Based on the donated and anonymized data of 881 iPhones Michael Kreil http://crowdflow.net/blog/2011/07/12/fireflies-hd/ https://www.youtube.com/watch?v=EI7yQnRy7QQ

The geo data of the iPhones are quite accurate, but only the locations at specific points in time are known. So for example the accurate position of an iPhone at 12:03 and at 14:27 but not how this iPhone had moved in the meantime. The estimation is that an iPhone moves from the last known location at an average speed of 30km/h – in all possible directions. It’s like a diffusion process. That’s why the estimated location becomes more and more blurry and the light fades away. And vice versa: If an iPhone will appear in one hour at a specific location, it should be somewhere nearby now – in a blur with a radius of 30km. (30km in 1 hour = 30km/h) And that’s why the image becomes blurry during the night. Most iPhones are not moving in the night; therefore they do not collect data; their positions are vaguer and the lights dissolve.