630 feet or 192 meters, to be exact. Taken from one of the narrow slit windows in the observation deck on top. Before the new work, the area in the center used to be open to the highway below (which wasn't the best idea in the first place). Now there is a continuous plaza from downtown into the Arch grounds.
We saw some of the circle at bottom center in yesterday's post. It slopes downward as you get closer to the new entrance. There are a few inches of water in the middle where kids can splash. You can see the half-circle of glass awning also mentioned yesterday. The entrance to the visitors' center and the tram to the top is at the bottom center.