Global Positioning
    System
 The System
 The Position Solution
 Hardware
 Ionospheric And
    Atmospheric Delays
 The Signals
 Dynamic Positioning
 Static Positioning
 Coordinate Systems
 Conclusions

Coordinate Systems

The type of coordinate system used in GPS surveying is an earth-centred cartesian system, of which a detailed discussion is beyong the scope of these reference notes in Plane Surveying. Generally coordinates produced by GPS units are geocentric coordinates which appear nothing like geographic coordinates. Geocentric coordinates are based on the centre of the earth and have an X, Y and Z component. The Z axis is from the centre of the the earth through the north pole, the XZ plane passes through the Greenwich meridian and the XY plane passes through the equator. Geographic coordinates are in the form latitude and longitude and are located on a spheroid of 'best fit' over the surface of the planet. Transformations between the cartesian coordinates and the spheroidal coordinates can easily be performed in the receivers, as well as transformations between grid mapping systems like AMG. There is also a selection of spherical models that can be used as a coordinate datum, some of these can be seen during the set-up options on the navigational GPS units. If one wishes to output AMG coordinates in Australia one selects the Australian National Spheroid as the datum, UTM coordinates as the type and the appropriate zone if necessary.

There will be considerably more information provided on coordinate datums in the later years of the geomatics program. We have only looked at the matter superficially, the problem of coordinate frames of reference is part of on-going investigations and developments.