One World Trade Center hit a milestone today as construction crews set the 408-foot spire on top of the building. Newspapers are calling it a historic moment for the project, as it brings the structure's height to a symbolic 1,776 feet. That would make it the tallest building in the western hemisphere and the third-tallest building in the world — if it holds up.

But making that official is a good deal more complicated. It won't happen for months, and when the final figure comes in, the building could fall as far as to 16th in the global rankings, below both the Willis (formerly Sears) Tower and the Trump International Hotel in Chicago.

It could fall as far as 16th in global rankings

It's up to the Council on Tall Buildings and Urban Habitat, the recognized authority on building heights, which will issue an official ruling later this year. The biggest issue is the massive spire, which doubles as a radio antenna. Unfortunately for the Port Authority, antennas are considered non-essential telecom equipment and don't factor into the "architectural top" of the building. If the council decides One WTC's peak is more antenna than spire, it could go on the books as reaching only 1,368 feet. The Willis Tower, in contrast, is 1,450 feet to its rooftop, with an antenna reaching up to 1,730 feet.

It's a point that's drawn controversy throughout the process. The plan was originally for a 400-foot antenna encased in an ornamental white shell, which would have been easier to acknowledge as a permanent architectural element of the building. Last May, the planned casing was dropped after the Port Authority decided it would be too expensive to construct and maintain, a move the building's lead designer described as "unfortunate." The inner antenna remained, but One WTC officials took care to refer to it as a spire in subsequent descriptions. Whether the council will agree has been the subject of much debate.

Even if the spire is recognized, other factors could rob the building of its symbolic 1,776-foot designation. The council measures the bottom of the building from the "lowest significant open-air pedestrian entrance," which could be interpreted a number of ways for One WTC. The building has entrances on four sides — from the pedestrian plaza and three adjacent streets — and the Vesey Street entrance is several feet lower than the other three. A few feet won't make much difference in the global rankings, but it illustrates the difficulty of nailing down an exact height before the final drawings are in.

The Council on Tall Buildings and Urban Habitat is expected to address the issue after its awards symposium on November 7th.