## 2 Materials and Methods
### Evaluation Strategy  
The evaluation approach to analyze the absolute accuracy and precision in this study is generally separated into the analysis based on single strip measurements without additional multi-strip or GCP corrections, and the analysis of measurements with a cross-flight pattern, referred to as multiple strip measurements. Single strip measurements are performed to focus the analysis of system performance without additional GCP input or corrections like strip adjustment, whereas the cross-flight pattern with multiple strips is used to evaluate the full benefit of the post-processing provided by the manufacturer's software, such as strip adjustment. The limiting factor for the absolute accuracy in the investigation of a single strip is clearly given by the GNSS processing within trajectory estimation. The accuracy achieved in the differential processing of GNSS observations is further dependent on the GNSS master station, the satellite geometry, and the measurement conditions. Since it is almost impossible to evaluate the accuracy of estimated UAV trajectory directly, the calculated point clouds are used for the evaluation of the system. The individual research aspects for the evaluation approach are detailed in the following with the assessment parameters and strategies used, which are investigated for the single strip and the multiple strip measurements. 1. **Noise** The noise level is analyzed based on different parts of the resulting point clouds. This parameter primarily gives a quantification of the noise of the laser scanner measurements (included in Equation (1)) and an understanding of how much it contributes to the error budget of the system. The noise evaluation parameter is based on a plane fit and the corresponding residuals. Therefore, planar objects with good reflective properties are necessary with an appropriate scan geometry. The noise level is investigated only at the given flight height without a detailed evaluation of the systematic dependence with increasing distance to the object. 2. **Absolute accuracy** The absolute accuracy is the main part of this study. For this, the UAV-based laser scanning point clouds are analyzed in comparison to a georeferenced point cloud measured with TLS. This evaluation is supposed to determine the absolute accuracy of the system in the vertical and horizontal direction using a point-based evaluation approach. Since the accuracy in the vertical direction is in most cases lower than in the horizontal direction [41], a special focus is given on the height component. Besides the point-based evaluation, the orientation of the point cloud in the global reference frame is analyzed using a parameter-based approach. Concerning the parameters in Equation (1), trajectory estimation, laser scanner and system calibration are evaluated simultaneously in this investigation. \begin{table}
\begin{tabular}{l c} \hline \hline
**Accuracy** & **Values** \\ \hline trajectory estimation-position vertical & \textless{}0.10 [m] \\ trajectory estimation-position horizontal & \textless{}0.05 [m] \\ trajectory estimation-roll \& pitch & 0.015 [deg] \\ trajectory estimation-heading & 0.035 [deg] \\ laser scanner & 0.015 [m] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Error budget of the UAV-based laser scanning system based on RIEGL miniVUX-SYS according to manufacturer’s specification [39; 40; 41]. 3. **Precision** Additionally, the precision is evaluated with repeated measurements using the same flight plan for each repetition, spread over several hours. This approach is intended to indicate the precision of the system and, in particular, its performance under changing GNSS conditions and constellations. This investigation uses point-based as well as parameter-based evaluation strategies. 4. **GNSS master station** This aspect examines the importance of the GNSS master station used for the trajectory estimation of the UAV. In principle, there are different possibilities of master raw data which are used for the relative GNSS processing.