Why does the acceleration graph not take direction into account? Seems like you could get much cleaner data if you looked only at the directions orthogonal to gravity.
They're using Fourier transforms to focus on frequencies of acceleration associated with subway travel; gravity should mostly fall out as an independent factor.
Is there always a great reference for which way down is, given that the phone can be in various orientations? You might have to do some other heuristics / build some other model for that to figure out which axis to subtract your 9.8 m/s^2 from - maybe it's easier not to worry about it.
