I know the browser shares my IP address and details of nearby wireless networks to determine my location, but what sort of info does it know about nearby wifi networks?

For example, where I live are no public wireless networks, and from my home I can see about 5 private networks, yet my location is determined within 20 meters. With there being no open networks nearby, how does Google determine my location? The only way I can think of is that Google mapped the locations of all wireless networks when they mapped streets for Street View.

I've searched about online, but all I can't find any specific details.