Spectra Precision Survey Pro v4.6 Recon and Nomad User Manual User Manual

Page 238

User’s Manual – GPS Mode

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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.

• 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.

Below is a description of some common horizontal and vertical

datums used by Survey Pro.

http://164.214.2.59/GandG/tr8350_2.html