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try to subscribe imu topic with rostopic echo :

---
header:
seq: 288
stamp:
secs: 1330631562
nsecs: 789304161
frame_id: xsens_mti_imu
orientation:
x: 0.00401890464127
y: -0.00402884092182
z: 0.679586052895
w: 0.73357373476
...
---

If you observe the orientation field, you will see four variables instead of three, as you would probably expect. This is because in ROS, the spatial orientation is represented using quaternions. You can find a lot of literature on this concise and nonambiguous orientation representation on the Internet.

try to subscribe imu topic with rostopic echo :

---
header:
seq: 288
stamp:
secs: 1330631562
nsecs: 789304161
frame_id: xsens_mti_imu
orientation:
x: 0.00401890464127
y: -0.00402884092182
z: 0.679586052895
w: 0.73357373476
...
---

If you observe the orientation field, you will see four variables instead of three, as you would probably expect. This is because in ROS, the spatial orientation is represented using quaternions. You can find a lot of literature on this concise and nonambiguous orientation representation on the Internet.quaternions.