On-site camera calibration with sub-block of images for UAS Photogrammetry corridor mapping with on-board GNSS-RTK

Abstract. UAS photogrammetry has gained popularity due to its efficiency and automation in acquiring spatial data. Among its applications, corridor mapping is essential for road and railway planning, transmission line inspection, coastal monitoring, and river geomorphological analysis. High spatial accuracy in corridor mapping typically requires dense GCP distribution or precise sensor position and orientation measurements. When using the direct sensor position obtained by GNSS-RTK, camera calibration is essential in reducing vertical bias on the photogrammetric intersection. Although previous studies have demonstrated the benefits of incorporating multi-height oblique images for improving vertical accuracy, there is a lack of research on more feasible data acquisition strategies for corridor mapping, such as using sub-blocks of oblique images for calibration. Thus, this study addresses this gap by evaluating the impact of different sub-block configurations on IOP estimation and spatial data accuracy. The results show that incorporating oblique images into on-site calibrations significantly improves IOP estimation, particularly for focal length, compared to an on-the-job calibration with only nadir images. Vertical accuracy improves by up to 79% in GNSS-AAT experiments using oblique images, making it possible to achieve checkpoint RMSEs for each coordinate (X, Y, and Z) of approximately 1 GSD, without the need for GCPs. In contrast, on-the-job calibration with nadir images alone resulted in higher Z-axis errors. These findings suggest that using nadir and oblique sub-blocks for on-site calibration can improve vertical accuracy, reduce reliance on GCPs in corridor mapping, and maintain high spatial accuracy.