Harry's GPS Suite Forum

I assume the g force calculation is based on the iPhone's internal accelerometer. If so, does the placement of the iPhone within the car affect its recording of g force?

Deven

LapTimer uses the iPhone's (or Android phone's) internal accelerometer. All these accelerometers deliver values relative to the device's three axles of potential rotations. As long as the phone is mounted at least slightly upwards (i.e. not completely flat), LapTimer will detect gravity and driving forces so they get translated and aligned to the car's moving / force dimensions (lineal + lateral) and calculate values accordingly. Search for "calibration" here in the forum to find more information if needed.

- Harry

I'm not sure why the device orientation matters at all. You can determine the relationship of the three accelerometer axes to the vehicle frame of reference with two vectors, gravity with the vehicle level and stationary and vehicle acceleration or deceleration in a level straight line. With only a little more sophistication you don't even need straight line acceleration. You can use GPS yaw rate and vehicle velocity to calculate true lateral acceleration. Rather than throwing out the accelerometer data and re-calculating accelerations from the GPS data, you could have an option to use the GPS data to refine the accelerometer calibration. The XGPS160 seems to have precise enough elevation data to include vertical velocity too.