Track user speed with CoreMotion without GPS

Question

I want to track user's position and update it in the offline map based on his movement without using GPS and having to rely on location updates.

I have tried CMMotionManager and got acceleration in G's. However, this is acceleration rather than valocity. The manager also allows to get gravity, rotation and attitude.

Is there a way to calculate the user's speed in m/s ? If so, how would I go about it? Any formulas / code samples?

Answer 1

The only way to do this is to assume that the phone is at rest when the app starts. With an accelerometer there's no way to tell the difference between being at rest and moving at a constant rate. For example if you were on a jet plane you'd have no way to tell that you were traveling at 800 kph and not sitting still.

If you do assume that you are at rest when you start it's possible to come up with very crude estimates of speed by tracking acceleration, but in practice, the results are prone to large amounts of "drift error", were small measurement errors quickly add up to a completely wrong current speed result, and so your position drifts around hopelessly.

So in practice, the answer to your question is "no, not really."

Edit:

Thinking about this a little more, you might be able to get usable results if you can impose some assumptions.

Say we assume that the user is on foot. We rule out traveling on a bike/in a car/train/plane. On foot, you really don't "drift". You move in fits and starts as the user takes steps. In fact, you could likely use the accelerometer to recognize the characteristic bounce of a person walking. There are pedometer apps that already do that. For walking, you could probably assume that in the absence of acceleration (ignoring gravity, which is constant), the phone is stationary, so zero out the speed and keep it at zero until there is an acceleration above a certain threshold. That would enable you to reduce drift error.