Which coordinate system does the DMP quaternion for the MPU-9250 initalise in?

[andymichael09]

When I use your library to fetch the DMP quaternion, I get different values in different initialising orientations. So, I was wondering which coordinate system the DMP quaternion for the MPU-9250 initalise in? Was thinking that it might just initialise with respect to a gravity vector. Could you confirm whether this is the case/provide any insight?

[gonzaarcr]

Z is vertical, always, but the plane XY (paralel to the ground) is moving in my case, I think is because the gyro drift. It probably uses the gravity to put it in a more accurate frame, but because it is perpendicular to the ground it can't do anything with that playing.

I don't know why it doesn't use the magnetometer to adjust that; probably because an external magnetic field would broke everything, but I'm just guessing.