CODEWITHSUNDEEP
androidandroid-sensorsmagnetometer
Nielg Woo
Nielg Woo

Reputation: 21

How to transform Android magnetometer readings to the the world's coordinate space?

I am writing some programming on the Android sensors, where I am confused by the readings of magnetometer sensor.

Magnetometer reports the magnetic strengths on the three axes of the phone. And I observe that at a same location, if the phone's heading changes, the magnetic readings dramatically change. In my understand, however, the earth's magnetic field at a specific location should be relative stable, regardless of the phone's placement gesture.

So, my question is, is there any way to transform the raw readings from the 3-axis magnetometer sensor to the world's coordinate system? The accelerometer and orientation data are also available on mobile phones. If so, I suspect the transformed magnetism should be the same even the phone's heading direction changes.

I have referred to the Android source codes, specially, the getOrientation() function and the getRotationMatrix() function. I hoped to get some help from their code implementation. But I did not understand very well. Could someone give any explains on the algorithm principle of these functions?

Link to the code of the functions: http://www.netmite.com/android/mydroid/cupcake/frameworks/base/core/java/android/hardware/SensorManager.java

Thanks! I am really anxious to the solution to this question.

Upvotes: 2

Views: 1671

Answers (1)

Marius
Marius

Reputation: 113

This is impossible, since the device does not know its orientation in world space.

Of course, the orientation can be guessed by the sensor input, and that is what getOrientation() and getRotationMatrix() do. However, on a long timescale only the measurement of acceleration (by gravity) and the magnetic field provide the necessary information. Gyroscope data can be used to refine the estimate for shorter periods, but getOrientation is not guaranteed to use it, and maybe that sensor is not even existent on the particular device. This means backtransforming using getOrientation would use the exact same data which you want to correct, rendering it useless.

Upvotes: 0

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