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

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24 3. Calibration of Terrestrial Laser ScannerTable 3.1: Specifications of terrestrial laser scanner, Imager 5003 of Zoller+Frohhch.manufacturer :Zoller+Frohlich GmbH (Wangen i.A., Germany)type name : Imager 5003type number : 53500serial number : 161laser number : 09014laser class :3Rproduction year : 2003Calibration is a set of operations that establish, under specified conditions, the relationshipbetween valuesof quantities indicated by a measuring instrument or measuring system, or values represented by a materialmeasure or a reference material, and the corresponding values realized by standards.Notes:(1) The result of a calibration permits either the assignment of values ofmeasurands to the indications orthe determination of corrections with respect to indications.(2) A calibration may also determine other metrological properties such as the effect of influence quanti¬ties.(3) The result of a calibration may be recorded in a document, sometimes called a calibration certificateor a calibration report.Performance assessment/evaluation is a voluntary assessment and would be conducted to determine howwell the instrument and the processing software meet a user's specific requirements [NIST, 2002]. Certifica¬tion has legal connotations and would involve testing the instrument in accordance with a set of protocolsand measuring the results against a metric pass/fail [NIST, 2002].ducted in a certified laboratory.The testing would, in general,be con¬The procedures of calibration and performance assessment/evaluation deal with the investigationof mea¬suring systems. Investigating measuring systems, e.g.automated total stations, digital levels, cameras,laser scanners, usually require manymeasurements to derive measurands as well as to assess the results.For the assessment of measurements, several standard definitions were developed by different organisa¬tions and institutes, such as the International Organization for Standardization (ISO), the American Na¬tional Standards Institute (ANSI) and the American Society of Testing and Materials (ASTM International).Furthermore, [Heister, 2006] introduced parameters to assess the acuracy of laser scanners and to makethem comparable.The standard definitions cover a wide range of terms. Unfortunately,there are some terms for which stan¬dard definitions are missing. Such terms are related to the topic of laser scanners. The reasons for this aredue to the relatively young age of the instrument developments and the differing working principles theywere based on.Examples of such missing terms include sample rate, horizontal resolution and verticalresolution. Each vendor uses different definitions and specifications for qualifyingthe characteristics of itslaser scanner. To avoid misunderstanding, definitions for two pairs of confusable but importantgiven. They are precision versus accuracy and repeatability versus reproducibility.Precision isdefined as the closeness of the agreement between independentterms aretest results obtained understipulated conditions [ASTM E456-02, 2002]. Usually, precision is expressed numerically bymeasures ofimprecision, such as standard deviation, variance, or coefficient of variation [ASTM E456-02, 2002]. Thenotes included with the definition also states that precision dependson random errors and does not relateto the true value. The precision can be described by the empirical standard deviation [Moser et al., 2000]:
251\.=1M = -y^a;»in(3.2)where jj, is the mean, xt represents the test results (data) and n describes the number of data.Accuracy is defined as the closeness of the agreement between the result of a measurement and a true valueof the measurand and is a qualitative concept [VIM, 1993]. Accuracy can be described bysquare (RMS) [Deakin and Kildea, 1999]:the root meanRMS =\n_yVa-x,)2 (3.3)n z—'i=\where a is a constant, xt represents the test results (data) and n describes the number of data. Therefore, theconstant a represents a true or nominal value which is based on surveying with high accuracy. Accuracycan also be expressed by a deviation A between the nominal value a, i.e.resulting in a bias (comparison principle):constant, and the real value /x,A = a-fj,. (3.4)In the following, numeric values describing the accuracy refer to the comparison principle expressed byEquation (3.4).Repeatability is defined as the closeness of the agreementbetween the results of successive measurementsof the same measurand carried out under the same conditions of measurement, e.g. expressed by dispersionparameters [VIM, 1993]. Reproducibility is defined as the closeness of the agreement between the resultsof measurements of the same measurand carried out under changed conditions of measurements [VIM,1993]. This is important since frequently the conditions under which the instrument is operatedthose under which the instrument was originally evaluated or calibrated.differ fromSeveral lab calibration facilities at the IGP (Institute of Geodesy and Photogrammetry, ETH Zurich, Switzer¬land) allow for the performance of component calibration procedures.These installations include a calibra¬tion track line with a length of approx. 52 m, an electronic unit for frequency measurements, test field ofobservation pillars, 3D test fields, etc. They facilitate the investigationgarding various properties. The following sections introduce the different typesby the discussion of the calibration procedures applied on the different componentsscanner. The focus is on the two main system components defining a surveyingand calibration of instruments re¬of installations followedof the terrestrial laserinstrument to measuredistances and angles, the distance measurement system and the angle measurement system, respectively.Furthermore, additional instrumental and non-instrumental errors affecting both the distance measure¬ment system and the angle measurement systemare discussed. Finally, an overall precision for a singlepoint and an accuracy for modeled objects that can be achieved bythe laser scanner are derived.The discussed calibration procedures give an idea of the possible errors that can influence the precisionand the accuracy of terrestrial laser scanners and their measurements, respectively. The completenesspossible influencing parameters is not guaranteedof alland is not the aim of this research. Different researchersand institutions deal with the investigation of terrestrial laser scanners to achieve a better understanding ofthese sensors and to establish their use in geodetic applications.
Page 1: DISS. ETH NO. 17036Calibration of a
Page 5 and 6: ZusammenfassungSeit einigen Jahren
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136 A. Imaging System of Imager 500
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152 BibliographyWunderlich, T. A. [
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List of Figures3.22 Residuals of th
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200220072002Curriculum VitaePersona
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