Quality assessment of combined IMU/GNSS data for direct georeferencing in the context of UAV-based mapping

Within the past years, the development of high-quality Inertial Measurement Units (IMU) and GNSS technology and dedicated RTK
(Real Time Kinematic) and PPK (Post-Processing Kinematic) solutions for UAVs promise accurate measurements of the exterior
orientation (EO) parameters which allow to georeference the images. Whereas the positive impact of known precise GNSS coordinates
of camera positions is already well studied, the influence of the angular observations have not been studied in depth so far. Challenges
include accuracies of GNSS/IMU observations, excessive angular motion and time synchronization problems during the flight. Thus,
this study assesses the final geometric accuracy using direct georeferencing with high-quality post-processed IMU/GNSS and PPK
corrections. A comparison of different data processing scenarios including indirect georeferencing, integrated solutions as well as
direct georeferencing provides guidance on the workability of UAV mapping approaches that require a high level of positional
accuracy. In the current research the results show, that the use of the post-processed APX-15 GNSS and IMU data was particularly
beneficial to enhance the image orientation quality. Horizontal accuracies within the pixel level (2.8cm) could be achieved. However,
it was also shown, that the angular EO parameters are still too inaccurate to be assigned with a high weight during the image orientation
process. Furthermore, detailed investigations of the EO parameters unveil that systematic sensor misalignments and offsets of the
image block can be reduced by the introduction of four GCPs. In this regard, the use of PPK corrections reduces the time consuming
field work to measure high quantities of GCPs and makes large-scale UAV mapping a more feasible solution for practitioners that
require high geometric accuracies.