In NDOF mode, after magnetic calibration is complete the heading will be 0 degrees when facing magnetic North. Since you are complaining about "not getting absolute values", then I would check the quality of the magnetic environment. For indoor robots like this, we recommend using the IMU mode, and mitigating the integration error by alternating left and right turns when possible. alternatively, the robot can keep track of the number of complete rotations and do a 360 degree rotation backwards.
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