Spectra Precision Survey Pro v4.6 Recon and Nomad User Manual - Manual (Page 239)

GPS Overview
229
Horizontal Datums
• NAD27
The North American Datum of 1927 (NAD27) horizontal datum was
established in the early part of the twentieth century to define a
horizontal coordinate system in North America. The datum originated
at a central point, Meades Ranch in Kansas. From there,
conventional triangulation and trilateration networks radiated
outward to establish new monuments in the system. The datum was
based on the Clarke 1866 ellipsoid, which was the best fitting
ellipsoid for the North American continent at the time.
Survey Pro performs a grid transformation for NAD27 in the United
States using the NADCON datum sets in *.DGF (datum grid file)
format. Several specific Molodensky datum transformations are also
available for other areas in North America.
Note: To use a grid datum, you must have the pair of *.dgf files for
latitude and longitude shift in the Geodata folder on the data
collector.
• NAD83 = WGS84
Later in the twentieth century, satellite and Very Long Baseline
Interferometry (VLBI) measurements were added to the numerous
conventional measurement networks and re-adjusted to define the
North American Datum of 1983 (NAD83). NAD83 was created to
conform to the new global datum, WGS84, and uses the same
reference ellipsoid.
Survey Pro uses no datum transformation for NAD83. Therefore,
NAD83 = WGS84 in Survey Pro projection calculations.
• NAD83 ≠ ITRF WGS84(1996.0 , 1997.0 , …)
Continuing improvements in GPS and VLBI technology, as well as
increased cooperation among world wide agencies, like the
International Earth Rotation Society (IERS), led to a much better
solution for the Earth’s center of mass and spin axis. The IERS’s
solution is adopted as the International Terrestrial Reference Frame
(ITRF). Because the earth’s center of mass and spin axis drift over
time, you will often see the WGS84 datum followed by brackets