Twitter recently launched their Geotagging API which allows third party Twitter apps to publish the location from where a tweet was posted.
We've had the ability to view a stream of tweets on Atlas for a while now. Tweets that match a search query (or the public timeline) are placed on the map based on what the Twitter user specified as their location in their Twitter profile.
The addition of geotagging to the Twitter API provides Atlas with the exact location from where a tweet was made. Now, if Atlas sees a tweet with a geotag we'll use it instead of the user's location specified in their Twitter profile.
As the Twitter Geotagging FAQ points out, "Geotaggging is disabled by default for everyone. You can enable it from the Settings>>Account tab".