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Brief History of North American Horizontal Datums New England Datum of 1879 United States Standard Datum of 1901 North American Datum of 1913 North American Datum of 1927 North American Datum of 1983
The first transcontinental arc of triangulation was begun in 1871 and completed in 1899. It connected the east coast network with independent triangulation networks on the west coast.
The New England Datum of 1879 had been extended to the west and south without major readjustments of the surveys in the east.
Ellipsoid – a slightly flattened sphere – an oblate spheroid of revolution – an ellipse that is rotated about it’s shorter axis.
The earth is not an exact ellipsoid – no single ellipsoid will match the earth’s surface.
Originally the method was to measure ‘arcs of meridians’ in various regions of the world, and create different reference ellipsoids, that ‘best fit’ the regional curvature of the earth, and use these for mapping datums. These are called Regional Datums .
GEOID – An abstract surface – the surface of the earth if it’s surface was equal to sea level everywhere . This is an undulating surface that varies no more than ~ 100 meters above or below a well fitting ellipsoid.
It is the equipotential surface of the earth’s gravity field which best fits global mean sea level .
It was first mathematically defined by Stokes in 1849, when he computed the “surface of the earth’s original fluidity” from surface gravity measurements.
The GEOID is constantly being revised – due to sea level changes, tectonics, and better measurements of the earth’s gravity field.
We now use GEOID 99 & GEOID 03 for our work in the U.S.
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Ellipsoid – Geoid surfaces in comparison with local topography
Meades Ranch in Osborne County, Kansas was chosen because it is very close to the geographic center of the lower 48 states, and the station was also in the 1899 arc of triangulation.
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New England Datum based on Clarke 1866 ellipsoid
Ellipsoidal Parameters
Clarke 1866
a = 6378206.40 meters
b = 6356583.80 meters
f = 1 / 294.978698 meters
‘ f’ the degree to which the ellipsoid is flattened to match the earth’s shape.
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Geodetic Latitude & Longitude are converted to Planar Coordinates for Triangulation and extension of the control network.
Originally – early planar coordinate systems were tangent plane coordinates, and the geodetic positions were projected onto various map projections which fit into one of 5 zones.
The National Geodetic Survey developed the State Plane Coordinate System in 1933. Every state is now covered – separate systems are assigned to zones. Each zone uses it’s own projection and coordinate center. 120 zones cover the U.S.
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Preparing a station for observations – North Dakota - 1913
Since 1927 ~ 99,000 stations in the U.S., Canada, Mexico, and Central America have been added to the network, and ‘forced’ to fit into the adjustment.
Control in Alaska was connected to NAD-27 during World War II by single arc triangulation along the Alaskan Highway.
By the 1940’s through the 1950’s, technological advances in surveying & geodesy revealed weaknesses in the existing network of control points based on NAD-27.
Differences were noticeable when linking existing control with newly established surveys.
The North American Datum of 1983 (NAD-83) is based upon terrestrial & satellite observations. [triangulation –trilateration, satellite positioning with doppler methods & GPS.]
The origin of NAD-83 is the earth’s center of mass. It is a geocentric datum.
NAD-83 is referenced to the Geodetic Reference System of 1980 ellipsoid (GRS-80).
The World Geodetic System (WGS-84) and GRS-80 are very similar, the origin and parameters for both are nearly identical.
Positions referenced to WGS-84 will differ from NAD-83 values by ~ 1mm in 15 km.
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There are differences in the two datums ranging from ~100m in the western U.S., to ~1m – 3m in the central and eastern U.S. as show below.
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Readjustment of the North American Datums NAD-27 to NAD-83
Geodetic positions of control points can change between 10m – 100m within the United States – more than 200m in Alaska – and more than 400m in Hawaii.
The shifts between the two datums is not uniform across the United States – and there is no single value that can be applied to convert Latitude & Longitude in NAD-27 to Latitude & Longitude in NAD-83.
NADCON was developed in order to help in conversions between the two datums.
The grids used by NADCON is based on more than 150,000 horizontal control points.
CORPSCON - U.S. Army program that provides datum transformation in the Continental U.S. It is based on NADCON & VERTCON software developed by the Nation Geodetic Survey.
CORPSCON converts from State Plane Coordinates or Geographic coordinates from NAD-27 to NAD-83, or NAD-83 to NAD-27
Also converts vertical datums as well, between NGVD-29 & NAVD-88.
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Datum Transformation Parameters from NAD-27 to NAD-83
NAD-27 (General) Dx = -5 m Dy = 135 m Dz = 172 m
NAD-27 (CONUS) Dx = -8 m Dy = 160 m Dz = 176 m
NAD-27 (East U.S.) Dx = -9 m Dy = 161 m Dz = 179 m
NAD-27 (West U.S.) Dx = -8 m Dy = 159 m Dz = 175 m
Local distortions in the NAD-27 datum were adjusted, so the magnitude of grid shift vales varies from region to region. [ Local grid shift values are more precise .]
Datum shifts are necessary to convert from one geodetic coordinate system based on a one ellipsoid, to another geodetic coordinate system referenced to a completely different ellipsoid.
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