This was misconception on my part, as i recalled from a documentary. Geoid is something different. There's good account on these pages. Interesting subject.
Sea level. That’s it! We’ll determine the shape of the earth by measuring the shape defined by the surface of the oceans which cover two thirds of the Earth’s surface. Good idea; but not very practical. The level of the oceans changes dramatically as the gravitational pull of the sun and the moon produce the phenomena known as tides. Weather patterns, ocean currents, and a whole host of other factors also contribute to a rather complicated picture. Thus, while we often talk about sea level, this concept is rather vague when we propose to accomplish some accurate measurements of the earth.
The answer geodesists have come up with is the concept of gravitational potential. That is, imagine a surface defined by all points where the force of gravity is the same, and which also approximates sea level. Visualize this surface as a three dimensional contour of all points at which the force of gravity is the same somewhat "arbitrary value". Then, adjust the "arbitrary value" so that the resulting surface approximates your best determination of the average level of the oceans. This surface, imaginary and somewhat subjective as it is, then is accepted as the surface which defines the shape of the earth.
Is this surface equivalent to an ellipsoid? Well, almost. It turns out that the material of which the Earth consists is not completely uniform throughout. Regions of denser material exists here and there intermingled with regions of material which is less dense. The gravitational force, which is dependent on mass, varies unpredictably from one place to another. The resulting shape, therefore, is a rather complex undulating shape with many peaks and valleys distributed over the entire surface of the Earth. However, geodesists have been able to establish an ellipsoid such that the height of the highest peaks and the depths of the deepest valleys are on the order of 100 meters from this best fitting ellipsoid.
So while the mathematically specific ellipsoid is a pretty good representation of the true shape of the earth, it is only a very good approximation. There is no mathematically defined surface which truly represents the shape of the earth; the earth is a unique shape all of its own. Geoid, a term derived from the Greek word geoeides meaning earthlike, is used to refer to the rather odd shape of the imaginary, somewhat subjective, surface which is taken to be the shape of the earth.