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Information on message sensor_msgs/Imu

Hi guyz,

I am new with ROS, and I am trying to use the output of the hector_mapping and my IMU data and fuse them into the robot_pose_ekf filter.

I wanted to have more information on the message units and frame of the data.

1)The angular_velocity is in rad/s or in deg/s ? 2)The linear acceleration are in g or in m/s^2 ?

3)What is the meaning of the covariance matrix? Shouldn't it be called the variance matrix? By definition :

The variance is a measure of how far a set of numbers is spread out.

The covariance is a measure of how much two random variables change together.

Since the measure of, for exemple, the acceleration, only depends on the accelerometers in the IMU, there is no point of using the covariance between the accelerometers, the gyrometers and the magnetometers since they are not related ...

Also, what are the units of the different covariance matrix?

I have also a suggestion: It could also be great documentation if each message description explained those information and not only the data types.