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

of0.000001s5.2 Rotation Time of Rotating Mirror of Laser Scanner 1015.2.3 Discussion and ComparisonThe rotation time T of the rotating mirror were derived by two independentmethods. The direct methodrequires a more careful consideration of different influencing parameters caused by the use of two photodiodes.The autocollimation thereby defines the main problem, which is to achieve the claimed precisionfor the rotation time T. Comprehensive considerations regarding different parameters for carryingautocollimation yield acceptable results. On the contrary, the indirect method onlyout thedeals with the numberof full rotations and the total time. By using the tools of adjustment the rotation time T as well as the specifictime delay St for the three supported angular velocities can be derived indirectly. These time delaysfurther interest in performing kinematic applications.are ofTable 5.9 summarizes the results and derives a mean value for the rotation time T.It can be seen that amaximal difference of 5 /xs was achieved between the two methods. The claimed precision was not achievedcompletely,but the results are close.Table 5.9: Comparison of the results of both methods deriving the rotation time T. Homogeneousmaximum difference o/5/xsbetween the direct method and the indirect method can be seen.results with aRPSDirect MethodIndirect MethodMeanTifs]T2[s]T[s]12.5 0.080635 0.080639 0.08063725 0.040318 0.040323 0.04032033 0.030544 0.030544 0.030544Finally, a short discussion shall be presented to clarify the meaning of (1) a precision and (2) a systematicerror of 1 /xs for the rotation time T. If the rotation time has a precision of 1 /xs, then the 'true' value isoscillating around the derived value of 1 /xs. In addition, a systematic deviation of the derived value fromthe 'true' value means that with a growing measuring time, the error caused by the systematic1 /xs is increasing. Supposing a velocity—of v « 0.2deviation ofa test trolley on which the laser scanner is mounted,the observation time can be estimated at which an error in the position of the test trolley of 1 mm is achieved.In this example, each rotation generates an error of As = v At = 0.2 .= 0.0002 mm. Thismeans 5000 rotations yield the supposed position error of 1 mm. ConsideringT the observation time covers a time period of 2.5 min upthe different rotation timesto 6.7min. In terms of a file size for a laser scanof 2.5 min, depending on the scan resolution, several hundreds of Megabytes up to some Gigabytes areobtained.In summary, three rotation times T are derived with high precision. Based on the desired spacing betweentwo adjacent vertical profiles an appropriate velocity of the test trolley has to be chosen. Additionally, thespacing can also be influenced by selectinganother rotation time T. On the contrary, the scan resolutionwithin each vertical profile is only correlated to the sampling interval and cannot be changed by the velocityof the test trolley or by the rotation time. Considering a mean velocity of 0.2—, two adjacent vertical profileshave a spacing of• 1.6cm (for ui k, 0.08s),• 0.8 cm (for ui k, 0.04 s), and• 0.6cm (for ui k, 0.03s).In general, there are three different parameters to achieve the most appropriate point densitylaser scanning: the rotation time, the velocity of the test trolleyprofile.for kinematicand the scan resolution within each vertical