of targets andinstruments. Figure 3.5 shows the situation of the observation pillars. The 3D coordinates were periodicallysurveyed with respect to a relative reference frame with a high degree of accuracy. The achieved accuracyin the horizontal position is less than 0.5 mm and in the vertical position less than 0.05 mm.Figure 3.5: Situation of the observation pillars. The observation pillars are aligned in a 3 x 3 pattern and span anarea of approximately Unix 5 m.3.1.4 Electronic Unit for Frequency MeasurementThe unit for frequency measurement allows for the verification of a modulation frequency via knowledgeof its nominal frequency. For this purpose a frequency counter with a high degree of accuracy regarding anabsolute time base has to be derived. Therefore, the clock frequency of a frequency counter (HP 53131A2)is controlled by a standardized frequency.This standardized frequency is emitted by longwave sources2 A two-channel counter that is capable of producing a frequency resolution of ten digits persecond and a bandwidth of 225 MHz.The time interval resolution is specified at 500 ps. An optional third channel provides frequency measurements of up to 3 GHz, 5 GHz,or 12 4 GHz. Standard measurements include frequency, period, ratio, time interval, pulse width, rise/fall time, phase angle, dutycycle, totalize, and peak voltage.
3.1 Laboratories and Tools for Calibration 29and can be received by the frequencymeasurement unit. Basedon this received standardized frequency,an absolute time base for the frequencycounter can be defined.The modulation frequency of a sensor is transmitted to an APD. The detected signal is amplifiedand de¬modulated. The frequency counter calculates the detected modulation frequency. Additionally,ulated signal can also be visualized by an oscilloscope. The electronic unit for frequencyshown in Figure 3.6. The APD, the frequency counter, the amplifier and the oscilloscopethis figure.the demod¬measurement iscan also be seen inFigure 3.6: Electronic unit forfrequency measurement including an APD, an amplifier, a frequency counter, and anoscilloscope (from right to left).3.1.5 Calibration of SpheresIn contrast to traditional geodetic instruments, e.g.total station, GPS, levels, a laser scanner does notsupport the direct determination of coordinates of discrete points. For the calibration purposes of a laserscanner, itis essential to work with discrete points to derive the coordinates of these points. Several geo¬metrical objects are suitable for laser scanning, e.g. planar coded targets, cylinders and spheres.appropriate object for laser scanning in terms of deriving coordinates of a discrete point is a sphere.advantages of a sphereare manifold. First,each sphere is absolutely defined in 3D bythe center point and its diameter. Second, a sphere is featured by a homogenousThe mostThethe coordinates ofsurface and is invariantregarding the viewing angle. Thus, spheres are well-suited for use as targets for laser scanning. However,there is one major disadvantage. The angle of incidence worsens with increasingand vice versa. The introduction of weights can reduce this problem. However, these pointsas they define the geometry of the sphere.distance from the centerConsequently, massive wooden spheres in two sizes were prepared as targets for laser scanning.are essentialThe diam¬eters are of approximately twelve and fifteen centimeters. Adapters were fixed on the spheres for attachingthem to prism holders. When installing the adapters, caution was taken to avoid an offset in position andin height between the prisms and the spheres. Furthermore, the spheres were paintedwith a white colourfor achieving high intensity values for the reflected laser beams. Overall, each sphere had to be calibratedwith respect to• the diameter and the symmetry of the diameter, in horizontal and vertical direction,• the central position of the adapter, and• the vertical offset between prism center and sphere center.The calibration processwas performed by usinga theodolite (Wild T 2000). The theodolite and the spherewere set up on two pillars of the test field of observation pillars, cf. Section 3.1.3. By knowingthe distance
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'28 3. <strong>Calibration</strong> <strong>of</strong> <strong>Terrestrial</strong> <strong>Laser</strong> <strong>Scanner</strong>The origin isdefined by the observation pillar on the track line (pillar number 2000) <strong>and</strong> the orientation<strong>of</strong> the x-axis is along the track line towards pillar number 1000. The coordinates <strong>of</strong> all control points weresurveyed with total station from different observation pillars to achieve both high redundancy <strong>and</strong> highaccuracy.All points show an accuracy in the horizontal position <strong>of</strong> less than 0.7mm <strong>and</strong> in the verticalposition <strong>of</strong> less than 0.2 mm. « • @ « «200 201 202 203 204Si B a H m il m100 110 120 130 140 146 150—«*x <strong>Calibration</strong> Track Une Bt300$301 302 303•304m® control pointHobservation pillarFigure 3.4: Configuration <strong>of</strong> the control points. The control points are aligned alongsides <strong>and</strong> at different heights. The track line definesthe x-axis.the calibration track line on both3.1.3 Test Field <strong>of</strong> Observation PillarsThe test field <strong>of</strong> observation pillars consists <strong>of</strong> 3 x 3 pillars. They span an area <strong>of</strong> approximately 11 m x5 m. The pillars can be equipped with tribrachs <strong>and</strong> allows for mounting several kinds <strong>of</strong> targets <strong>and</strong>instruments. Figure 3.5 shows the situation <strong>of</strong> the observation pillars. The 3D coordinates were periodicallysurveyed with respect to a relative reference frame with a high degree <strong>of</strong> accuracy. The achieved accuracyin the horizontal position is less than 0.5 mm <strong>and</strong> in the vertical position less than 0.05 mm.Figure 3.5: Situation <strong>of</strong> the observation pillars. The observation pillars are aligned in a 3 x 3 pattern <strong>and</strong> span anarea <strong>of</strong> approximately Unix 5 m.3.1.4 Electronic Unit for Frequency MeasurementThe unit for frequency measurement allows for the verification <strong>of</strong> a modulation frequency via knowledge<strong>of</strong> its nominal frequency. For this purpose a frequency counter with a high degree <strong>of</strong> accuracy regarding anabsolute time base has to be derived. Therefore, the clock frequency <strong>of</strong> a frequency counter (HP 53131A2)is controlled by a st<strong>and</strong>ardized frequency.This st<strong>and</strong>ardized frequency is emitted by longwave sources2 A two-channel counter that is capable <strong>of</strong> producing a frequency resolution <strong>of</strong> ten digits persecond <strong>and</strong> a b<strong>and</strong>width <strong>of</strong> 225 MHz.The time interval resolution is specified at 500 ps. An optional third channel provides frequency measurements <strong>of</strong> up to 3 GHz, 5 GHz,or 12 4 GHz. St<strong>and</strong>ard measurements include frequency, period, ratio, time interval, pulse width, rise/fall time, phase angle, dutycycle, totalize, <strong>and</strong> peak voltage.
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