Measuring walking speed with iPhone accelerometer As part of a research project, I have been tasked with determining gait speed (walking speed) for a subject using a handheld iPhone. The main constraint is that walking distances are very short (10-30 meters), and accuracy is critical (data is used for research.) Using a pedometer and measuring step distance is not an option (too time consuming.) The only plausible way I foresee this could be work is using accelerometer data. However, I'm not sure whether this will succeed. I was wondering if anyone has experience with these sensors and could answer the following two questions:


Will an iPhone accelerometer plausibly be able to accurately measure walking distance for such short distances? Are there any figures on accelerometer accuracy for this kind of sensor? (I couldn't find any.)
Are there any established protocols for converting accelerometer data into distance measurements (calibration, noise correction, sampling rates, etc.) that have been shown to have good accuracy for such short distances? I have been able to find this, http://www.freescale.com/files/sensors/doc/app_note/AN3397.pdf but I'm not sure if this will apply well to the iPhone sensors.
I don't have any idea about Accelerometer in iPhone, but converting Accelerometer data into distance is fairly difficult because the offset in double integration causes drift as already told. Check this interesting presentation sensor fusion for further details.
As far as I  know this can be avoided by using Digital Accelerometers( with high sampling rates), double integration has to be performed using electronic integrators with appropriate filters at every stage to remove the offset as well as the low frequency noise.
Also in that presentation he mentions the possibility of using more sophisticated algorithms like use of Kalman Filter for object
tracking or Pedometer Algorithm for pedometer design.  

Hope this gives little more insight into your problem:)  

PS: This does not accurately address your question still I thought I will share my knowledge, I want to comment but I don't have enough reputation !