Andy,

> I am trying to create a nav_msgs/Odometry publisher, to be used by
> the robot_pose_ekf  node.  My robot only give me 3 numbers: x,y and theta
> while the nav_msgs/Odometry message needs covariance and many other data.
> My question is what variables correspond to the covariance matrices
> in nav_msgs/Odometry?

Check out the message documentation here:
<http://www.ros.org/doc/api/geometry_msgs/html/msg/PoseWithCovariance.html>.
You'll basically need a 6x6 matrix to represent the Gaussian
uncertainty on your robot pose. If you only have 3 values from your
odometry, you probably want to use high covariance values
corresponding to the other 3 values in the robot pose.  If you want to
see an example, you can start up a pr2 robot in Gazebo, and do a
rostopic echo on the output of the base_odometry controller.

Hope this helps,

Wim





-- 
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Wim Meeussen
Willow Garage Inc.
<http://www.willowgarage.com)