geoid

The problem of the form and dimensions of the sea level surface of the earth has been one of peculiar difficulty. The combined efforts of the ablest mathematicians of the past two centuries, supplemented by the most laborious and costly geodetic measurements have yielded us the first approximation only to the complete solution. […] This spheroid, or reference ellipsoid, as it is sometimes called, has its minor axis coincident with the earth's axis of rotation and is usually regarded as sensibly fixed in position and dimensions. With respect to it the actual sea surface or geoid must be imagined to lie partly above and partly below by small but unknown amounts, the determination of which, if possible, will constitute a second approximation to the figure of the earth.