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

New England Datum of 1879

• U.S. Congress created the Survey of the Coast on February 10, 1807.

• First field surveys carried out in 1816 – 1817, near New York City.

• First surveys were concerned with coastal charts, coastal hazards, and the mapping of harbors for navigation & commerce.

• Triangulation networks were extended to Dauphin Island, Alabama, and then west to New Orleans, Louisiana between 1833 – 1898.

Fire Island Triangulation Network – Long Island, New York – Point Judith, Rhode Island. 1851

U.S. Coast Survey - Chart of Boston Harbor – 1857 – Based on early New England Datum.

Transcontinental Arc of Triangulation

• 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.

Transcontinental Arc of Triangulation - 1899

Basic Definitions & Concepts

• 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.

Basic Definitions & Concepts

• 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.

Ellipsoid – Geoid surfaces in comparison with local topography

Geodetic Datums

• A geodetic datum consists of a reference ellipsoid fixed in some manner to a physical position on the earth’s surface.

• The ellipsoid that a datum is referenced to is NOT considered a datum.

• An ellipsoid that approximates the shape of the earth in a limited region, and is fixed to a specific point (the origin); forms a local datum.

• The North American Datum of 1927 is just such a datum.• NAD-27 is not an earth centered (geocentric) datum.

Regional Datums are Relative Datums

• NAD-27 is referenced to Clarke 1866 ellipsoid.

• Clarke 1866 ellipsoid is offset from the earth’s center to “best fit” the earth’s surface in the region of interest.

Relative Datums

Geodetic Latitude & Longitude

Origin of NAD-27 – Meades Ranch,Osborne County, Kansas

• Station Meades Ranch origin for NAD-27.

• Deflection of the vertical = 0.00 degrees

• Astronomic Latitude = Geodetic Latitude.

• Perpendicular to Earth’s surface = Perpendicular to ellipsoid.

Meades Ranch As the Origin of NAD-27

• 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.

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.

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.

Preparing a station for observations – North Dakota - 1913

Triangulation setup – Kaibab Plateau, Arizona - 1914

Triangulation crew, Idaho-Montana - 1922

Backsighting with a Heliotrope – Snake River, Idaho - 1922

Measuring a baseline with an Invar tape, Southeastern Oregon - 1916

Measuring a baseline with a microwave distance meter – Cook Inlet, Alaska - 1970

NAD-27 Triangulation Network in The U.S. as of 1931

North American Datum - 1927

• 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.

North American Datum - 1983

• 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.

NAD-83 is based on the GRS-80 ellipsoid

• Ellipsoidal Parameters

• GRS-80

• a = 6378137.000 meters

• b = 6356752.314 meters

• f = 1/298.2572221 meters

• ‘f’ is the flattening ratio – the degree to which the ellipsoid is flattened to match the earth’s shape.

WGS-84 ellipsoidal parameters

NAD-27 relative to NAD-83

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.

Readjustment of the North American DatumsNAD-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.

Datum Transformations

• 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.

Datum Transformation Algorithm

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.

Order & Class for horizontal stations

• Horizontal A order – stations that have a relative accuracy of ~ 5 mm relative to other A order stations; i.e. ( 1: 10,000,000).

• Horizontal B order – stations that have a relative accuracy of ~ 8 mm relative to other B order stations; i.e. (1:1,000,000).

• First order – stations with an accuracy of 1:100,000• Second order (Class I)– stations with an accuracy of

1:50,000.• Second order (Class II) – stations with an accuracy of

1:20,000.• Third order (Class I & II) – local control – stations with an

accuracy of 1:10,000 for Class I and 1:5000 for Class II.

HARN & HPGN Networks

• HARN – High Accuracy Reference Network; a statewide geodetic network – upgrade of positions in NAD-83 using GPS observations.

• NAD-83 coordinates based on the HARN surveys changed ~ 0.2m to ~1.0 m, relative to the original NAD-83 (1986 adj.) positions.

• HPGN & HARN are synonymous acronyms.

• 16,000 stations in the U.S. were upgraded to Horizontal A & Horizontal B stations.

NGS - Data Sheets

KG0640 *********************************************************************** KG0640 FBN - This is a Federal Base Network Control Station. KG0640 DESIGNATION - MEADES RANCH RESET KG0640 PID - KG0640 KG0640 STATE/COUNTY- KS/OSBORNE KG0640 USGS QUAD - MEADES RANCH (1978) KG0640 KG0640 *CURRENT SURVEY CONTROL KG0640 ___________________________________________________________________ KG0640* NAD 83(1997)- 39 13 26.71169(N) 098 32 31.74579(W) ADJUSTED KG0640* NAVD 88 - 600.3 (meters) 1969. (feet) GPS OBS KG0640 ___________________________________________________________________ KG0640 X - -734,972.577 (meters) COMP KG0640 Y - -4,893,188.523 (meters) COMP KG0640 Z - 4,011,982.818 (meters) COMP KG0640 LAPLACE CORR- -2.96 (seconds) DEFLEC99 KG0640 ELLIP HEIGHT- 573.99 (meters) (08/12/03) GPS OBS KG0640 GEOID HEIGHT- -26.35 (meters) GEOID03 KG0640 KG0640 HORZ ORDER - B KG0640 ELLP ORDER - FOURTH CLASS I