North American Datum
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The North American Datum is the official datum used for the primary geodetic network in North America. In the fields of cartography and land-use there are currently two North American Datums in use: the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83). Both are geodetic reference systems, but each is based on different measurements.
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[edit] North American Datum of 1927
The North American Datum of 1927 (NAD27) is a datum based on the Clarke Ellipsoid of 1866. This Ellipsoid was created by way of manual surveying of the entire continent. The geodetic "center" of NAD-27 is a base station at Meades Ranch in Kansas.
[edit] North American Datum of 1983
As satellite and remote sensing technology both improved and were made available for civilian applications, it became obvious that the NAD-27's appromixations were not sufficiently accurate.
The North American Datum of 1983 was created to meet requirements for better accuracy and more precision. It is based on the GRS80 ellipsoid; an ellipsoid derived from satellite geodesy.
| Ellipsoid | Semi-major axis | Eccentricity Squared |
| NAD 83 | 6,378,137 m | 0.006694300229 |
[edit] Comparisons Between NAD-27 and NAD-83
A point having a given latitude and longitude in NAD27 may be displaced on the order of many tens of meters from another point having the identical latitude and longitude in NAD83. So it is important to specify the datum along with the coordinates. The North American Datum of 1927 is defined by the latitude and longitude of an initial point (Meades Ranch in Kansas), the direction of a line between this point and a specified second point, and two dimensions that define the spheroid. The North American Datum of 1983 is based on a newer defined spheroid (GRS80); it is an Earth-centered datum having no initial point or initial direction.
[edit] External links
- NADCON – a free utility to convert between NAD27 and NAD83
- nadcon.prl – a web-based utility for NADCON
- NAD27/NAD83 Conversion Tool - free online conversion between NAD27 and NAD83 datums
