A geodetic datum is the tool used to define the shape and size of the earth, as well as the reference point for the various coordinate systems used in mapping the earth. Throughout time, hundreds of different datums have been used - each one changing with the earth views of the times.
True geodetic datums however, are only those which appeared after the 1700s. Prior to that, the earth's ellipsoidal shape was not always taken into consideration, as many still believed it was flat. Since most datums today are used for measuring and showing large portions of the earth, an ellipsoidal model is essential.
Today, there are hundreds of different datums in use; but, they are all either horizontal or vertical in their orientation.
The horizontal datum is the one that is used in measuring a specific position on the earth's surface in coordinate systems such as latitude and longitude. Because of the different local datums (i.e. those having different reference points) the same position can have many different geographic coordinates so it is important to know which datum the reference is in.
The vertical datum measures the elevations of specific points on the earth. This data is gathered via tides with sea level measurements, geodetic surveying with different ellipsoid models used with the horizontal datum, and gravity, measured with the geoid. The data is then depicted on maps as some height above sea level.
For reference, the geoid is a mathematical model of the earth measured with gravity that corresponds with the mean ocean surface level on the earth- such as if the water were extended over the land. Because the surface is highly irregular however, there are different local geoids that are used to get the most accurate mathematical model possible for use in measuring vertical distances.
As previously mentioned, there are many datums in use around the world today. Some of the most commonly used datums are those of the World Geodetic System, the North American Datums, those of the Ordinance Survey of Great Britain, and the European Datum; however, this is by no means an exhaustive list.
Within the World Geodetic System (WGS), there are several different datums that have been in use throughout the years. These are WGS 84, 72, 70, and 60. The WGS 84 is one of the most widely used datums around the world.
In the 1980s, the United States Department of Defense used the Geodetic Reference System, 1980 (GRS 80) and Doppler satellite images to create a new, more accurate world geodetic system. This became what is known today as WGS 84. In terms of reference, WGS 84 uses what is called the "zero meridian" but because of the new measurements, it shifted 100 meters (0.062 miles) from the previously used Prime Meridian.
Similar to WGS 84 is the North American Datum 1983 (NAD 83). This is the official horizontal datum for use in the North and Central American geodetic networks. Like WGS 84, it is based on the GRS 80 ellipsoid so the two have very similar measurements. NAD 83 was also developed using satellite and remote sensing imagery and is the default datum on most GPS units today.
Prior to NAD 83 was NAD 27, a horizontal datum constructed in 1927 based on the Clarke 1866 ellipsoid. Though NAD 27 was in use for many years and still appears on United States topographic maps, it was based on a series of approximations with the geodetic center being based at Meades Ranch, Kansas. This point was chosen because it is near the geographic center of the contiguous United States.
Also similar to WGS 84 is the Ordinance Survey of Great Britain 1936 (OSGB36) as the latitude and longitude positions of points are the same in both datums. However, it is based on the Airy 1830 ellipsoid as it shows Great Britain, its primary user, the most accurately.
The European Datum 1950 (ED50) is the datum used for showing much of Western Europe and was developed after World War II when a reliable system of mapping borders was needed. It was based on the International Ellipsoid but changed when GRS80 and WGS84 were put into use. Today ED50's latitude and longitude lines are similar to WGS84 but the lines do become farther apart on ED50 when moving toward Eastern Europe.
When working with these or other map datums, it is important to always be aware of which datum a particular map is referenced in because often there are large differences in terms of distance between place to place on each different datum. This "datum shift" can then cause problems in terms of navigation and/or in trying to locate a specific place or object as a user of the wrong datum can sometimes be hundreds of meters from their desired position.
Whichever datum is used however, they represent a powerful geographic tool but are most important in cartography, geology, navigation, surveying, and sometimes even astronomy. In fact, "geodesy" (the study of measurement and Earth representation) has become its own subject within the field of earth sciences.