Decreasing the Uncertainty of the Target Center Estimation at Terrestrial Laser Scanning by Choosing the Best Algorithm and by Improving the Target Design

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.

Sensitivity Analysis and Minimal Measurement Geometry for the Target-Based Calibration of High-End Panoramic Terrestrial Laser Scanners

“…This is opposite to the expectations based on the known uncertainty behavior of the rangefinder measurements [44,45] and contrary to the results from Medic et al 2019 [24]. However, this result is in accordance with the values presented in Janßen et al 2019 [46], and the reason this result lies in the improved target design of the newly developed BOTA8 targets used herein [46]. The input values for the stochastic model are linearly interpolated with respect to the TLS-to-target distance from the values given in the figure.…”

“…The targets used were black and white BOTA8 targets, which assure high precision of the target center estimation [46]. The target centers were estimated using the algorithm based on the template matching described by Janßen et al 2019 [46]. From each scanner station, two-face measurements were made and the compensator was turned on.…”

“…We implemented the measurement configuration from Figure 16 in the facility that we have at hand for the calibration of TLSs with approximate dimensions of 33 x 75 x 9 m (Figure 17). The targets used were black and white BOTA8 targets, which assure high precision of the target center estimation [46]. The target centers were estimated using the algorithm based on the template matching described by Janßen et al 2019 [46].…”

Designing and Evaluating a User-Oriented Calibration Field for the Target-Based Self-Calibration of Panoramic Terrestrial Laser Scanners

“…The positioning was based on the known 3D model of the facility and the measurements with an electronic distance measurement device and a measuring tape. The implemented targets were planar black and white, 8-fold BOTA8 targets with dimensions of 30 × 30 cm, which guaranteed high precision target center estimation [55]. The targets had a semisphere mounting at the back side of the plate that was compatible with the laser tracker magnetic nests.…”

“…Further details of each calibration are summarized in Table 5. The target centers were estimated using the algorithm based on template matching described by Janßen et al (2019) [55]. These observations were processed in the calibration adjustment as described in Sections 2.1.2 and 2.1.3, with one modification.…”