The Secrets of Snapchat Filters: How Do Apps Track Speed?

[guitarphysics]

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 $\pm$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?

 

[Borg]

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 $\pm$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?

Based on the replies in this thread, it looks like it done using various tools (including GPS) and isn't very accurate.
How can I find the velocity using accelerometers only?

 

[russ_watters]

It's GPS. And while it is true that it isn't as accurate as one would hope, over a long walk/run, the error becomes negligible. For example, if you run 3k and the distance is off by 10m, that's only 1/3 of 1%.

 

[Borg]

It's GPS. And while it is true that it isn't as accurate as one would hope, over a long walk/run, the error becomes negligible. For example, if you run 3k and the distance is off by 10m, that's only 1/3 of 1%.

The OP was looking for instaneous speed at the time that a picture was taken.

 

[russ_watters]

The OP was looking for instaneous speed at the time that a picture was taken.

Fair enough: I don't know the algorithm by which it calculates "instantaenous speed", but suffice to say, is isn't instantaneous speed. Just a guess, but I suspect it uses a multi-reading average or decaying average.