Sossa B. Methods and models of terrestrial laser scanning accuracy improvement with calibration data

The components of the systematic error of determining the coordinates are singled out. A model of the preliminary assessment of the influence of the components of the systematic error in the context of the universal model of instrument errors has been developed. The effect of the specified components on the obtained coordinates was considered by performing a priori simulation modeling of the obtained results using the Monte Carlo method with simulated systematic error (data before calibration) and with its absence (data after calibration). The influence of each of the specified components was determined and the significant influence of the component for the measured distance on the accuracy of coordinate determination was proved. In order to obtain a range of distances at which it is possible to carry out calibration, a study of the minimum size of the planar CTO was conducted and the minimum necessary dimensions of the CTO were established for the unambiguous determination of a flat, spherical and cylindrical plane. A study was conducted with the simulation of an error in one of the points defining the sphere or cylinder and the subsequent calculation of the number of points needed to determine the plane. Acceptability criteria of 5%, 10%, and 20% of the center coordinates offset relative to the offset of the coordinates of the point in which the error was simulated were considered. It is proposed to use a 10% acceptance criterion for TLS calibration problems. The maximum distances to planar CTOs are also determined, taking into account their standard sizes and the most common scanning modes. The criteria for the selection of test objects were considered and it was proposed to use point targets for calibration as those that allow the calibration of the EDM unit. A system of statistical evaluation of manual and automatic modes of determining the targets coordinates is proposed by comparing the MSE of distances between points, and an algorithm for assessing the quality of measurements as well as choose the optimal mode during calibration. A methodological bases for the design of a calibration polygon have been developed based on the investigated types of CTO and according to the determined maximum distances. It is proposed to take the values of the minimum vertical and horizontal angles between the test objects as the basis of the design. According to the developed methodical bases, the number of CTO was calculated and their configuration was proposed for the specified minimum angles. It is proposed to carry out a separate calculation of the transformation and calibration parameters, for which a modification of the mathematical model was carried out. To find the parameters of the transformation between the coordinate systems of the polygon and the scanner, it is suggested to use the Kabsh-Umeyama algorithm. It was proposed to simplify the numerical solution of the calibration problem by the method of least squares by carrying out the inverse transformation of the coordinates from the CS of the polygon to the CS of the scanner and then computing for the calibration parameters through the defined errors model. A non-linear method of finding calibration parameters was also considered and a program was written in the Mathcad software for the non-linear solution of this problem. A practical approbation of the conducted research was made and its results were analyzes. A calibration test site was created for calibration. The coordinates of the calibration test objects were determined from each scanning station and with the high-precision electronic total station. According to the developed modification of the mathematical model, calculations were carried out in a linear and non-linear way. According to the results of the assessment of the accuracy of the data before and after the calibration, it was proved: •increasing the completeness of the data due to the rotation of the scanner allows to partially solve the problem of the impossibility of fixing the designed targets and improves the accuracy of determining the calibration parameters; •the use of the Kabsh-Umeyam algorithm allows one to calculate the optimal rotation matrix and transition vector in the presence of gross errors in measurements and simple numerical solution; •solving the problem of computing calibration parameters in a linear and non-linear way gives practically the same results; •the proposed method of determining calibration parameters is available to end-users of TLS systems in normal conditions without the use of high-precision equipment and specialized software. A general conclusion was also made that using the calibration parameters into the data obtained by the TLS method allows to increase the accuracy of these systems to the level that meets the regulatory requirements for the performance of engineering and geodetic works during construction support.