model states topic in gazebo

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Hi, I wanted to understand the information printed by the gazebo in the model_states topic of gazebo. "rostopic echo /gazebo/model_states" - prints this:

name: ['ground_plane', 'jackal'] pose:

position: 
  x: 0.0
  y: 0.0
  z: 0.0
orientation: 
  x: 0.0
  y: 0.0
  z: 0.0
  w: 1.0

position: 
  x: 0.0664383614036
  y: 0.00389383920109
  z: 0.0634964523388
orientation: 
  x: -1.04535799417e-07
  y: -1.3819443258e-05
  z: -0.00528291143972
  w: 0.99998604523

From this, it looks like, the first set of position information is related to the ground_plane of the gazebo. The second set of position information is for the jackal. The first set of position information always shows "0.0" for position. Why is that so? The second set of positions increases as the jackal moves which is to be expected.

If the model_state topic of gazebo gives the position of the robot in the gazebo world frame already, is it necessary to transform the co-ordinates between frames? Can we just use the position information provided by the second set of position values as the robots position with respect to the world frame at that point?

Please help.

Regards, rsmitha.