Quality Assessment of Combined Imu/GNSS Data for Direct Georeferencing in the Context of Uav-Based Mapping

Abstract

ABSTRACT: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.

GPS precise point positioning for UAV photogrammetry

“…Firstly, regarding the PPK‐BBA solutions, check point RMSEs are better than 1 pixel in all coordinates for Flights 1 and 2 (equivalent to less than 3 cm on the ground). These results reflect the high accuracy and precision of the GPS PPK camera positions, and are similar to the check point errors obtained in comparable studies (Rehak and Skaloud, ; Gerke and Przybilla, ; Benassi et al., ; Stöcker et al., ). In addition, they also suggest that the principal distance (focal length) has been estimated with pixel‐level (equivalent) accuracy for both flights.…”

“…As with conventional GPS‐supported aerial triangulation, if UAV imagery comprises a strong block geometry, with good, consistent scene texture to enable a dense and regular distribution of image tie points (for example, obtained using the scale‐invariant feature transform (SIFT) algorithm of Lowe, ), camera attitudes may be computed in the bundle block adjustment with high accuracy. Moreover, in this configuration, the relatively low accuracy of current lightweight IMU observations offer little additional constraint in the bundle block adjustment, as per conventional integrated sensor orientation (Rehak and Skaloud, ; Stöcker et al., ). For these reasons, approaches to remove GCPs in UAV photogrammetry are usually concerned with camera positioning only (Gerke and Przybilla, ), and such GPS‐supported aerial triangulation will be referred to in this paper hereafter as direct georeferencing.…”

Evaluating the potential of post-processing kinematic (PPK) georeferencing for UAV-based structure- from-motion (SfM) photogrammetry and surface change detection

“…The open-source software package RTKLib was used for computing differential positioning (Takasu and Yasuda, 2009). Raw GPS data from the UAV-mounted cameras and the base station were then extracted and corrected by postprocessing using RTKLib.…”

“…The utilization of such an approach has the potential to avoid or mitigate the need for GCPs. Several studies already investigated the application of RTK-PPK direct georeferencing by the integration of sensor orientation with onboard RTK GPS (Fazeli et al, 2016;Forlani et al, 2018;Stöcker et al, 2017). In a study performed by Gerke and Przybilla (2016), block orientation accuracy was significantly enhanced by using an onboard RTK GNSS solution.…”

Unmanned Aerial System Imagery, Land Data and User Needs: A Socio-Technical Assessment in Rwanda

“…However, the measurements of additional Ground Control Points (GCPs) indispensable. In contrast, the DT18 stood out for its high-quality navigation sensor that records precise attitude logs including both, angular observations (< 10 arcmins) as well as camera positions (<2.3 cm) [41]. However, former test flights showed, that the DT18 requires additional GCPs to correct for (minor) systematic errors [41], particularly when no GNSS corrections are applied.…”