What units does the IMU message use?

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Quaternion is different from euler angles, but as always you can convert between one another.

You can check here for sweet and short basic of quaternion. Let me quote from it:

A quaternion rotation is made up of 4 numbers, whose values all have a minimum of -1 and a maximum of 1, i.e (0, 0, 0, 1) is a quaternion rotation that is equivalent to 'no rotation' or a rotation of 0 around all axis.

Euler angle to Quaternion Given an Euler rotation (X, Y, Z) using orthogonal axes:

x = sin(Y)sin(Z)cos(X)+cos(Y)cos(Z)sin(X)

y = sin(Y)cos(Z)cos(X)+cos(Y)sin(Z)sin(X)

z = cos(Y)sin(Z)cos(X)-sin(Y)cos(Z)sin(X)

w = cos(Y)cos(Z)cos(X)-sin(Y)sin(Z)sin(X)

• You don't need to implement those by yourself as there are also available functions in ROS api for converting from quaternion to euler and vice versa

You might also want to check :

• coordinate conventions used within ROS

• Quaternion orientation representation