Unmanned aerial vehicles (UAVs) are more and more employed in arch bridge safety inspections. Nonetheless, the process of data acquisition during UAV flights often confronts challenges such as insufficient flight safety assurance, suboptimal flight efficiency, and data redundancy. These challenges can be effectively mitigated through the strategic planning of inspection waypoints and the utilization of UAVs for autonomous flight operations. This paper introduces a method for the automatic planning of UAV inspection waypoints for long-span arch bridges, grounded in the Geometric Digital Twin (GDT) models. In comparison to alternative methodologies, this approach is distinguished by its streamlined workflow, heightened automation, and time efficiency. The method is delineated into three sequential steps: (1) the generation of a 3D reconstruction model of the arch bridge; (2) the development of the GDT model for the arch bridge using the Polyfit framework; (3) the strategic placement of coverage waypoints across the surface of the arch bridge. Ultimately, the practical viability of the proposed approach is empirically verified through its application to real world arch bridges.