iPhone and iPad: Calibrating Compass
You may need to calibrate the compass on your iPad or iPhone 3GS or later before using Maps, Compass, or other apps that use this feature.
To learn more about Compass, see the user guide.
The built-in digital compass works like a magnetic-needle compass. The accuracy of digital compass headings can be affected by magnetic or other environmental interference, including interference caused by proximity to the magnets contained in the iPhone earbuds. The compass may need to be calibrated from time to time.
When the device displays the calibration alert, tilt the screen to move the red ball around in a circle. You should use the digital compass for basic navigation assistance, but you shouldn't rely on it to determine precise locations, proximity, distance, or direction.
Some areas have more magnetic interference than others. For example, your car's dashboard may have a high level of magnetic interference.
Note: It may need to periodically recalibrate while you're driving, depending on the level of magnetic interference present. If you wish to use the device in a vehicle, disregard the message to calibrate. The device will eventually recalibrate after several turns.
Warning: For important information about driving and navigating safely, see the user guide.