Accuracy Assessment of UAS Photogrammetry with GCP and PPK-Assisted Georeferencing

Abstract

Abstract

Establishing a dense, well-distributed ground control point (GCP) network for unmanned aerial system (UAS) surveys can be time-consuming and impractical. Recent availability of UASs capable of GNSS-assisted aerial triangulation (AAT) has provided an alternative method, wherein the refinement of the positional accuracy of camera stations via, for example, post-processing kinematic (PPK) correction reduces the need for GCPs. Studies have highlighted how AAT can provide nearly equal accuracy to GCP-based georeferencing, especially if at least one GCP is utilized for bias correction. However, results on the utility of more than one GCP together with AAT are scarce or mixed. This study explores how the number of GCPs affects model accuracy when mapping a ~1 km2 site with a UAS capable of PPK correction. Also, a comparison between two different local base stations and a virtual reference station (VRS) is provided. Based on analysis with 3D checkpoints, increasing the number of GCPs provided only negligible improvements in horizontal accuracy. However, significant improvement is seen in vertical accuracy when increasing the number of GCPs, with the VRS providing the most accurate results. The results indicate that UAS surveys with AAT may benefit from utilization of multiple GCPs.