## 2 Materials and Methods
### Data Acquisition and Reference Point Cloud
#### 2.3.3 UAV-Based Laser Scanning Measurements and Processing  
The UAV measurements are divided into the flights for single and multiple strip measurements. For both approaches we collected data from four identical flights, using the UgCS software for flight planning. Right before and after the measurements, a recommended flight pattern is included, which is later used for sensor initialization in the trajectory estimation process. For the single strip measurements, the flight path is shown in Figure 3 with a straight flight from the start in the north-east direction and on the same path back. The flight plan was chosen due to the challenging conditions for the trajectory  
Figure 5: Second study are for the analysis of multiple strip measurement with included objects and the flight pattern of the individual measurement, © [[https://www.google.de/maps/](https://www.google.de/maps/)]([https://www.google.de/maps/](https://www.google.de/maps/)) (accessed on 14 May 2021). estimation with a constant heading of the UAV and, thus, performance evaluation under less optimal conditions similar to corridor mapping. For the measurements with multiple strips, the corresponding flight path is shown in Figure 4. The flight height and speed were chosen according to the buildings and objects to cover the entire area with one flight and the highest possible point density. The corresponding flight parameters for the individual approach are listed in Table 3.  
The flight height for the single strip measurements was 10 m, the flight speed was 1 m/s and the line speed of the laser scanner was 55 lines per second. With this configuration, the average point spacing is about 2.2 cm leading to about 2495 points per square meter. With this setting of the flight parameters, the evaluation of the point cloud should mainly include the errors from the trajectory estimation and the system calibration, since the influence of the errors due to the laser scanner is expected to be within the manufacturer's specification of 1.5 cm. Based on this, the evaluation of the system mainly refers to the aspects of trajectory and system calibration. Thus, the most important aspect is the trajectory accuracy and especially the absolute accuracy resulting from the differential GNSS processing. To evaluate the influence of the choice of the GNSS master station on the result, we compare the different GNSS master stations listed in Table 4.  
The master station mainly used within all experiments is the own master station set to pillar \(P_{2}\) (Figure 3) next to both study areas. The pillar is part of the network of control points and, therefore, known with an accuracy of a few millimeters. The receiver used as the master station is a Leica GS10 with a Leica AS10 multi-GNSS antenna. Besides this own master station, two additional VRS next to the study area and a CORS station from the SAPOS NRW network in 16 km distance are included, which are only used in the comparison between master stations. The explained measurements are afterward processed using the calculation scheme described in Figure 1 divided in trajectory estimation and the following georeferencing of laser scanner data. The first step of trajectory estimation is done using the software POSPac UAV providing the smoothed trajectory in the same coordinate system as the reference point cloud (ETRS89). The following processing of the point cloud is done with RIEGL's software RiPROCESS without any additional optimization step or integration of GCPs. The outputs are the georeferenced 3D point clouds for the first study area, which are used in the following section for the evaluation of the system's performance. For the second study area, the full processing using RiPRECISION within RIEGL's RiPROCESS software is used for the optimization of the point cloud product based on