I don't know if many of you have snapchat, but it's basically an app for taking pictures and sending them to others. Something you can do is add a filter that shows what speed your phone was going while the picture was taken.. How is this done? Or, more generally, how do things like running apps track speed? I'm guessing GPS, but the reason I'm doubtful is that usually my phone's GPS gives me my location to an accuracy of around [itex]\pm [/itex]10 meters, and that's after I've let the GPS settle down for a minute or so. So I don't know if that's the way they do it. Anybody know, or have any ideas?