Twitter Location Feature Does not work outside Major Countries using the web interface

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.

There are many parameters which a server can use to identify a visitor. IP address is the first, cookies is another possibility, but the browser itself provides a "signature" including its full version, the name and version of the OS, the languages and encoding it supports... Also, it is easy to use JavaScript or Flash to identify plugins, screen resolution and other aspects of your machine configuration.

If you use all of it, it is too discriminant and the simplest browser upgrade will have you identified as "a new location". But choosing wisely an appropriate subset can give websites a good idea about whether you are using the same platform as before.

On Panopticlick, a web demonstrator of the Electronic Frontier Foundation, you can get an overview of all the information your browser is leaking, and to which extent it can be used to identify you.