You can see in this picture that the glass flower vase is tilted, and that the water levels retain the same relationships.



However, a measurement from the top of the water to the unlevel cushion on which the glass flower vase is standing, will show a variety of distances.

This is the same as the voltage measurments in a typical pipe-to-soil potential survey. The potential of the pipeline metal is level but the potential of the ground is variable. However, we do not know the potential of either in relation to a common zero. This is shown by comparison to the water at the bottom of the glass flower vase tubes. We have nothing to compare this with, as it is tilted on an undulating cushion surface.