[nickmooney]

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.

[zokier]

Isn't merging gyro data with accelerometer data pretty standard approach?

[nickmooney]

I forgot phones have gyroscopes :)