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*** POP QUIZ ***

  Is the center of the GROUND (prop) at the center of the Universe?**

No, it isn't. :-)

 Bizarrely, the ground prop itself is modelled at Y = -0.37499991 PNU. I have no idea why. Poser is hard-coded to load it at Y = -0.1, if I remember right. (EDIT: no, I just checked; it's -0.001) Again, I have no idea why.

The centre of the ground prop is at X = 0, Z = 0 however.