The impact of oblique images and flight-planning scenarios on the accuracy of UAV 3D mapping

The developments in lightweight unmanned aerial vehicles (UAVs) and structure-from-motion (SfM)-based software have opened a new era in 3D mapping which is notably cost-effective and fast, though the photogrammetric blocks lead to systematic height error due to inaccurate camera calibration parameters particularly when the ground control points (GCPs) are few and unevenly distributed. The use of onboard Global Navigation Satellite System (GNSS) receivers (such as RTK- or PPK-based devices which use the DGNSS technique) to obtain the accurate coordinates of camera perspective centres has reduced the need for ground surveys, nevertheless, the aforementioned systematic error was reported in the UAV photogrammetric blocks. In this research, three flight-planning scenarios with oblique imagery in addition to the traditional nadir block were evaluated and processed with four different processing cases. Therefore, 16 various blocks with different overlaps, direct and indirect georeferencing approaches as well as flight-planning scenarios were tested to examine and offer the best imaging network. The results denote that the combination of oblique images located on a circle in the centre of the block with the nadir block provides the best self-calibration functionality and improves the final accuracy by 50% (from 0.163 to 0.085 m) for direct georeferenced blocks and by 40% (from 0.042 to 0.026 m) for indirect georeferenced blocks.

Automated summary

The developments in lightweight UAVs and SfM-based software have revolutionized 3D mapping by providing a cost-effective and fast approach. However, inaccurate camera calibration parameters can lead to systematic height error, especially when the number and distribution of ground control points are limited.