How could Google Latitude find my exact PC location with no GPS or public WiFi?

Question

I found a similar question here but I still don't get it.

You see, I live in a small town and every time I check my IP location via online services or speed test websites, my location appears to be my ISP server location (which in my case is 250 miles away). But when I tried Google Latitude, it pinpointed my exact location within less than 100 meters!

I use Windows Vista, Google Chrome, and when I got the message that "Google is trying to locate you", I agreed just to check what the result will be. It was scary, very scary!

What I've come up after reading the above link is that Google has a kind of extensive WiFi database locations. That could be understandable with the case of public and open WiFis that are used with a lot of people. Some of them might be using applications that could gather location data and somehow this information ends up in giant Google databases. From those, Google could pinpoint a WiFi location based on its MAC address along with these bits of info that have been gathered via various sources.

The issue here is that my WiFi is private, I don't even broadcast my WiFi name. So how on earth did Google find my exact PC location?

Please break down the answer in layman's terms as possible.

Update:

I think that I found how they did it. First, we don't have Google Street View in the whole country, so the possibility that Google obtained WiFi info through that mean is not there. Second, when I tried the Latitude, I did it using Google accounts belonging to me (for experimenting on how it works). The first account I used on my PC. The second account I used on my cellphone which has GPS and was connected to the same WiFi that my PC was connected to. When Latitude got the cellphone location, it got hold of the WiFi location with its MAC address, hence the PC location since it was connected to the same WiFi.

I just need a confirmation of whether what I think is reasonable, I'm just a novice.

Solution

First, just because your SSID is not broadcasting, that does not mean you are not emitting radio transmitter waves. That said, they do not seem to use that when you are doing it from a browser. Here is the exact explanation of how they do it when you use a browser:

http://en.wikipedia.org/wiki/W3C_Geolocation_API

Of course, when installed on a phone, I believe every phone today comes with a locator.