Abstract
During the registration and georeferencing of terrestrial laser scans, it is common to use targets to mark discrete points. To improve the accuracy of the registration, the uncertainties of the target center estimation (TCE) have to be minimized. The present study examines different factors influencing the precision of the TCE. Here, the focus is on the algorithm and the target design. It is determined that, in general, the uncertainties of the TCE are much smaller than those indicated by the manufacturers. By comparing different algorithms for the first time, it was possible to clearly determine that an algorithm using image correlations yields the smallest standard deviations for the TCE. A comparison of different target designs could not identify an ideal commercially available target. For this reason, a new target, the BOTA8 (BOnn TArget with 8-fold pattern) was developed, which leads to smaller standard deviations than the previous targets. By choosing the best algorithm and improving the target design, standard deviations of 0.5 mm in distance direction and 1.2 arcsec in angular direction for a scan distance up to 100 m were achieved with the laser scanner Leica ScanStation P20. The uncertainties could be reduced by several millimetres and angular seconds compared to the manufacturer’s targets and software.
Highlights
The influencing factors on the precision of the target center estimation (TCE) can be defined analogously to the influencing factors of the single point measurement according to Zogg [8] and Soudarissanane [9], whereby the four influencing variables are extended by the influence of the algorithm estimating the target center (Figure 2): 1. Laser scanner: The influence of the laser scanner on the uncertainty of the TCE essentially depends on two variables: (a) The uncertainty of the scan points: Random errors of the angle and distance measurements lead to random errors in the three-dimensional point cloud, which is used for the TCE [10]
The present study examined the target center estimation (TCE) in all its aspects regarding its precision
An attempt was made to improve the precision of the TCE by means of the BOTA8
Summary
Terrestrial laser scanning (TLS) has become an established measuring technique in engineering geodesy. For many TLS applications, laser scanning targets are employed, as for example in the data acquisition of a measured object from different stations or in deformation monitoring [1,2]. In these cases, target coordinates extracted from the 3D point cloud are used to register the single point clouds to a joint point cloud. This study discusses the influence of the laser scanner, scan geometry, atmosphere, target design, and algorithm on the uncertainty of the TCE.
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