GDA94 is the official geodetic datum adopted nationally across Australia on 1 January 2000. It replaced the Australian Geodetic Datum 1966 (AGD66) used in Victoria. Coordinates in GDA94 can be expressed as geographical coordinates (latitude and longitude), Universal Transverse Mercator (UTM) projection coordinates (easting, northing and zone), or Cartesian coordinates (X,Y and Z). The standard map projection associated with GDA94 is the Map Grid of Australia 1994 (MGA94). Coordinates on MGA94 are derived from a Transverse Mercator projection of GDA94 latitudes and longitudes that conforms to the internationally accepted Universal Transverse Mercator Grid system. The UTM zone system is used to enable coordinate references between zones to be unique. A spreadsheet to assist with computing Point Scale Factor, Height Factor and Grid Convergence calculations is available for download. GDA94 is a static coordinate datum based on the International Terrestrial Reference Frame 1992 (ITRF92), held at the reference epoch of 1 January 1994. At this time GDA94 and ITRF were coincident with WGS84 – the datum used for GPS. However, over time, both ITRF and WGS84 realisations have diverged from GDA94, primarily due to tectonic motion of the Australian plate (approximately 70 mm per year in a NNE direction) and improvements to IRTF. The absolute difference between GDA94 and ITRF is now approximately one metre. Since the WGS84 reference frame is aligned to the ITRF at the centimetre level, the absolute difference between WGS84 and GDA94 is also approximately one metre. For most practical applications, where an accuracy of only a metre or greater is required, GDA94 coordinates can continue to be considered the same as WGS84 or ITRF. However, to allow for the divergence in coordinate reference frames, users working at the centimetre level are encouraged to use the transformation parameters available on the Geoscience Australia website to transform between GDA94 and the ITRF and WGS84 reference frames. For many differential GPS applications the divergence can be overlooked because both ends of a baseline move at the same rate. However, users working at the centimetre level over long distances (greater than 50 km) should apply the appropriate transformation parameters to overcome time-dependent variations in the orientation of differential vectors. Coordinate transformation software to assist in applying transformation parameters is available on the ICSM website . For users working with AGD66 coordinates, ICSM produced a GDA Technical Manual and Land Victoria developed GDA transformation tools to support transition from AGD66 to GDA94. A brief description of some of the transformation methods between AGD66 and GDA94, and the recommended Land Victoria approach are given in GDA Further Transformation Options. GDA94 Transformation Tools include a software program called GDAit, which can be downloaded and used on a variety of Windows platforms on your PC. GDA it will transform coordinates (geodetic coordinates and grid coordinates) between AGD66 and GDA94 within the coverage area of the National Transformation Grid File, which replaces all preceding Victorian specific grid files. Source.