# Quaternion rotation around only two axes.

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Adam Levschuk on 11 Dec 2020
Commented: Amrtanshu Raj on 23 Dec 2020
Hello all,
I am working on a problem where i am using quaternions to put meaured accelerations into the global reference frame (North-East-Down). When I rotate the acceleraion vectors by the conjugate of the quaternions it does this perfectly. However, what if i wanted to keep the Heading (the yaw, or the k component on the quaterion) the same and not rotate my accelerations about this axis as to not change the heading? How would i go about doing this? is this possible?
Adam Levschuk on 11 Dec 2020
Aboslutly, here we go.
acc = quatrotate(quatconj(quat), acc);
quat is a 4 x m array where the first columns go [0 i j k] and describes the orinetation of the sensor relative to NED reference frame.
thank you

Amrtanshu Raj on 23 Dec 2020
Hi,
You can convert the quaternion to euler angles and then set the yaw(z axis) to zero and then convert it back to quaternion and do the required rotations.
quat = quaternion(0,.1,.2,.3) %Sample data
ang = quat2eul(quat) %get euler angles
ang_new = ang
ang_new(1) = 0 %set yaw to zero
quat_new = quaternion(eul2quat(ang_new)) %convert it back to quaternion
Hope this helps !!
##### 2 CommentsShowHide 1 older comment
Amrtanshu Raj on 23 Dec 2020
Directly getting the desired quaternion after eliminating rotation along a specific axis is not possible in Matlab. However, you can derive a set of equations using basic quaternion and vector calculations to get rotation quaternion after eliminating rotation along a specific axis.

R2020a

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