Understanding spheroids

A geographic coordinate system attempts to model the shape of the earth as accurately as possible. Many models of the earth's shape have been made over the years, and each has its own geographic coordinate system. All are based on degrees of latitude and longitude, but the exact latitude-longitude values assigned to individual locations will vary.

Two shapes that are commonly used to model the earth are a sphere and a spheroid.

The shape of the earth can be approximated by a sphere or a spheroid.

Assuming that the earth is a sphere greatly simplifies mathematical calculations and works well for small-scale maps (maps that show a large area of the earth). A sphere does not provide enough accuracy, however, for large-scale maps (maps that show a smaller area of the earth in more detail). For those, it is preferable to use a spheroid.

A spheroid is a more accurate model of the earth, but it's not perfect.

More about the shape of the earth

Planet Earth is slightly pear-shaped and bumpy. Satellite technology has revealed several dents and undulations. In addition, the south pole is closer to the equator than the north pole. There is a model for this complicated shape—it's called the geoid.

The geoid is too mathematically complicated to use for practical purposes, so the spheroid is used as a compromise.

Different spheroids are currently in use, in part because newer technology has provided more accurate measurements of the earth's shape. Some spheroids were developed to model the entire earth, while others were developed to model specific regions more accurately.

For example, the World Geodetic System of 1972 (WGS72) and World Geodetic System of 1984 (WGS84) spheroids are most commonly used to represent the whole world, while in North America, the Clarke 1866 and Geodetic Reference System of 1980 (GRS80) spheroids are most commonly used.

Why do you need to know about spheroids? Because ignoring deviations and using the same spheroid for all locations on the earth could lead to measurement errors of several meters or, in extreme cases, hundreds of meters.