Real-time updated free station as a georeferencing method in terrestrial laser scanning

Sammanfattning: Georeferencing is an important task in terrestrial laser scanning (TLS) applications. It means transformation of the 3D data (point clouds) into an external coordinate system so that it can be combined with other spatial data. The aim of this study is to investigate the accuracy and precision of real-time updated free station (RUFS) as a georeferencing method in TLS applications, and to evaluate its efficiency. The RUFS is used in total station surveys, implies determination of the instrument position and orientation based on measurements of two or more backsight targets, whose coordinates are determined with Real Time Kinematic (RTK) GNSS. The field surveying was carried out in May and June 2010. A control point was established based on static GNSS measurements in order to have reference data to evaluate the accuracy of station coordinates. Three different configurations of 10 backsight targets were tested: the targets were evenly spread over the sectors of 200 and 100 gon on one arc and 100 gon on two arcs. The measurements were repeated ten times for each configuration. The precision and accuracy of the station position were then derived by processing the surveying data. The results show that with increasing the number of backsight points from 2 to 10, the planimetric precision of the station position improved from 6 to 3 mm; the height precision was at the level of 3−5 mm. The accuracy of the station position improved from 10 to 2 mm in planimetry, and from 28 to 11 mm in height. The time expense for one set of measurements with RUFS was approximately 15 minutes.