DATUMS

A datum is any numerical or geometrical quantity or set of quantities that serves as a reference or base for other quantities. In surveying, mapping, and charting, datums are essential for correlation of measurements and for determining and portraying relative positions and elevations (or other characteristics) of points at different locations.

The two basic types of survey datums are horizontal and vertical. A datum is defined by quantities (for example, position, elevation, direction, a reference surface) at an initial point from which measurements are made to other points. The reference surface, particularly for the horizontal datum, usually is not the physical surface of the Earth but some easily defined mathematical surface such as an ellipsoid.

HORIZONTAL DATUMS

A horizontal datum is defined by five quantities: the latitude and longitude of an initial point; the azimuth of a line from that point; and two constants that define the reference ellipsoid most nearly approximating the Earth in the area of interest. In the conterminous United States and Alaska, the current horizontal datum is the North American Datum (NAD) of 1927; its reference surface is the Clarke Spheroid of 1866. The following datums are used for the State of Hawaii and for the Territories, all on the Clarke Spheroid of 1866:

of other points on the same datum, tying them into a unified survey.

VERTICAL DATUMS

A vertical datum is the surface to which heights, elevations, or depths are referred. It can be a leveling datum, a tidal datum, a chart datum, or a geodetic vertical datum.

Tidal datums

The simplest datum is the tidal datum—a base elevation defined by a certain phase of the tide (fig. 1) and used as a reference from which local heights and depths are reckoned. Local datums should not be extended into adjacent areas where a different tidal regime may exist. Mean high water, mean low water, and mean lower low water (fig. 2), are the tidal datums most significant to mapping and charting in the coastal zone. Local mean sea level (that is, mean sea level at a tide station) is the basic tidal datum for a harmonic analysis of observations at that station. It does not have a direct relationship to either charting or mapping. Half-tide level (fig. 3) is halfway between mean high water and mean low water and is computed for each tidal station. However, it also has no direct effect on mapping and charting. Mean sea level and half-tide level may be equal or may differ depending on the tidal characteristics at the specific location.

International Great Lakes Datum

The International Great Lakes Datum (IGLD) of 1955, established jointly with Canada, is the basic level datum for charting water depths of the Great Lakes and connecting waterways. The datum is used by NOS and the Corps of Engineers in Great Lakes operations that require a vertical datum. Although some bench marks are common to the IGLD of 1955 and the National Geodetic Vertical Datum (NGVD) of 1929, the two datums are not the same and cannot be converted mathematically.

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