4.3 Standard Geographical Coordinate System of GDA94

In terms of consistent spatial positioning, the Intergovernmental Committee on Surveying and Mapping (ICSM) adopted a new coordinate system for Australia, called the Geocentric Datum of Australia (GDA94), in 1994 (ICSM, 1997). This allows global compatibility for geographical coordinates and the direct linkage with the Global Positioning System (GPS), following the trend in positioning, navigation and information systems being related to a global Earth model (Steed, 1995).

A datum is used to define coordinates for a point on the earth’s surface and consists of:

• the coordinate reference frame;
• the ellipsoid defining the geometric shape of the earth; and
• one or more origin sites used to locate the ellipsoid to the reference frame.

Table 4.2 summarises the technical specifications for the new datum, including the reference frame and ellipsoid that is used. The GDA94 as the name suggests, is geocentric so that its reference frame (ITRF92) has its origin at the centre of mass of the earth. The international reference frames are regularly refined by using more observations and better computational techniques, however the differences are now at the centimetre level. The ellipsoid used to approximate the shape of the earth is defined by its semi-major (a) and semi-minor (b) axes. The flattening (f) describes the relationship between these axes (f = 1-b/a).

The earth-centred nature of GDA94 differs from its predecessor, the Australian Geodetic Datum (AGD84) which was adopted to fit the Australian region. The original version (AGD66) of this datum was later adjusted and introduced as AGD84. There is a displacement of about 200m across Australia in a northeast direction, between coordinates of points on the existing AGD84 compared with coordinates of the same points on the new GDA94. This will be reflected in a shift in map details, with the magnitude depending on map scale. At 1:1,000,000 the shift is 0.2mm but at 1:100,000 the shift will be about 2mm. GDA94 coordinates are practically the same (± 10cm) as WGS84 coordinates, the geocentric datum commonly used in GPS receivers and developed by the US Department of Defence.

The ICSM recommends that Australia convert to GDA94 by the year 2000, with some authorities having already started the conversion process. Depending on the accuracy required and the data held, coordinates might be re-computed or transformed with national transformation parameters and software becoming available to do so.

The Map Grid of Australia (MGA94) is the Universal Transverse Mercator projection of the GDA94 latitudes and longitudes. As such, these grid coordinates will supersede the current Australian Map Grid (AMG) coordinates.

Table 4.2 Technical Specifications for the Geocentric Datum of Australia (AUSLIG,1997)