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What is a datum?

A datum is a framework that enables us to define coordinate systems. The framework includes the ellipsoid and other parameters. For information about these see the Geocentric Datum of Australia (GDA). The way a datum is defined has changed with the improvement of measurement techniques through technology. The AGD66/84 was defined differently to the GDA94 method. The two methods are briefly outlined below.

Traditional geodetic datums (AGD66/AGD84)

To describe positions on the Earth accurately, two datums (horizontal and vertical) were needed.

Vertical datum

• A vertical datum is used to fix a position in the vertical direction, up and down the Z axis
• A vertical datum is a line, value or set of values from which heights are measured. Australia's vertical datum is the Australian Height Datum (AHD) which approximates mean sea level and was determined by monitoring tide gauges around the Australian coastline. The change of horizontal datum to the GDA94 will NOT affect the height of the currently used vertical datum

Horizontal datum

• A horizontal datum is used to fix a position in the X and Y directions
• A horizontal datum was traditionally defined using an ellipsoid and an origin. The ellipsoid was generally chosen for a finest match to the nearest geoid
• The origin of the AGD66/AGD84 is the Australian Geodetic Datum. More information about this datum can be found on the ICSM GDA Homepage

Modern geodetic datums (GDA94)

Today's technology means that a new datum can be based on several well determined points. The eight points used to define the GDA94 are known as the Australian Fiducial Network (AFN).

The adopted coordinates of the AFN were computed in terms of the International Terrestrial Reference Frame (ITRF) and from these AFN coordinates the geodetic network that covers the whole continent was recomputed. The ITRF is based on a number of very accurate observations all over the Earth, including Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and the Global Navigation Satellite System (GNSS).

Though the AFN provides coordinates in three dimensions, only the horizontal components have been taken on. Heights in Australia remain those of the AHD.

Why did we change to a geocentric datum?

The main reason was so that we could use the Global Navigation System (GNS) more easily. The Global Navigation Satellite System is becoming more widely used for navigation by both the spatial sciences industry and the general public world-wide.

Geocentric datums have been used for most aeronautical and shipping applications due to its international capabilities.

As the Global Positioning System (GPS) is based on a geocentric datum, to use the coordinates here in Australia means we have to convert the coordinates to our datum, the AGD66/84. With the introduction of the GDA94, the datums will both be based on the same ellipsoid and this will eliminate the need for a complex conversion as the coordinates read from the GPS will be the same as those of the GDA94.