Spectra Precision Survey Pro v3.80 Recon GPS User Manual - Manual (Page 7)

GPS Coordinates
3
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.
WGS84 Geodetic v. Local Geodetic
When the coordinate system is a mapping plane in a datum other
than WGS84, positions measured in WGS84 latitude, longitude and
height, must be transformed into local latitude, longitude, and height
before they can be used to calculate northing and easting with the
specified map projection.
There are three methods of datum transformation supported by
Survey Pro.
•
Molodensky Transformation: Is the most commonly used
transformation. Three parameters specify an X,Y,Z shift between
WGS84 and the local datum origin. Survey Pro uses the
Molodensky datum transformation algorithms specified in the
National Imagery and Mapping Agency Technical Report
8350.2.
1 .
•
Similarity Transformation: The most precise method of datum
transformation. The seven-parameter similarity transformation,
also called the Helmert transformation, uses a shift of XYZ origin,
a rotation about XYZ axes, and a scale to transform from WGS84
and the local datum.
•
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.
1 http://164.214.2.59/GandG/tr8350_2.html