Spectra Precision Survey Pro v4.6 Recon and Nomad User Manual - Manual (Page 237)

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GPS Coordinates
To represent positions in space you need two things. First, you need a
datum to define an origin, an orientation, and a scale. Second, you
need a coordinate system to specify the locations in the datum. GPS
positions are in a global geocentric datum, using latitude and
longitude angles to specify location. Most engineering and surveying
jobs require positions in a 2D Cartesian coordinate system. In order
to use GPS with most coordinate systems, you must transform the
GPS measurements into local coordinates.
This section offers a more detailed description of some theory and
terminology used to describe geodetic coordinate systems and datums
in Survey Pro.
Datums
A datum consists of three components: an origin, an orientation, and
a scale. The origin defines the start point, the orientation defines the
direction of the bearings, and the scale defines the relative magnitude
of the distance units. For example, a surveyor shows up at a new job
site, places a monument in the ground and calls it (5000, 5000, 100).
This establishes the origin of the datum. The surveyor does a sun
shot and calculates the azimuth to a reference object. Astronomic
north at this meridian establishes the orientation of the datum.
Finally, the surveyor begins measuring distances with a total station.
The EDM establishes the scale of the datum.
GPS measurements are taken in a global geocentric datum, the World
Geodetic System of 1984 (WGS84). The WGS84 datum has its origin
at the earth’s center of mass, its orientation defined by the earth’s
spin axis and the intersection of the mean meridian of Greenwich
with the mean equatorial plane, and its scale defined by metric
standard measurement.
Geocentric datums such as WGS84 use a rotational ellipsoid to model
the shape of the earth. The WGS84 ellipsoid was based on and is
virtually identical to the Geodetic Reference System of 1980 (GRS80)
ellipsoid. The ellipsoid origin is at the earth’s center of mass. Its
minor axis corresponds with the earth’s rotation axis and its major
axis corresponds to the mean equatorial plane.