# Quality Assessment of DSMs Produced from UAV Flights Georeferenced with On-Board RTK Positioning
## 5 Conclusions  
An experimental investigation on the repeatability of DSM generation from UAVs was performed by repeating the same flight plan eight times from about 90 m aboveground, with a GSD of 2 cm. Using camera station positions measured at exposure time by a senseFly eBee RTK, the photogrammetric blocks were successfully oriented, without any GCPs, with an accuracy of the horizontal coordinates that was better than the GSD in most cases, and within 1 cm of the accuracy obtained using GCPs. However, dispensing entirely with GCPs makes block orientation unreliable, as significant biases, especially in elevations, can arise. Indeed, in three of the eight repetitions, an elevation bias of about 10 cm and more was registered. On the other hand, in all of the tests that were carried out, adding at least one GCP to the camera stations successfully removed most of the bias, bringing the RMSE in elevation to about 3 cm. As checkpoints must in any case be provided for an independent quality check, and using a local master is recommended, this does not appear to be a serious limitation of AAT. An average repeatability of 6 cm in DSM generation with different GNSS datasets and changing lighting conditions (from noon to late afternoon) was found for the **RTK+IGCP** configuration, in areas suited to image matching (smooth, textured surfaces); the discrepancy ranges were also quite uniform over the block, with a standard deviation of only 3 cm. Exactly the same values were obtained using GCPs, almost irrespective of the surface type (hard surfaces or meadows); the average range was instead larger (by 5-7 cm) if only camera stations (**RTK** configuration) were used. As far as the quality of the eight 10 cm-resolution DSMs generated by dense matching is concerned, using either GCPs or all of the camera stations and one GCP yielded similar results. Indeed, on roads, car parks, and meadows the mean RMSE in elevation turned out to be very close, in the order of 5.4 cm, with both types of block control configurations. In all eight flights, using at least one GCP as well as AAT yielded good results. However, further investigations are needed to find out whether the location of the GCP in the survey area can be critical (in our test, the GCP was located in the middle of the area). It is also of special importance for high mountain surveys to find out whether an on-site small calibration block can be used when the survey area is not accessible. Moreover, though the benefits of a very dense control from the camera stations are clearly apparent from the simulations in [PERSON] and [PERSON] et al. (2012); [PERSON] and [PERSON] et al. (2012), more experience should be gained about the effect of GNSS systematic errors and residual inner orientation errors. It is not yet clear whether AAT will supersede GCPs to become the standard referencing technique for UAV blocks. Indeed, as the results shown in this test confirm, the incentives towards using it are strong not only for improving survey efficiency, but also because the accuracy loss compared with the traditional case seems negligible in horizontal coordinates and, with a small additional effort, also very limited in elevation. In any case, AAT will certainly contribute to further enlarging the domain of UAV photogrammetric surveys. **Acknowledgments:** This study was supported by the Interreg V-A \"PrevRiskHauteMontagne\" project (No. 427), co-funded by the European Regional Development Fund, under the operational program for European Territorial Cooperation (ALCOTRA) France-Italy 2014-20. No funds have been received for covering the costs to publish in open access. **Author Contributions:** Experiment conception and design: F.D., G.F., R.R. and U.M.d.C. ; Ground data acquisition: F. D., G.F. and R.R. ; UAV flight planning: F.D., R.R. and U.M.d.C. ; Imagery acquisition: U.M.d.C. ; Data processing of UAV imagery: E.D., M.S., G.F.; Manuscript drafting: G.F.; Analysis and interpretation of results: G.F. and R.R. **Conflicts of Interest:** The authors declare no conflict of interest.