For more accurate measurements the iPhone's accelerometers, local vertical and orientation can be calibrated. For best results it is also recommended that a good vibration free mounting is used.
The raw iPhone acceleration values can show an error of about 10%. This can be corrected for by pressing the Accelerometer button at the bottom of the display and following the on screen instructions. Calibrated value bars are shown over the uncalibrated ones allowing you to judge if the calibation is still current. To check this move the iPhone as if calibrating it. The calibrated bar should line up with the white lines at zero and 1G.
When TrakPal starts and you press GO the local vertical is measured. Use this option to preset upright before pressing GO or if you have moved iPhone mounting once GO was pressed.
If your iPhone is orientated so that the home button points directly backwards in your car you will not need to set the orientation. However you may find that during constant acceleration, for example while going round long corners, the G display is not quite square. This can be corrected for by tilting the iPhone in the appropriate direction and pressing calibrate.
A red guidance line provides an indication of the direction of acceleration. The closer this line is to the outer ring, the more time has been spent accelerating in that direction. Driving at constant speed round corners should provide peaks to the left and right.
Alternatively having calibrated the accelerometers and local vertical with your car stationary on flat ground tilt the car to find the orientation (for example by parking on a steep hill.)
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