NAD

NAD 27 - The System Prior to 1986
Let's begin where most stories start, at the beginning. The North American Datum 1927 was established long before we had any ability to locate the centre of mass of the earth by tracking satellite orbits.

NAD 27
4-28. NAD 27 is a horizontal datum based on a comprehensive adjustment of the US National Control Network of traverse and triangulation stations. NAD 27 is a best fit for CONUS.

NAD 83 is based on the Earth's centre of gravity and also is a more accurate mathematical representation (ellipsoid) of the shape of the earth than was NAD 27.

NAD 1983
Acronym for North American Datum of 1983. A geocentric datum and graphic coordinate system based on the Geodetic Reference System 1980 ellipsoid (GRS80).

NAD: North American Datum
NAPP: National Aerial Photography Program (NAPP)
NAVD: North American Vertical Datum ...

The NAD 83 is the culmination of 12 years of automation and analysis of every major horizontal control survey performed. The readjustments to NAD 83 has resulted coordinate shifts that exceed 400 meters in some parts of the country.

If your DOQ is in NAD 83 and your DRG is in NAD 27, the files will not exactly overlay; they may be up to a few hundred meters off. You will need to edit the header file of the image you wish to change. The header files are 6 lines long.

A given set of bits might mean a set of coordinates with respect to the North American Datum of 1927 (NAD 27) to one user, ...

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 (Kansas)." ...

North American Datum (NAD)
There are two phases of the North American Datum - NAD27 and NAD83 - one local the other geocentric. Local datum align a spheroid to closely fit the earth's surface in a particular area.

North American Datum (NAD): There are two North American Datums: North American Datum of 1927 (NAD 27) and North American Datum of 1983 (NAD 83). Both are geodetic reference systems, but each is based on different measurements.

For data concerning North America, your choice is typically between the North American Datum (NAD) of 1927 or 1983. If you you think that your data was based on a map made before 1983, then pick NAD27. If not, choose NAD83.

More recent maps commonly use the NAD-83 referencing system which is based on the GRS-80 ellipsoid. The datum used for a map is printed on the front of a map.

Changing datums (for example, from NAD 27 to NAD 83) can be difficult or cumbersome.
Projections are usually implemented using power series approximations that might be inadequate for high precision work.

[geodesy] Acronym for High Accuracy Reference Network. A regional or statewide resurvey and readjustment of NAD 1983 control points using GPS techniques. The resurvey date is often included as part of the datum name: NAD 1983 (1991) or NAD91.

Geodetic Datum - WGS 84 - NAD 83 - GPS
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For the Geographic Transformation, select NAD_1927_To_NAD_1983_NADCON.
Click OK. A progress dialog will indicate when the process is complete. Click Close.

2. Ensure that the Shapefile coordinate system is Geographic (decimal degree latitude/longitude) and the Datum is North American Datum (NAD 83). If your data are not Geographic NAD 83, contact your GIS Specialist for help in converting it.

The geocentric reference ellipsoid / datum used by LRIS to replace NAD 27. (See also Datum)
Azimuth
Direction of a line given as an angle measured clockwise from a reference direction, usually north.

It has been georeferenced using the EPSG 26910 projection for NAD 83 / UTM Zone 10N.

[D]
Click for larger version, 173 KB
Figure 1. Maximum Error (in metres) for Single Point Determination per 1:50 000 NTS Map Sheet for NAD 27-83 Coordinate Conversion ...

Only adjusted positions on the North American datum of 1927 and NAD 1983 may be properly transformed into plane coordinates on a state system.

NAD - North American Datum: The official reference ellipsoid used for the primary geodetic network in North America.
Node: The beginning and ending locations of a line on a digital map.

See also: Coordinate, Geographic, Information, Mapping, Model