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A second great arc of triangulation, the [:TranscontinentalTriangulation:Transcontinental Arc] along the 39th parallel, was completed in 1899 and finally allowed the regional datums to be connected. In 1901, the CGS established the U.S. Standard Datum. The CGS reduced the impact of the new datum on the many charts and positions that had already been published by basing the new datum on the existing New England Datum, keeping station Principio fixed. At the same time, a new triangulation arc along the 98th meridian was being extended north and south of the Transcontinental Arc. In order to have a standard reference point near the intersection of those arcs, the station Meades Ranch was selected as the new standard point, after its position relative to Principio and the New England datum had been computed. A second great arc of triangulation, the [:TranscontinentalTriangulation:Transcontinental Arc] along the 39th parallel, was completed in 1899 and finally enabled the regional datums to be connected. In 1901, the CGS established the U.S. Standard Datum. The CGS reduced the impact of the new datum on the many charts and positions that had already been published by basing the new datum on the existing New England Datum, keeping station Principio fixed. At the same time, a new triangulation arc along the 98th meridian was being extended north and south of the Transcontinental Arc. In order to have a standard reference point near the intersection of those arcs, the station Meades Ranch was selected as the new standard point, after its position relative to Principio and the New England datum had been computed.

History of the North American Datums

Regional Datums

Before a national triangulation network connected the various portions of the U.S., regional datums had their own origins and standards for latitude and longitude, usualy determined by astronomic observations. The U.S. Lake Survey based their surveys on the station in Detriot. The U.S. Geological Survey based their surveys west of the 100th meridian on the station in Ogden, Utah, which had been surveyed by Lt. Walker in 1877. The coastal surveys of California were based on local astronomic observations, but when telegraphic longitude was determined at San Francisco's Washington Square station, it became the standard of longitude for the West Coast surveys.

In New England, the Harvard Observatory in Cambridge, Massachusetts was designated by the CGS as the "cardinal point of longitude" in 1845 and its position was determined by exchange of chronometers with Greenwich and by astronomic observation. In 1866 and 1867 the longitude of Calais, Maine and Harvard Observatory were determined telegraphically by exchange of signals with Europe over the first transatlantic telegraph cable. In 1872, the difference in longitude between Harvard Observatory and San Francisco was determined telegraphically.

New England Datum

Over the years from 1840 to 1878, a triangulation network, the Eastern Oblique Arc, was extended from Maine to Georgia and was connected to the coastal triangulations. Adjustments were made as additional observations were obtained. In 1863, latitude had been computed from 20 astronomic stations between Maine and Connecticut and was standardized at Blue Hill, Massachusetts. The azimuth from Blue Hill to Copecut was the standard for orientation. In 1867, longitude had been determined telegraphically at Harvard Observatory as previously mentioned, and was approved as a standard in 1869. The Bessel 1841 spheriod was used for all CGS charts.

By 1879, the Eastern Arc had been extended to Georgia and the additional observations merited another adjustment. The station Principio at the upper end of the Chesapeake Bay was approximately midway along that triangulation, and in 1879 the CGS performed a least squares adjustment of the 58 stations of the Arc and established a standard position for station Principio. All other stations were referenced to that station. This became the initial "New England Datum" used by the CGS for their coastal charts. The datum was specified in terms of the position of Principio, the azimuth from it to station Turkey Point, and the use of the Clarke 1866 spheriod as the standard surface.

Subsequent adjustments were done in 1880 and 1884 as more stations were connected to the Eastern Oblique Arc and telegraphic longitudes were expanded. During this time, station Principio remained the standard fixed point of the datum.

U.S. Standard Datum

A second great arc of triangulation, the [:TranscontinentalTriangulation:Transcontinental Arc] along the 39th parallel, was completed in 1899 and finally enabled the regional datums to be connected. In 1901, the CGS established the U.S. Standard Datum. The CGS reduced the impact of the new datum on the many charts and positions that had already been published by basing the new datum on the existing New England Datum, keeping station Principio fixed. At the same time, a new triangulation arc along the 98th meridian was being extended north and south of the Transcontinental Arc. In order to have a standard reference point near the intersection of those arcs, the station Meades Ranch was selected as the new standard point, after its position relative to Principio and the New England datum had been computed.

The 1901 U.S. Standard Datum was then specified in terms of the position of Meades Ranch, the azimuth from Meades Ranch to station Waldo, and the Clarke 1866 spheriod. Five thousand survey points were incorporated into the datum.

North American Datum

By 1913, the 98th meridian arc had been extended to both Mexico and Canada, and Canada and the U.S. had jointly established the eastern boundary of Alaska at the 141st meridian, and work was in progress on the 49th parallel survey of the Canada-U.S. boundary. Being involved with these geodetic collaborations with the U.S., Canada and Mexico were convinced of the merits of a continental datum, and they adopted the U.S. Standard Datum, which was renamed the North American Datum.

NAD27

Once the U.S. Standard Datum of 1901 had been established and perpetuated as the North American Datum, all subsequent triangulation work was fit into that network, and all closure errors were taken up by adjustments to the new work, leaving the stations of the 1901 datum unchanged. By 1925, it was apparent that the new work was requiring adjustment far in excess of what could be attributed to measurement error. A decision was made to perform a new adjustment that included all the completed first order triangulation. The work was divided into two parts -- the Western network, consisting of triangulation west of the 98th meridian, would be done first because minimal additional field work would be required before the adjustment could be done. The western field work was accomplished by 1927 and the necessary field work for the Eastern network was finished by 1931. The adjustments were largely complete by 1933.

The adjustments were the first to make use of Laplace stations to control azimuths and correct for deflections of the vertical. The datum of 1927 retained the position of Meades Ranch as the reference point, but its azimuth to station Waldo was changed based on the corrections from Laplace stations. The datum, instead of being oriented solely by the azimuth at one station, was now oriented by 175 Laplace azimuths throughout the network. The Clarke 1866 spheriod was retained as the best fit to the data.

NAD83

North American Datum (last edited 2010-08-28 18:19:18 by Holograph)