![]() ![]() 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. By looking at the movement of markers in the horizontal datum, geodesists can determine just how much the surface of the Earth has moved after an earthquake.Datum- 1Any numerical or geometrical quantity or set of such quantities which may serve as a reference or base for other quantities. The fault is approximately 800 miles long and extends 10 miles below the Earth's surface. Extending from northern California south to San Bernardino, the San Andreas Fault is where two plates of the Earth's crust meet. This type of monitoring is often used in places like the San Andreas Fault in California where many earthquakes occur. One application of the horizontal datum is monitoring the movement of the Earth's crust. The new datum, called NAD 83, is the most commonly used horizontal positioning datum today in the United States. In 1983, NAD 27 was adjusted to remove inaccuracies and to correct distortions. ![]() This datum was used extensively during the next 60 years as the primary reference for horizontal positioning. Coast and Geodetic Survey, the predecessor of the National Geodetic Survey, "connected" all of the existing horizontal markers together and created the North American Datum of 1927 (NAD 27). Learn more and view a larger version of this image. ![]() When positioning markers are located on flat ground, large towers such as this were often erected above them allowing geodesists to observe the monument's accurate position over large distances. As technology has improved, surveyors now rely almost exclusively on the Global Positioning System (GPS) to identify locations on the Earth and incorporate them into existing datums. To "connect" the horizontal markers into a unified network, or datum, surveyors have used a variety of methods, including triangulation. When markers were set on flat land, towers were built above them to aid surveyors in locating them. To maximize the line-of-sight between markers, they were usually set on mountaintops or at high elevations. These markers were placed so that surveyors could see one marked position from another. To create the horizontal datum, or network of horizontal positions, surveyors marked each of the positions they had identified, typically with a brass, bronze, or aluminum disk or monument. At its most basic level of definition, the horizontal datum is a collection of specific points on the Earth that have been identified according to their precise northerly or southerly location (latitude) and easterly or westerly location (longitude). ![]()
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