Pantheon, Rome
The span of the Pantheon, Rome, is not 43.3 m because there is a hole at the top of 9.1 m, so the span has been reduced with the size of the hole to 34.2 m. The span of any structure is measured in the following way: Place the largest possible imaginary horizontal circular disk under or inside the structure, barely touching any load-bearing pillars or walls, or parts used to stabilize the structure like wires. The disk must also not encircle any objects of this kind. At least one diameter of the disk have to be completely covered, that is rain-sheltered, by the structure. The span of the structure is diameter of the disk. Now, if the structure contains a hole at the center of the disk, as in the case of the Pantheon, the span is measured by using a second-largest possible imaginary horizontal circular disk that is smaller than the first disk and completely encircled by it. At least one diameter of the second disk has to be completely covered by the structure. The span of the structure is diameter of the second disk. Applying this method of measurement to Pantheon gives the previously stated result, 34.2 m. The method works for many types of structures.