SNAP! We've got a compass base over at STN as well. Ours is a circular piece of concrete with one lead-n from the Juliet taxiway. I thought (and it is a very simplistic thought) that it was used to check/align the onboard compasses.
I'm quite possibly wrong on this one, we'll have to wait for it to be confirmed. Interestingly though this is where we tend to 'store' any hijacked aircraft that visit us, I think it's used as the area around it is raised and therefore good for hiding troops in, and it is away from the southside passenger aprons and the northside GA.