Optimizing Bundle Block Adjustment for High-Overlap Small-Format Multi-Head Camera Systems

Abstract. Small-format multi-head camera systems integrating nadir and oblique cameras are becoming standard in photogrammetry, offering the versatility needed for UAV-based mapping and surveying applications to collect high-quality data in challenging environments. A key consideration for these systems is the inherent limitations due to their small size, which may lead to various trade-offs, such as reduced ground coverage, overlap, and image quality, as well as constraints on using high-precision instruments like GNSS and INS. In this contribution, we propose the compass direction (CD) frame as a superior reference frame to avoid confusion regarding oblique cameras and oblique viewing directions. This approach supports continuous and consistent ground coverage, similar to the nadir camera, and facilitates easier analysis. We focus particularly on relative orientation, as it is a decisive factor for the consistency of subsequently derived information. We evaluate various influencing factors, including image connectivity, image ray intersection angle, and tie point multiplicity, in both single-camera (nadir) and penta-camera (nadir and oblique). The analysis reveals significant deviations in image connectivity from proportionality to image overlap. The percentage of overlap and side lap for nadir images influences the overlap for oblique cameras. While, there are no gaps at the near line between overlapping frames in each individual compass direction, the nadir images overlaps do not result in the same symmetry in the oblique image overlaps. Achieving such symmetry could enhance the visibility of facades in oblique images. In terms of the image ray intersection angle, the maximum intersection angles per tie point are well below the theoretical maximum, indicating a moderate exploitation of the accuracy potential offered by oblique images. Furthermore, analysis of the RMS error suggests that by adding the oblique images to the nadir image block and removing the 2-fold tie points, vertical accuracy improved considerably, while planimetric accuracy remained consistent.