civil engineering--(survey): the relation between any differnt geodetic datums

name :ahmed wazer Nov 2012

category: the relation between any differnt geodetic datums

What is a geodetic datum? The Geodetic Glossary (National Geodetic Survey, National Ocean Service, National Oceanic and Atmospheric Administration, Rockville, MD, September 1986) pp. 54, 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."
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. 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 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.
What is the relationship between the geodetic vertical datums (NGVD 29 and/or NAVD 88) and the various water level/tidal datums? NGS develops and maintains the current national geodetic vertical datum, NAVD 88. In addition, NGS provides the relationships between past and current geodetic vertical datums, e.g., NGVD 29 and NAVD 88. However, another part of our parent organization, NOS (National Ocean Service), is the Center for Operational Oceanographic Products and Services (CO-OPS). CO-OPS publishes tidal bench mark information and the relationship between NAVD 88 and various water level/tidal datums (e.g., Mean Lower Low Water, Mean High Water, Mean Tide Level, etc.). The relationships to NGVD 29 are not published, but may be calculated independently from specified tidal bench mark sheet links to the NGS data base. Tidal bench mark information, water level/tidal datums, and their relationship to geodetic vertical datums are available at the CO-OPS web site: