Digital Compass Calibration
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Most new smartphones contain electronic magnetometer,
gyroscope, and accelerometer chips that, either
individually or when combined, provide orientation
information about the device. These electronic
components vary in quality and are subject to a number of
limitations. For example, magnetometers can be strongly
influenced by the magnetized iron, steel or other
permanent magnets in the device (the hard iron effect),
as well as being influenced by external magnetic material
in the natural and built environment. Gyroscopes often
suffer from gyro-drift – an error caused by the
accumulation of the unique bias present in each
gyroscope. We can counteract these limitations by
attempting to calibrate the sensors, or using a
combination of their output data to provide an improved
result.
However, some smartphones perform more consistently than others. This variation between
devices can be caused by:
differences in phone hardware and firmware
differences in the built-in noise filtering and sampling frequency
the different sensor combinations (sensor fusion) or compensation algorithms
There are also differences between the mobile operating systems and how they allow
developers to access and manipulate the sensors available on each device.
We are reluctant to recommend specific devices to run FieldMove Clino, however there are
significant variations in the quality of the components that are currently being used by the
various manufacturers, and we feel strongly that we should make users aware of some of
our recent findings.
Calibration for iPhones
The iPhone 4S, 5 and 5C contain good quality components and the compass readings are
generally accurate to ±1 degree – better than can be achieved with a traditional hand-held
Brunton or Silva compass. Unfortunately the magnetometer/gyroscope/accelerometer
combination in the iPhone 5s is of lower quality and the compass is only accurate to ±5
degrees. This is a known issue which has been widely documented online and in the press.
Apple have produced a firmware fix for this issue but we have not yet had the opportunity
to test the device with the installed update.
To calibrate, Apple phones display a black
screen with a small red ball that must be rotated within the confines of a graduated circle.
The compass clinometer within FieldMove Clino will not become active until this operation
has been performed.
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