Various visibility maps that we compared to each other. (a): LOD2-60 visibility map with 250 m pixel⁻¹ at the first LOD spanning 20 × 20 km and a resolution of 750 m pixel⁻¹ at the second LOD covering an area of 60 × 60 km. (b): NOLOD-20 visibility map resembling the first LOD of the map shown in (a). (c): NOLOD-60 visibility map covering the same area as the map shown in (a) with a resolution of 250 m pixel⁻¹.