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

102 5. Kinematic Laser Scanning5.3 Position-Fixing Using Total StationThe use of a total station for kinematic applications requiressome considerations that differ from those forstatic applications. The main restriction is based on the spatial resolution of the measurements. The spatialresolution depends on both the velocity of the moving trolley and the sampling frequencystation.of the totalTypical sampling frequencies are from 5 Hz to 10 Hz. The velocity of the moving trolley dependson the application.The operating range for total stations is defined by the line of sight and the environment. Thus, typicalranges for kinematic applications do not exceed 200 m [Glaus, 2005]. The operationof the total station ismostly performed via a personal computer, which controls the total station. The data transfer between com¬puter and total station is carried out via radio communication. After setting up the total station, the prismis automatically searched for within a user-defined window. For a precise positioning of the telescope, theautomated target recognition (ATR) has to be activated.This fine search uses digital image processing orfurther scan techniques [Ingensand and Bockem, 1997]. If the positioning is successful, the prismin and the trackingsearched for again.the lost target reliably.mode can be started. Ifthe target is lost due to the loss of signals, the prismMost total stations provide search procedures and extrapolation algorithmsis lockedhas to beto locateThe data read-out of the total station contains the angle measurements and the distance measurement.Therefore, the angle measurement can be corrected by applyingin onlyone face. Foryielding fast encoder readings, the two-axis compensatorinstrumental errors due to measurementshas to be switched off orhas to be used in a planar model [Leica Geosystems, 2004]. Furthermore, distance measurements, ATR andthe compensator readout depend on exterior influences that can result in lags [Hennés, 1999].The deter¬mination of the distance takes more time than the readings of the direction encoders and thus, defines thesampling frequency of the total station. Depending on the chosen distance measurement mode, e.g. single,standard or fast, the range provided bythe total station is more or less accurate. The more accurate the dis¬tance the more time is required for the determination of the distance. Readings of the angle measurementsand the distance measurement are not carried out at the same time. They are asynchronousa time delay.and result inThis time delay has the largest influence, if the moving trolley passes the total station, andthe smallest influence, if the moving trolley drives awayfrom or towards the total station. Asive investigation regarding tracking total stations and the synchronization of the anglemeasurement system can be found in [Stempfhuber, 2004].comprehen¬and the distanceGlobal Navigation Satellite Systems (GPS, GLONASS) can alternatively be used for kinematic applicationsor can complement tracking total stations.Gaps produced either by GPS or bytotal station can be filled.The combination of both GPS and total station leads to an improved accuracy of the trajectory.The raw data acquired by the total station have to be processed regarding several aspects.These stepsof processing contain the detection of blunders and smoothing of raw data, the derivation of equidistantmeasurements, the filling of data gaps, i.e.interpolation, and trajectory smoothing. In the following, thedifferent steps of processing are discussed. Figure 5.9 shows the work flow for processingdata acquired in kinematic applications.the total station5.3.1 Blunder Detection and SmoothingA signal xl(t) can be considered as being superimposed by additive Gaussian noise nt{t) and impulsivenoise il{t)x%{t)=st(t) + nt(t) + it(t) (5.14)