Data Standards - Datum
 

A datum is a mathematical model of the earth used to calculate the coordinates on a map, chart, or survey system. This includes a set of defining parameters, which forms the basis for computing positions on the surface of the earth. These parameters include the dimensions of a reference ellipsoid and the coordinates of a point of origin. Most datums are created for use only in specific areas of the earth, but the World Geodetic Systems (WGS) can be used globally.

Datums are important because to accurately represent horizontal positions on maps and charts, we need a mathematical model of the earth that takes into consideration the size and shape of the earth. It is important to know which datum is being used on a map because the coordinates for a point on the earth’s surface in one datum will not match the coordinates from another datum for that same point. These different systems can be translated into each other very rapidly through the use of sophisticated computer algorithms.

Geodetic datums are a part of geodesy [See Table 1]. Prior to World War II, geodesy was primarily concerned with the creation of national control surveys each of which had it's own geodetic datum. This created a lack of accuracy and made inconsistencies with actual location of a point on the earth. There were so many different datums in use that a same position could be found in different locations on a map. Therefore, new datums were created which covered larger areas and which have a higher accuracy. (Maling,1989)

Table 1: Datums and their principle areas of use

Datum

Area

Origin

Ellipsoid

WGS 1984

Global

Earth center of mass

WGS 84

NAD 1983

North America, Caribbean

Earth center of mass

GRS 80

NAD 1927

North America

Meades Ranch

Clarke 1866

European 1950

Europe, Middle East, North Africa

Potsdam

International

 

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