A reference datum is a known and constant surface which can be used to describe the location of unknown points. On Earth, the normal reference datum is sea level. On other planets, such as the Moon or Mars, the datum is the average radius of the planet.
The term “reference datum” was used rather than ‘above (or below) the earth’s surface’ or ‘above (or below) sea level’. The reason for this is simple once you think about it…If you use the term ‘above the earth’s surface’, what exactly does that mean? In other words, the earth’s surface where? Similarly, although we tend to think of sea level as a constant, it is not the same everywhere on the globe, so sea level where? and sea level when? (high tide or low) become pertinent questions. So, to avoid these problems, a reference datum is needed that represents the same surface or elevation at all points on the earth and that remains constant over time. An example of a datum that could be used for the earth is a sphere with a radius equal to the average radius of the earth.
Such a sphere would provide a constant surface to which elevations on the earth’s actual surface could be referenced. However, the earth is not a perfect sphere; the radius of the earth is greater at the equator and less at the poles. The resulting shape is what is known as an ‘oblate ellipsoid’. By using an oblate ellipsoid as a datum for the earth we have a shape that approximates the shape of the earth fairly well and provides a datum to which points all over the earth’s surface can be referenced (hence the term ‘reference datum’).
Most 7.5 minute topographic maps still in circulation use the NAD-27 (North American Datum, 1927) referencing system based on the Clarke ellipsoid of 1866. Technological advances that allowed more precise measurements of the earth resulted in modifications of the Clarke ellipsoid, producing the GRS-80 (Geographic Referencing System, 1980).
More recent maps commonly use the NAD-83 referencing system which is based on the GRS-80 ellipsoid. The datum used for a map is printed on the front of a map. Although the reference ellipsoids used in the NAD-27 and NAD-83 are different, the changes are slight on large-scale maps .