The North American Datum is the datum now used to define the geodetic network
in North America. A datum is a formal description of the shape of the Earth
along with an "anchor" point for the coordinate system. In surveying,
cartography, and land-use planning, two North American Datums are in use: the
North American Datum of 1927 and the North American Datum of 1983. Both are
geodetic reference systems based on slightly different assumptions and
measurements. First North American Datum
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.
North American Datum of 1927 As more data were gathered,
discrepancies appeared, so the datum was recomputed in 1927, using the same
spheroid and origin as its predecessor. The North American Datum of 1927 was
based on surveys of the entire continent from a common reference point that was
chosen in 1901, because it was as near the center of the contiguous United
States as could be calculated: It was based on a triangulation station at the
junction of the transcontinental triangulation arc of 1899 on the 39th
parallel north and the triangulation arc along the 98th meridian west that was
near the geographic center of the contiguous United States. The datum
declares the Meades Ranch Triangulation Station to be 39°13′26.686″ north
latitude, 98°32′30.506″ west longitude. NAD27 is oriented by declaring the
azimuth from Meades Ranch to Waldo to be 255°28′14.52″ from north. The latitude
and longitude of every other point in North America is then based on its
distance and direction from Meades Ranch: If a point was X meters in
azimuth Y degrees from Meades Ranch, measured on the Clarke Ellipsoid of
1866, then its latitude and longitude on that ellipsoid were defined and could be
calculated. †By definition. ††Calculated.
These are the defining dimensions for NAD27, but Clarke actually defined his
1866 spheroid as a = 20,926,062 British feet, b = 20,855,121 British feet. The
conversion to meters uses Clarke's 1865 inch-meter ratio of 39.370432.
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
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. †By definition. ††Calculated.
Comparing NAD27 to NAD83 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, 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; it is an Earth-centered datum having no initial
point or initial direction. North American Datum 1983 and WGS84
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
The United States National Spatial Reference System NAD 83(2011PA11) epoch
2010.00, is a refinement of the NAD83 datum using data from a network of very
accurate GPS receivers at Continuously Operating Reference Stations.
See also North American Vertical Datum of 1988
World Geodetic System References
External links NOAA-NGS-coordinates CORS-active network
- explanation of NAD 83(2011PA11) epoch 2010.00
NOAA-NGS-coordinates passive network - explanation of most recent adjustment of
passive network NADCON – a free utility for Microsoft
Windows to convert between NAD27 and NAD83
nadcon.prl – a web-based utility for NADCON
NAD83: What Is It and Why You Should Care by Dane E. Ericksen, P.E., Hammett
& Edison, Inc., Consulting Engineers. 1994 SBE National Convention and World
Media Expo.