Earth ground is used as a reference from which all voltages are measured in systems which are earth grounded. Because the earth absorbs or dissipates nearly unlimited electric charge, it should always be at an ideal zero or ground potential. (Barring such problems as corrosion or improper grounding.)
I understand that high voltage transmission lines do not simply have a ground at each end of the system; that multiple ground connections are utilized throughout the length of the line to keep things consistent. This is because conductors are imperfect, etc.
My question is, if you could measure the potential difference between two distant points, both earth grounded, using an ideal superconductor, would there exist any difference (if so, why)? Is the "zero reference" on one continent the same as another? Common sense tells me it should be, it's the same earth. Someone asked me whether there would ever be a reason that they differ, and I couldn't say definitively.