We call geoid to theoretical almost spherical shape assumed by planet Earth, in which the average level of the seas that run through it will be taken as the surface. It is spoken in an almost spherical way because there is a slight flattening at both poles, given by the equipotential surface of the Earth's gravitational field that coincides with the mean level of the seas. So if we consider the crust, the earth will not be one hundred percent a geoid, although it will be if it is represented with the mean level of the tides.
The idea of the earth as a geoid was anticipated by the scientist Isaac Newton in his work Principia in the year 1687. Newton would demonstrate it through a homemade exercise: if a viscous body is rotated rapidly in a liquid fluid, the equilibrium form that the mass will present under the design of the law of gravity and rotating around its own axis will be a spheroid flattened at their respective poles.
Meanwhile, Newton's proposal would be studied and verified in situ some time later by Domenico and Jacques Cassini; both carried out an exact measurement of the difference of one degree in the vicinity of the equator and compared the differences with European latitudes. Mathematical and geometric work carried out later, would also confirm the form originally proposed by Newton.
The shape of the geoid can be determined by: gravimetric measurements (measuring the magnitude of the intensity of gravity at various points on the surface of the earth. As a consequence that it is a flattened sphere at its poles, the acceleration of gravity will increase from the Equator to the Poles), astronomical measurements (They measure the vertical of the place in question and wait for its variants. The variation will be related to the shape) and measurement of the deformations produced in the orbit of the satellites caused by the fact that the earth is not homogeneous.
