# What do x, y, and z denote in mavros topic /mavros/imu/data_raw

hi, I am using mavros to read the signals from the APM2.5. However I am not sure what x, y, z denote.

For example, in angular_velocity, I think the signals should be pitch, roll, and yaw in rad. How do x, y, and z correspond to them. Also, in linear_acceleration, what is the coordinates for x, y, and z, and are they in meter? Many thanks.

$rostopic type /mavros/imu/data_raw sensor_msgs/Imu$ rosmsg show sensor_msgs/Imu
uint32 seq
time stamp
string frame_id
geometry_msgs/Quaternion orientation
float64 x
float64 y
float64 z
float64 w
float64[9] orientation_covariance
geometry_msgs/Vector3 angular_velocity
float64 x
float64 y
float64 z
float64[9] angular_velocity_covariance
geometry_msgs/Vector3 linear_acceleration
float64 x
float64 y
float64 z
float64[9] linear_acceleration_covariance

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Also, how can I know the frequency of the signals. Many thanks.

( 2015-11-03 00:36:47 -0500 )edit

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In IMU message, x,y and z values under angular_velocity correspond to roll rate, pitch rate and yaw rate respectively. Because, roll is rotation around x axis, pitch is around y and yaw is around z. Angles are represented as quaternion under orientation . They are not Euler angles directly. You should convert them from Quaternion to Euler angles. Check Wikipedia for description about Quaternions: https://en.wikipedia.org/wiki/Quatern... .

linear_acceleration values should be represented as m/s^2 unless otherwise stated. You should check REP-103 ( http://www.ros.org/reps/rep-0103.html ) for dimension and coordinate standards in ROS.

For publish frequency of a topic you can use, rostopic hz /mavros/imu/data_raw

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Hi @Akif, thanks for your answer. I am confused about the quaternion representation for angular_velocity. I mean if the angular_velocity are under quaternion, there should be four values, namely x, y, z, and w, similar to the representation of orientation. Any hints on it?

( 2015-11-03 10:02:39 -0500 )edit

Only orientation is represented as quaternion, angular velocity is not. Its message type is geometry_msgs/Vector3 and consists of x, y and z only.

( 2015-11-03 10:32:37 -0500 )edit

Thx @Akif. That helps a lot.

( 2015-11-03 16:12:10 -0500 )edit

@Akif, BTW, what is the output of a APM2.5 connected GPS. Is it a global NED position or a local position? Many thanks.

( 2015-11-03 17:44:11 -0500 )edit