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# What Is a Geodetic System?

E.A. Sanker
E.A. Sanker

A geodetic system, also sometimes known as a datum, is a system of coordinates used to describe locations on the earth. Since the earth is not a perfect geometric sphere or ellipsoid, geodetic systems must be used in cartography, surveying, and other related fields to create a fixed set of coordinates from which accurate maps can be created. The World Geodetic System (WGS) is a reference system in use worldwide. Global positioning system (GPS) devices measure location on the earth’s surface using this standard.

In order to navigate or create a map of locations on the earth’s surface, the geometry of that surface must be taken into consideration. A variety of geodetic systems were developed throughout the course of history to accomplish this purpose. It was only after the 18th century, when the earth’s shape was confirmed to be similar to an ellipsoid, that the systems began to take on their modern form.

Although the earth is geometrically irregular and not a perfect ellipsoid, a series of reference ellipsoids can be created to abstractly model its shape. Modern geodetic systems utilize this method. By overlapping several local ellipsoids, greater mathematical accuracy can be achieved in describing the earth's surface, and this allows for the creation of a standardized set of coordinates. In settings where only short distances must be measured, such as surveying or short-range navigation, a flat or spherical earth model can be used. Global navigation and mapping, however, require an ellipsoid-based geodetic system for acceptable accuracy.

Several geodetic systems are in use locally throughout the world. Some maps may use one of these local systems, rather than the global system, as a reference. Since each local geodetic system is defined differently, it is important to be aware of which is being used in a given map. Errors can result in faulty navigation.

The World Geodetic System (WGS) was first developed during the 1950s and 1960s to standardize geodetic data and create a worldwide system for mapping, navigation, and aviation. Advances in aeronautics and space travel encouraged the creation of this unified method for coordinating information. The last major revision of the WGS occurred in 1984, and this version was still in use as of 2011.

GPS receivers use the WGS in determining geographical location. These devices work by receiving signals from satellites orbiting the earth. By calculating the time it takes the signals to reach it, the GPS can determine its position on the earth’s surface. The coordinates the GPS displays are arrived at based on the standardized system of coordinates set up through the WGS.