The geoid is the shape that the surface of the oceans would take under the influence of Earth's gravitation
and rotation alone, in the absence of other influences such as winds
and tides. This surface is extended through the continents (such as with
very narrow hypothetical canals). All points on the geoid have the same
gravity potential energy (the sum of gravitational potential energy and
centrifugal potential energy). The force of gravity acts everywhere
perpendicular to the geoid, meaning that plumb lines point perpendicular and water levels parallel to the geoid.