in this tutorial 
http://www.ros.org/wiki/tf/Tutorials/Writing%20a%20tf%20listener%20%28C%2B%2B%29
Writing a tf listener 
i wanted to know what transform.getOrigin().x() and
transform.getOrigin().y() were.
first i just thought that they were the difference in the x and y
coordinates of turtle1 and turtle2. 
but i found out that the values are different.( by adding the line
"ROS_INFO("net displacement is /turtle1-/turtle2: (%.4f, %.4f, %.4f)",
transform.getOrigin().x(),transform.getOrigin().y() ,
transform.getOrigin().z());" to the turtle_tf_listener.cpp and running it
separately)
so what exactly are they.

and what is Quaternion w, x, y, z and how is it related to the turtle. i
guess it represents the turtles orientation(direction it faces) since its
position is already represented by its coordinates.
and in this 
http://www.ros.org/wiki/tf/Tutorials/Writing%20a%20tf%20broadcaster%20%28C%2B%2B%29
Writing a tf broadcaster   "transform.setRotation(tf::Quaternion(msg->theta,
0, 0));" how does theta change to w and z.

and finally i guess u represent btQuaternion as x,y,z,w rather than w,x,y,z
as rostopic echo /tf prints them that way.

even though my questions look simple(silly) please do answer them. still
there's no reply for my other queries 
http://ros-users.122217.n3.nabble.com/navigation-stack-tp923979p923979.html
navigation-stack 

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