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The Earth absolute potential it is not so important but only the voltage drop and the potential gradient since E[Electric Field Intensity]=-grad(V).

If an object is connected between two points of different potential the current passing through the resistance between these two points could be deadly.

The resistance between these 2 points depends on grounding resistance of the grounding electrode in each point [or grounding grid]- which is just a contact resistance between the electrode and the ground. The ground resistance between electrodes is considered 0 [somewhere I found this equation rE=π^2.freq/10^4 Ω/km].More important is the “depth” of this ground considered a conductor buried at this depth [according to Carson] in order to calculate the reactance from a wire to ground.

If we measure the ground resistivity the second spike could be anywhere in vicinity-10 fits. is good enough, for instance. So the infinite [where potential=0] it could be 10 ft. from the measuring point[!].