  Aerial mapping systems are important for many surveying applications (e.g.,
industrial inspection or agricultural monitoring). Aerial platforms that can
fly GPS-guided preplanned missions semi-autonomously are already widely
available but fully autonomous systems can significantly improve efficiency.
Autonomously mapping complex 3D structures requires a system that performs
online mapping and mission planning. This paper presents Osprey, an autonomous
aerial mapping system with state-of-the-art multi-session LiDAR-based mapping
capabilities. It enables a non-expert operator to specify a bounded target area
that the aerial platform can then map autonomously over multiple flights. Field
experiments with Osprey demonstrate that this system can achieve greater map
coverage of large industrial sites than manual surveys with a pilot-flown
aerial platform or a terrestrial laser scanner (TLS). Three sites, with a total
ground coverage of $2528$ m$^2$ and a maximum height of $27$ m, were mapped in
separate missions using $112$ minutes of autonomous flight time. True colour
maps were created from images captured by Osprey using pointcloud and NeRF
reconstruction methods. These maps provide useful data for structural
inspection tasks.
