How does Twitter implement geolocation so they don't have to ask the user permission every time they post?

Question

Most sites I know of that use geolocation cause the browser to show a dialog asking the user permission to gather location info. This is like it can be seen here:

http://html5demos.com/geo and the way it works with Google Maps.

But Twitter seems to be able to do it another way. From twitter.com, when you compose a Tweet, you can click a little button, and it will add your location to your Tweet.

But in the case of the twitter implementation, there is no user interruption.

How is this done?

Answer 1

The twitter website seems to do a XHR request to https://twitter.com/account/geo_location.json which returns the user's current location as a json object. Since location detail is about city-level, I'd guess they are using the client's IP address to locate them using a IP-Locations database.

There are numerous commercial and free API's available to match IP addresses to approximate locations, such as http://ipinfodb.com/ip_location_api.php, if you want to do something similar.