The Shape of Our Earth Our World is an Oblate Spheroid
The earth is not a perfect sphere. It is slightly flattened at the poles. The pole-to-pole diameter (12,714 kilometers) is slightly shorter than the equatorial diameter (12,756 kilometers) – a difference of 42 kilometers.
This slightly “squashed” shape is produced by the earth’s rotation. Spinning on its north-south axis – its axis of rotation - produces a small bulging-out at the equator and some compression at the poles due to centrifugal force.
The earth’s flattened spherical shape is not unique among planets. All spherical bodies that rotate on an axis produce some compression along that axis.
Because of the compressed or oblate shape of its polar plane, the earth is best considered an oblate spheroid rather than a perfect sphere.
A vertical cross-section through the earth along the polar axis has the shape of an ellipse.
By contrast, a horizontal cross-section through the earth along the equator, or any plane parallel to the equator, has the shape of a circle. Indeed, the equatorial (and any parallel) plane is for all practical purposes, a perfect circle. The deviation of the equatorial plane from a perfect circle is less than one kilometer (compared to its diameter of 12,756 kilometers) which, for virtually all practical purposes, is an insignificant error.
The perfect sphere model calculates latitude and longitude coordinates that can be several kilometres different from the actual location on the earth’s surface and is unacceptably inaccurate for purposes that require precision – geodetic calculations, navigation and so on.
Even an oblate spheroid is inadequate for certain purposes. The formula for an oblate spheroid produces a smooth curve. Clearly, the earth’s surface is not smooth. Land masses have mountains, hills, valleys and other irregularities. Water bodies have differing average water heights. For some specialised purposes, it is necessary to take into account the specific topography of the earth’s surface. The concept that seeks to adjust for these factors is known as the geoid.
The geoid is a model of the earth based on its surface, spanning both water bodies and land masses, being at mean sea level. In short, the geoid seeks to approximate mean sea level at all points on the earth's surface. By definition, the geoid is not directly observable; it is purely a hypothetical concept defined by large set of complex mathematical calculations. Its main practical purpose is to serve as a base line - a reference surface - from which topographic heights and depths are measured.
The scientific discipline concerned with defining and applying the geoid is known as geodesy.
It is important for users of electronic navigational aids, such global positioning systems (GPS), to be aware that they usually display results calculated on an oblate spheroid model rather than a geoid. This can result in the two different sources presenting the user with different readings for the height (depth) of given topographical feature above (below) mean sea level.