Geodetic datum

As understanding of the earth and mapping techniques advanced, a system was needed to translate the three dimensional curved surface of the earth to a two dimensional piece of paper. A geodetic datum is a coordinate reference system from which measurements are made defining the size and shape of the earth's surface. Geodetic datums were developed to assist the work of surveyors, cartographers and navigators by using a coordinate reference system to describe geographic positions. Many different datums have been used to frame position descriptions since the first estimates of the earth's size were made by Aristotle. Datums have evolved from those describing a spherical earth to ellipsoidal models derived from years of satellite measurements, which are now infinitely more accurate.

County Arbitrary Meridian adopted

In 1899, the use of the magnetic meridian as the datum was abandoned. Determining the bearings of lines in this way using a compass created problems, as the readings from compasses could vary due to external factors (e.g. location of iron ore deposits).

A more accurate system that used a County Arbitrary Meridian (CAM) was introduced. In each county a location was chosen by government surveyors, usually in the centre of the county, called the Initial Point for that county.

An arbitrary meridian was allocated in each county that approximated the mean magnetic declination (variation) from true north, to the nearest whole degree, at the Initial Point in each county.

The location of the Initial Point, its latitude and longitude as well as the county variation to shift true north bearings onto the County Arbitrary Meridian were recorded in a table published in the Rules and Directions for the Guidance for Surveyors.

For example, the County of Rodney location of the Initial Point was the south-west corner of Portion 2v, parish of Collier. Its latitude and longitude were 23°06'43.5" S 144°50'58.1" and its county variation was 6°.

There was also a table to allow true bearings taken by astronomical observations to have their convergence corrected for.

These tables allowed surveyors who had taken an astronomical observation to determine a bearing based on true north, as well as the latitude and longitude of the point, to convert their bearing to one based on the County Arbitrary Meridian.

Longitude values determined by time signals

In 1927, for the first time in Queensland, longitudes were determined by means of wireless time signals as broadcast by radio station 4QG.

During 1948, a change was made in the longitude values adopted for mapping in Queensland. This led to the introduction of a new datum value. The observations resulted in a mean adjustment of 6.46 seconds to the longitude shown on maps and 0.25 seconds to the latitude. (Duffy, LJ 1971, A history of mapping in the Survey Office Queensland)

A national change of datum

The coordinate system used in Australia has evolved through response to demands by business users and the availability of technology. The first datum adopted in 1966 by the National Mapping Council was refined in 1984 and then again in 1994 to establish the coordinate system presently used for all surveying, mapping and spatial information uses.

At the time it was adopted in 1966, the Australian Geodetic Datum (AGD) was based on the best available information and derived using the best available technology the AGD was revised in 1984. Not long after it was recognised that Global Positioning Systems (GPS) and computer information systems were advancing at unprecedented rates and would eventually require Australia to adopt a geocentric datum.

In 1992, the then Department of Natural Resources in Queensland became an integral body in developing and implementing the first airborne Global Positioning System (GPS) control mechanism in Australia.

The advancing technology during the 1990s saw Australian mapping organisations progressively adopt the Geocentric Datum of Australia (GDA). The geodetic co-ordinate of this datum is called GDA94.

The move from AGD to GDA saw coordinates shift in a north-easterly direction by about 200 metres (varying slightly in different parts of Australia).

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