Ask HN: How does Twitter do geolocation?

I've recently been traveling and along the way have been tweeting. I always check out my location as Twitter determines it. I'm really amazed with how specific their geolocation system gets. How are they doing geolocation on such a specific level? Also, where are they getting their spatial overlays? Sometimes I'm tracked to districts in my own town I never even knew the name of. I'd love to have access to the source they're getting this data from.