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

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.