## 3 Results
### Experiment (2): Point-Based Absolute Accuracy and Precision for Single Strip Measurements  
The second experiment analysis the precision and absolute accuracy based on the comparison to the TLS reference point cloud. The first quantification of accuracy is derived from the target coordinates of four TLS targets divided over the whole study area. The target centers are computed for TLS with \(\left[x_{TLS},y_{TLS},z_{TLS}\right]^{T}\) and the four flight repetitions with the algorithm by [PERSON] et al. [42]. This algorithm uses a robust plane estimation for the selected target and subsequent template correlation to detect the target center coordinates with a standard deviation in the millimeter range. The TLS point cloud as well the UAV-based laser scanning point clouds are given in the official German coordinate system ETRS89/UTM32 with ellipsoidal heights. The used relations for quality assessment are shown in Figure 7 simplified for one target and only two dimensions. The coordinates for the target centers from TLS with \(\left[x_{TLS},y_{TLS},z_{TLS}\right]^{T}\) are assumed to be the reference. Since each target is included multiple times in the four repetitions with the UAV, the precision \(\sigma\) can be estimated based on the mean target coordinates \(\left[\mu_{x},\mu_{y},\mu_{z}\right]^{T}\). The differences between TLS and the UAV-derived target center coordinates are used for the description of the absolute accuracy described with \(\delta\). Considering the differences for all target centers from the UAV data set, a corresponding root mean square error ( RMSE) is calculated. The histograms in Figure 8 combine the results for four UAV flights and the differences for target coordinates in East, North and height direction. The results are shown together for all targets indicated with the index \(i\) for the corresponding data set and index \(j\) for the target number. Figure 6: Evaluation of point cloud noise with residuals regarding plane estimation for an example plane (**a**) and the histogram of all range precision estimated based on four data sets (**b**). The mean absolute difference for the east direction \(\delta_{x}\) is 0.9 cm with a RMSE of 1.1 cm and for the north direction \(\delta_{y}\) it is 1.9 cm with 1.2 cm RMSE. The mean absolute difference of target coordinates describes the absolute accuracy of a single measurement without any additional information or correction within the processing. Based on the shown results a systematic error can be seen in the height coordinate by \(\delta_{z}\) with 3.3 cm, which should be analyzed also by the use of additional objects in the study area. Furthermore, the RMSE for the vertical direction is only 1.2 cm and of a similar order of magnitude compared to the horizontal direction. The precision of the system is determined with the standard deviation using \(dx_{i}\), \(dy_{i}\) and \(dz_{i}\). It is lower than 1.2 cm for all directions. Both results are better than the manufacturer's system specification. Especially the precision shows good results in position and height direction. Since the uncertainty in the vertical direction is typically worse than the horizontal direction, the tables within the study area are used for the additional analysis of the height component. Therefore, the described plane estimation from **Experiment (1)** is used for the determination of table heights for a given position on the table. The estimated ellipsoidal  
Figure 8: Histogram for East, North and height difference of estimated target coordinates for four flights. Results are separated in estimates for the precision and absolute accuracy compared to the TLS reference. Figure 7: Relation of target center coordinates from TLS to multiple estimates of the same target from UAV data sets. Included quality parameters are the absolute accuracy and precision. heights are compared with the reference from the TLS point cloud. The six tables within the study are measured again twice for one data set as before described for the target evaluation.