Horizontal Datums - Tripod Data Systems SURVEY PRO User Manual

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Grid File Datum Transformation: Is used when the datum
differences are not consistent over large areas. A grid file datum
transform uses a data set of shift values. For any location, an
approximate shift can be calculated by interpolating from the
data set.
For many surveying applications, the horizontal and vertical datums
are separate. This is because GPS heights are measured on the
ellipsoid with its origin at the earth's center of mass, while elevation
is a function of local gravity field, which is influenced by the unequal
distribution of mass in the earth.
Below is a description of some common horizontal and vertical
datums used by Survey Pro.

Horizontal Datums

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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 correct geodata directory on the
data collector. This will be Disk\Geodata on the Ranger, and Built In
Storage\Geodata on the Recon.
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NAD83 = WGS84
Later in the twentieth century, satellite and Very Long Baseline
Interferometry (VLBI) measurements were added to the numerous
GPS Coordinates
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