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. 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.ĭatum, National Geodetic Vertical, 1929 (NGVD 29)-A determination of the mean sea level datum that has been used as a standard datum for heights.
Usually identified by a proper name.ĭatum, mean sea level-A determination of mean sea level that has been used as a standard datum for heights or elevations. 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.ĭatum, local-The point of reference of the geodetic control used exclusively in a small area. A datum for a small area is usually called “local datum” and is given a proper name. The horizontal-control datum may extend over a continent or be limited to a small area.
Army Topographic Command publications for geodetic datums not described herein.ĭatum, 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. One should consult the National Geodetic Survey and the U.S. It forms the basis for the computation of horizontal-control surveys in which the curvature of the earth is considered. An assumed datum for elevation is sometimes used.ĭatum, 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. Others used are mean low water, mean lower low water, mean high water, mean higher high water. 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.ĭatum, elevation-Usually mean sea level as determined by hourly readings over an 18.6- or 19-year average. The adjusted datum is always parallel to the initial datum. 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.”ĭatum, 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. Datum- 1Any numerical or geometrical quantity or set of such quantities which may serve as a reference or base for other quantities.
0 kommentar(er)
