Spectra Precision Survey Pro CE v3.60 GPS User Manual - Manual (Page 6)

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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, we must transform the
GPS measurements into local coordinates.
Some jobs require a coordinate system based on a specified mapping
plane and geodetic datum, such as a UTM. For this case, the section
below describes how Survey Pro converts WGS84 GPS positions into
local north and east coordinates. Other jobs have an arbitrary
coordinate system, such as a re-survey of an old conventional traverse
or starting a new job from scratch. For this case, you can use TDS
Localization, and you will not need to know the details of anything in
this section.
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.