What to use with amcl to get the robot current Velocity, geometry_msgs or nav_msg/Odometry??

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I am not exactly sure what you really want to achieve but I have done something similar in my project.
I use actionlib to send goals to the robot and wait until the goal have been reached. Then I recorded the position of the robot for later analysis. You can see a part of the code below. It of cause requires that the whole navigation stack is running.

    goal.target_pose.header.frame_id = "/map"; 
    goal.target_pose.header.stamp = ros::Time::now(); 

    goal.target_pose.pose.position.x = 1.5;
    goal.target_pose.pose.position.y = -7.0; 

    angle = tf::createQuaternionFromRPY(0.0, 0.0, -M_PI_2 ); 
    goal.target_pose.pose.orientation.x = (float)angle.x(); 
    goal.target_pose.pose.orientation.y = (float)angle.y(); 
    goal.target_pose.pose.orientation.z = (float)angle.z(); 
    goal.target_pose.pose.orientation.w = (float)angle.w(); 

    ROS_INFO("Sending goal");
    ac.sendGoal(goal); 
    ac.waitForResult(); 

    try {

        listener.waitForTransform("/odom", "/base_link", ros::Time::now(), ros::Duration(0.5));
        listener.lookupTransform("/odom", "/base_link", ros::Time::now(), transform);  

    }
    catch( tf::TransformException ex ) {

        ROS_ERROR("%s", ex.what() ); 
    }

    ROS_INFO("Ground truth: %f %f", transform.getOrigin().x(), transform.getOrigin().y());