North American Datum

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Meades Ranch Triangulation Station, fundamental station for the North American Datum of 1927


First North American Datum[edit]

In 1901 the United States Coast and Geodetic Survey adopted a national horizontal datum called the United States Standard Datum, based on the Clarke Ellipsoid of 1866. It was fitted to data previously collected for regional datums, which by that time had begun to overlap. In 1913, Canada and Mexico adopted that datum, so it was also renamed the North American Datum.[1][2]

North American Datum of 1927[edit]

As more data were gathered, discrepancies appeared, so the datum was recomputed in 1927, using the same spheroid and origin as its predecessor.


Ellipsoid Semimajor axis† Semiminor axis† Inverse flattening††
Clarke 1866 6,378,206.4 m 6,356,583.8 m 294.978698214

†By definition. ††Calculated.


Most USGS topographic maps were published in NAD27 and many major projects by the United States Army Corps of Engineers and other agencies were defined in NAD27, so the datum remains important, despite more refined datums being available.

North American Datum of 1983[edit]

Because Earth deviates significantly from a perfect ellipsoid, the ellipsoid that best approximates its shape varies region by region across the world. Clarke 1866, and North American Datum of 1927 with it, were surveyed to best suit North America as a whole. Likewise, historically, most regions of the world used ellipsoids measured locally to best suit the vagaries of Earth's shape in their respective locales. While ensuring the most accuracy locally, this practice makes integrating and disseminating information across regions troublesome.

As satellite geodesy and remote sensing technology reached high precision and were made available for civilian applications, it became feasible to acquire information referred to a single global ellipsoid. This is because satellites naturally deal with Earth as a monolithic body. Therefore, the GRS 80 ellipsoid was developed for best approximating the Earth as a whole, and it became the foundation for the North American Datum of 1983. Though GRS 80 and its close relative, WGS 84, are generally not the best fit for any given region, a need for the closest fit largely evaporates when a global survey is combined with computers, databases, and software able to compensate for local conditions.

Ellipsoid Semimajor axis† Semiminor axis†† Inverse flattening††
GRS 80 6,378,137 m 6,356,752.3141 m 298.257222101

†By definition. ††Calculated.

Comparing NAD27 to NAD83[edit]

Datum Shift Between NAD27 and NAD83


North American Datum 1983 and WGS84[edit]

The initial definition of NAD83(1986) was intended to match GRS80 and WGS84, and many older publications indicate no difference. Subsequent more accurate measurements found a difference typically on the order of a meter over much of the United States. Each datum has undergone refinements with more accurate and later measurements.

One fundamental difference is that NAD83 is defined to remain essentially constant over time for points on the North American Plate, whereas WGS84 is defined with respect to the average of stations all over the world. Thus there is a change over time as to the difference between the systems. For much of the United States the relative rate is on the order of 1 to 2 cm per year.

Latest version of North American Datum[edit]


See also[edit]



External links[edit]