# Development and Performance Assessment of a Low-Cost UAV Laser Scanner System (LasUAV)
## 4 Discussion  
In this paper, the motivation and technical details of developing the LasUAV are described, and its performance is assessed in terms of accuracy and partitioning among multiple error sources. All the major error sources were independently considered by means of analysis made in both angular static and dynamic conditions, and in in-flight conditions. With respect to other studies that tested UAV LiDAR systems developed for agriculture or forestry application, the first and basic assessment of the performance of the LasUAV was conducted in controlled conditions and on a target with a simple geometric shape whose dimensions were exactly measured and whose state was not affected by external conditions, such as wind. In fact, for example, [13] assessed the effect of high-density point clouds (up to 62 returns/m\({}^{2}\)) produced by the UAV-LiDAR system on the measurement of location, height, and crown width of isolated trees. In this case, the standard deviation of tree height was 0.15 m, the standard deviation of tree location was 0.53 m, and the standard deviation of crown was 0.61 m. The assessment of reproducibility of the relative height distribution of 3D point-cloud data from the laser system developed by [14] was conducted in a rice paddy; they found that mapping accuracy was 1.56 \(\pm\) 0.12 m without ground control point (GCP) correction and 0.23 \(\pm\) 0.10 m using only one GCP. The UAV mapping system for agricultural field surveying designed by [15] was tested in crop volume estimates calculated using a voxel grid of the canopy heights of crop biomass with a spatial resolution of 0.04 m \(\times\) 0.04 m \(\times\) 0.001 m.  
In contrast to the above-mentioned studies, whose tests allowed assessment of the total uncertainty of the system, the study presented here allowed us to quantify the uncertainties associated with the different subcomponents of the system and their impact on total uncertainty. The uncertainty and MAE in Z of the LasUAV had small differences between angular static and dynamic conditions. In angular static and dynamic conditions, in the uncertainty scales with a distance from the target from 3 m to 10 m (i.e., typical flight altitude), there is no significant variation, because the measurement error scale is trigonometric. For this reason, the uncertainty at a 3 m distance is small and at 10 m is still not sizable. This means that if the laser scanner system is used at low heights over the target surface, an INS with high values in angular static and dynamic (pitch/roll)  
Figure 12: Uncertainty in Z as a function of number of passages over the target. Blue bars are the standard deviation of uncertainty. accuracy is not needed. These figures changed in in-flight conditions, where the uncertainty in Z of the measurements of LasUAV ranged between 5.1 cm and 9.8 cm for single passages, during which the target was reached from laser pulses with different angulations (ranging from near-nadir to very far-nadir). For the combination of nine passages, uncertainty ranged between 5 cm and 16 cm. In other words, uncertainty in a small number of passages (one or two) is dominated by uncertainty coming from both the laser and the GNSS RTK receiver, whereas for an increasing number of passages (three to nine), uncertainty becomes dominated by the GNSS RTK receiver. This allows us to conclude that if the user of the laser scanner system is interested in the lowest values of uncertainty, a single passage over the area of interest is recommended. This could be the case of UAV-borne laser scanner campaigns over wide forest areas with the goal of scanning the maximum surface in the shortest time. For UAV-borne laser scanner campaigns with repeated passages, the positioning uncertainty coming from the GNSS is in the running and the duration of the flight. In general, this kind of flight survey method is to be avoided, because if the GNSS signal reception is moving away from good conditions of acquisition, the uncertainty in Z measurements will increase markedly.