# Datum Definitions for Land Surveyors

datum-1Any numerical or geometrical quantity or set of such quantities which may serve as a reference or base for other quantities. For statistical references, the plural form is “data,” e.g., “geographic data for a list of latitudes and longitudes.” For particular geometrical concepts, such as “several geodetic datums have been used in the United States,” the correct plural form is “datums.” 2 A level surface to which elevations are referenced, e.g., “mean sea level.”

datum, adjusted—The datum (reference surface) of a system of plane coordinates in a raised or lowered position from the initial datum on which the plane coordinate system was initially projected by mathematical procedures. The adjusted datum is always parallel to the initial datum. Position of the adjusted datum, above or below the initial datum, is governed by the degree to which reduction is made in the inherent differences between distances measured on the ground, and distances computed using plane coordinates of points and features delineated on maps compiled, or survey distances computed and/or measured on the adjusted datum.

datum, elevation—Usually mean sea level as determined by hourly readings over an 18.6- or 19-year average. Others used are mean low water, mean lower low water, mean high water, mean higher high water. An assumed datum for elevation is sometimes used.

datum, geodetic—A datum consisting of five quantities: the latitude and the longitude of an initial point, the azimuth of a line from this point, and two constants necessary to define the terrestrial ellipsoid. It forms the basis for the computation of horizontal-control surveys in which the curvature of the earth is considered. One should consult the National Geodetic Survey and the U.S. Army Topographic Command publications for geodetic datums not described herein.

datum, horizontal—The position on the ellipsoid of reference assigned to the horizontal control (triangulation and traverse) of an area and defined by 1) the position (latitude and longitude) of one selected station in the area, and 2) the azimuth from the selected station to an adjoining station. The horizontal-control datum may extend over a continent or be limited to a small area. A datum for a small area is usually called “local datum” and is given a proper name. The horizontal-control datum primarily used on the North American continent is the North American Datum of 1983 (NAD 83), which is based on the Geodetic Reference System of 1980.

datum, local—The point of reference of the geodetic control used exclusively in a small area. Usually identified by a proper name.

datum, mean sea level—A determination of mean sea level that has been used as a standard datum for heights or elevations. The Sea Level Datum of 1929 (now called the National Geodetic Vertical Datum of 1929) was based on tidal observations over a number of years at various tide stations along the coasts of the United States.

Datum, National Geodetic Vertical, 1929 (NGVD 29)—A determination of the mean sea level datum that has been used as a standard datum for heights. The sea level is subject to some variations from year to year, but, as the permanency of any datum is of prime importance in engineering work, a sea-level datum should, after adoption, be maintained indefinitely even though it may differ slightly from later determinations of mean sea level based on longer series of observations. See also Datum, North American Vertical, 1988 (NAVD 88); mean sea level; datum, tidal.

Datum, New England—Identical with the horizontal geodetic datum for which the United States standard datum was adopted in 1901, whose name was later changed to North American datum. This datum was defined by the geographic position of the triangulation station Principio and the azimuth from that station to station Turkey Point, on the Clarke ellipsoid of 1866:

Source: U.S. Coast and Geodetic Survey Report for 1879, Appendix 8, pp. 112-114

Datum, North American—The horizontal geodetic datum which is defined by the following geographic position of triangulation station Meades Ranch and the azimuth from that station to station Waldo, on the Clarke ellipsoid of 1866:

The North American datum is identical to the United States standard datum.   It’s name was changed in 1913, when its adoption by the governments of Canada and of Mexico for their control surveys gave it an international character.  See also datum, North American, 1927 (NAD 27); datum, North American, 1983 (NAD 83).

Datum, North American, 1927 (NAD 27)—This datum is identical to the North American datum at triangulation station Meades Ranch, except that the azimuth from Meades Ranch to Waldo was changed to 75° 28′ 09.64″. The datum was adopted in 1927 after a readjustment of the triangulation of the entire country in which Laplace azimuths were introduced. See also datum, North American; datum, North American, 1983 (NAD 83).

Datum, North American, 1983 (NAD 83)—A horizontal control datum differing from the NAD 27 in that its reference system is based on a geocentric origin rather than the station at Meades Ranch, and its reference ellipsoid is the Geodetic Reference System 1980 (GRS 80), rather than the Clarke Ellipsoid of 1866. NAD 83 is based on an adjustment including more than 250,000 points whose locations were precisely located after 1927. See also Geodetic Reference System 1980 (GRS 80).

Datum, North American Vertical, 1988 (NAVD 88)—A vertical control datum determined by a minimal constraint adjustment of leveling observations taken throughout Canada, the United States and Mexico. The height of the primary tidal benchmark at Father Point/Rimouski, Quebec is held fixed.

Datum, Sea Level, 1929—Renamed in 1976 to the National Geodetic Vertical Datum of 1929 to distinguish between a local mean sea level and a national geodetic vertical datum [Federal Register, May 17, 1976, Vol. 41, No. 96, p. 20202]. See Datum, National Geodetic Vertical, 1929 (NGVD 29).

datum, state plane coordinates—The surface onto which each point of concern is transferred mathematically from the corresponding point on the ellipsoid representing the Earth to give its map position. For illustrative purposes, the Lambert conformal projection datum is thought of as being represented by a cone and the Transverse Mercator projection datum by a cylinder, after each has been rolled out flat, or as a plane for other projections. Projection of distances between points on the ground to the datum is a two-stage process, first from the surface of the ground to the ellipsoid and, second, from there to the datum. Thus, the map, in effect, is a scale reproduction of the cone or cylinder rolled out flat. These datums are below the sea level arc of the earth between the lines of intersection of the cone or the cylinder with the ellipsoid, and are above the sea level arc for all segments of their surface outside such lines of intersection with the ellipsoid.

datum, tidal—Specific tide levels which are used as surfaces of reference for depth measurements in the sea and as a base for the determination of elevation on land. Many different datums have been used, particularly for leveling operations. Also called tidal datum plane. See also Datum, Sea Level, 1929; datum, National Geodetic Vertical, 1929 (NGVD 29); datum, North American Vertical, 1988 (NAVD 88).

datum, vertical—Any level surface (as for example, mean sea level) taken as a surface of reference from which to reckon elevations. Although a level surface is not a plane, the vertical datum is frequently referred to as the datum plane; datum level, reference level, reference plane, vertical control datum, vertical geodetic datum.

datum, United States Standard—See Datum, North American; Datum, North American, 1927 (NAD 27); Datum, North American, 1983 (NAD 83).

datum plane—A surface used as a reference from which to reckon heights or depths. Called “tidal datum” when the plane is defined by a certain phase of the tide. The datum in most general use is based upon mean sea level and this is used as the reference for the first-order level net extending over the whole country. For hydrographic work, including soundings on charts and tidal predictions, a low-water datum is preferred. For this purpose the datum adopted is mean low water for the Atlantic coast of the United States and lower low water for the Pacific coast of the United States, including Alaska and island possessions. In many other parts of the world low water springs are used for hydrographic purposes. In order that they may be recovered when needed, datum planes are referenced to fixed points known as bench marks.

Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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