Datum

 

A datum defines the position of the spheroid relative to the center of the earth. It provides a frame of reference for measuring locations on the surface of the earth by defining the origin and orientation of latitude and longitude lines. Whenever you change the datum, or more correctly, the geographic coordinate system, the coordinate values of your data will change. Local datum, such as the North American Datum of 1983 (NAD 83) are designed to be used in a specific area.

The North American Datum of 1927 uses the Clarke 1866 spheroid to represent the shape of the earth. The origin of this datum is Meades Ranch in Kansas. Many NAD 1927 control points were calculated from observations taken in the 1800s. These calculations were done manually and in sections over many years. The result of this approach was errors between stations.

The North American Datum of 1983 is based upon both earth and satellite observations, using the GRS80 spheroid. The center of this datum is the center of the earth. Locations between NAD 27 and NAD83 shift by as much as 500 feet, however NAD 83 is compatible with GPS data. The Geodetic Glossary defines geodetic datum as:

  1. “A set of constants specifying the coordinate system used for geodetic control, i.e., for calculating the coordinates of points on the Earth.”
  2. “The datum, as defined in (1), together with the coordinate system and the set of all points and lines whose coordinates, lengths, and directions have been determined by measurement or calculation.” – From National Geodetic Survey, National Ocean Service, National Oceanic and Atmospheric Administration, Rockville, MD, September 1986, pp. 54.

These differing definitions require caution when using the word “datum.” The first definition makes datum synonymous with the selection of a reference coordinate system (origin and orientation). The second definition makes datum synonymous with a list of coordinates of the control points. When the first definition is used, the published coordinates of control points can change when better measurements allow better determinations. With the second definition, a change in coordinates should result in a new datum. The National Geodetic Survey (NGS) has used the first definition for NAD 1983.

What are NAD 27 and NAD 83?

The North American Datum of 1927 (NAD 27) is “The horizontal control datum for the United States that (was) defined by (a) location and azimuth on the Clarke spheroid of 1866, with origin at (the survey station) Meades Ranch.” … The geoidal height at Meades Ranch (was) assumed to be zero. ” Geodetic positions on the North American Datum of 1927 were derived from the (coordinates of and an azimuth at Meades Ranch) through a readjustment of the triangulation of the entire network in which Laplace azimuths were introduced, and the Bowie method was used.” (Geodetic Glossary, pp. 57).

The North American Datum of 1983 (NAD 83) is “The horizontal control datum for the United States, Canada, Mexico, and Central America, based on a geocentric origin and the Geodetic Reference System 1980.

“This datum, designated as NAD 83, is the new geodetic reference system. … NAD 83 is based on the adjustment of 250,000 points including 600 satellite Doppler stations which constrain the system to a geocentric origin.” (Geodetic Glossary, pp 57).

Why did National Geodetic Survey (NGS) change from NAD 27 to NAD 83?

NAD 83 was computed by the geodetic agencies of Canada (Federal and Provincial) and the National Geodetic Survey for several reasons. The horizontal control networks had expanded piecemeal since 1933 to cover much more of the countries and it was very difficult to add new surveys to the network without altering large areas of the previous network. Field observations had added thousands of accurate Electronic Distance Measuring Instrument (EDMI) base lines, hundreds of additional points with astronomic coordinates and azimuths, and hundreds of Doppler satellite determined positions. It was also recognized that the Clarke Ellipsoid of 1866 no longer served the needs of a modern geodetic network. For an in-depth explanation see NOAA Professional Paper NOS 2 “The North American Datum of 1983”, Charles R. Schwarz, Editor, National Geodetic Survey, Rockville, MD 20852, December 1989.

How do the horizontal datum differ? Which should I use?

The NAD 27 was based on the Clarke Ellipsoid of 1866 and the NAD 83 is based on the Geodetic Reference System of 1980. The NAD 27 was computed with a single survey point, MEADES RANCH in Kansas, as the datum point, while the NAD 83 was computed as a geocentric reference system with no datum point. NAD 83 has been officially adopted as the legal horizontal datum for the United States by the Federal government, and has been recognized as such in legislation in 44 of the 50 states. The computation of the NAD 83 removed significant local distortions from the network which had accumulated over the years, using the original observations, and made the NAD 83 much more compatible with modern survey techniques.

What is HARN or HPGN?

A High Accuracy Reference Network (HARN) and a High Precision Geodetic Network (HPGN) were two designations used for a statewide geodetic network upgrade. The generic acronym HARN is now used for both HARN and HPGN and was adopted to remove the confusion arising from the use of two acronyms. A HARN is a statewide or regional upgrade in accuracy of NAD 83 coordinates using Global Positioning System (GPS) observations. HARNs were observed to support the use of GPS by Federal, state, and local surveyors, geodesists, and many other applications. The cooperative network upgrading program began in Tennessee in 1986. The last field observations were completed in Indiana in September 1997 after horizontally upgrading some 16,000 survey stations to A-order or B-order status. Horizontal A-order stations have a relative accuracy of 5 mm +/- 1:10,000,000 relative to other A-order stations. Horizontal B order stations have a relative accuracy of 8 mm +/- 1:1,000,000 relative to other A-order and B-order stations Of these 16,000 stations, NGS has committed to maintaining about 1,400 survey stations, named the Federal Base Network, and the various states will maintain the remainder.

What are NGVD 29 and NAVD 88?

The National Geodetic Vertical Datum of 1929: The name, after May 10, 1973, of (the) Sea Level Datum of 1929.” (Geodetic Glossary, pp. 57).

“Sea Level Datum of 1929: A vertical control datum established for vertical control in the United States by the general adjustment of 1929.” “Mean sea level was held fixed at the sites of 26 tide gauges, 21 in the U.S.A. and 5 in Canada. The datum is defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. A total of 106,724 km of leveling was involved, constituting 246 closed circuits and 25 circuits at sea level.” “The datum (was) not mean sea level, the geoid, or any other equipotential surface. Therefore it was renamed, in 1973, the National Geodetic Vertical Datum on 1929.” (Geodetic Glossary, pp. 56).

The North American Vertical Datum of 1988 (NAVD 88) is the vertical control datum established in 1991 by the minimum-constraint adjustment of the Canadian-Mexican- U.S. leveling observations. It held fixed the height of the primary tidal bench mark, referenced to the new International Great Lakes Datum of 1985 local mean sea level height value, at Father Point/Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., the fact that mean sea level is not the same equipotential surface at all tidal bench marks. (“Results of the General Adjustment of the North American Datum of 1988,” Surveying and Land Information Systems Vol. 52, No. 3, 1992 pp. 133-149).

Why did NGS change from NGVD 29 to NAVD 88?

NAVD 88 was computed for many of the same reasons as NAD 83. About 625,000 km of leveling had been added to the NGVD since 1929. Thousands of bench marks had been subsequently destroyed and many others had been affected by crustal motion, postglacial rebound, and subsidence due to the withdrawal of underground fluids. Distortions amounting to as much as 9 meters had been seen due to forcing the new leveling to fit the NGVD 29 height values. (“Results of the General Adjustment of the North American Datum of 1988,” Surveying and Land Information Systems Vol. 52, No. 3, 1992 pp. 133-149).

What is WGS 84? Does it change?

WGS 84 is the World Geodetic System of 1984. It is the reference frame used by the U.S. Department of Defense (DoD) and is defined by the National Imagery and Mapping Agency (NIMA) (formerly the Defense Mapping Agency). WGS 84 is used by DoD for all its mapping, charting, surveying, and navigation needs, including its GPS “broadcast” and “precise” orbits. WGS 84 was defined in January 1987 using Doppler satellite surveying techniques. It was used as the reference frame for broadcast GPS Ephemerides (orbits) beginning January 23, 1987. At 0000 GMT January 2, 1994, WGS 84 was upgraded in accuracy using GPS measurements. The formal name then became WGS 84 (G730) since the upgrade date coincided with the start of GPS Week 730. It became the reference frame for broadcast orbits on June 28, 1994. At 0000 GMT September 30, 1996 (the start of GPS Week 873), WGS 84 was redefined again and was more closely aligned with International Earth Rotation Service (IERS) Terrestrial Reference Frame (ITRF) 94. It is now formally called WGS 84 (G873). WGS 84 (G873) was adopted as the reference frame for broadcast orbits on January 29, 1997.

I have heard rumors of a new reference system/datum. What are NGS’s plans?

Between 1987 and 1997, the National Geodetic Survey, in cooperation with other Federal, State and local surveying agencies has conducted a resurvey of the United States using Global Positioning System (GPS) observations often referred to as the High Accuracy Reference Networks (HARNs). All 50 states, American Samoa, Guam, Puerto Rico and the Virgin Islands have now been connected with a network of A-order and Border horizontal control points. Continued improvements in GPS technology and requirements from users of spatial data will eventually require a transition to an improved global reference frame based on the International Terrestrial Reference Frame (ITRF). Positions relative to ITRF differ from the existing North American Datum of 1983 (NAD 83) by approximately 1 meter in horizontal position and 1 meter in ellipsoidal height. NGS already publishes ITRF coordinates for all Continuously Operating Reference Stations (CORS), and will over the next 3-5 years implement an adjustment to include the HARNs and other GPS data that have been submitted to NGS for adjustment and publication. NGS will continue to maintain and improve NAD 83 as the official datum of the United States, until such time as it will no longer support requirements for surveying, mapping and navigation. NGS is currently conducting workshops and seminars around the country to educate data users concerning these and other improvements to the National Spatial Reference System.