Coordinates, Datums & Ellipsoids

• Introduction
• Measurement bases
• Early mapping origins
• Origin values
• Laplace stations
• References

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Introduction

In Australia prior to 1966, there were some twenty different datums, using four different figures of the earth, but the traditional spheroid in general use until 1961 was the Clarke 1858:

Semi-major axis (a):	20 926 348 feet
Flattening (f):	1/294.26

The foot used by Clarke in this definition was, 1 Clarke Foot = 0.3047 9726 54 metres, giving a = 6 378 293.645 metres. This value was used in the Tasmanian implementation of the Clarke 1858 datum.

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Measurement bases

However, most triangulation was computed from bases measured in British feet: 1 British Foot of 1926 = 0.3047 9947 metres so that the resulting latitudes and longitudes lay on a spheroid whose semi-major axis was:

Semi-major axis (a):	20 926 348 British Feet
	6 378 339.78 metres

Thus, there was often some uncertainty in the exact meaning of the term "Clarke 1858 spheroid". It was still used for mapping control until the end of 1965.

On the Clarke 1858 spheroid there were several distinct origins of which the more important were:

Sydney Observatory:	S 33° 51' 41.10", E 151° 12' 17.85"
Perth Observatory 1899:	S 31° 57' 09.63", E 115° 50' 26.10"
Darwin Origin Pillar:	S 12° 28' 08.452", E 130° 50' 19.802"
Lochmaben Astro Station (Tas):	S 41° 38' 23.389", E 147° 17' 49.725"

Astronomical longitudes differed from geodetic longitudes on either the Sydney or Perth origins by about 10 seconds on average, indicating deflections of the vertical of this order at the two observatories.

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Early mapping origins

While much early mapping was based on these origins, some 1:250,000 maps were based only on astronomical observations with an accuracy of the order of 100 metres or more, or by a mixture of astro and conventional surveying. A comparison of coordinates based on different origins of this kind will include differences due to the uncertainty of the astronomical observation as well as the deflections of the vertical and could show differences of several hundreds of metres.

For a short period in 1962, geodetic computations were performed on the so-called photo zenith tube "NASA" spheroid with an origin at Maurice as below, but these computations were completely superseded:

Semi-major axis (a):	6 378 148 metres
Flattening (f):	1/298.3

From the end of 1962 until April 1965, the computation and adjustment of the Australian Geodetic Survey was done on the "165" spheroid:

Semi-major axis (a):	6 378 165 metres
Flattening (f):	1/298.3

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Origin values

Prior to April 1963, the "Maurice" origin used with the NASA spheroid was retained. As a result of these computations, new origin values were determined and from April 1963 to April 1965 computations were made on the 165 spheroid and this new "Central" origin. Computations still emanated from Maurice whose various coordinates were:

165 Central:	S 32° 51' 13.979", E 138° 30' 34.062"
165 Maurice:	S 32° 51' 13.000", E 138° 30' 34.000"
Clarke 1858, Sydney:	S 32° 51' 11.482", E 138° 30' 42.29"
Astronomic:	S 32° 51' 11.341", E 138° 30' 25.110"

The Central origin was based on the best mean fit to 155 Laplace stations spread over the whole of Australia with the exception of Cape York and Tasmania. The residual mean deflection was less than 0.1" in both latitude and longitude whether isostatic topographic corrections were applied to the astronomic values or not. It was therefore considered unlikely that there was a significant artificial component in N with the Central origin. As no observed values of N from geoid surveys existed, it was assumed that N is zero everywhere.

In April 1965, it was decided to change to the spheroid adopted by the International Astronomical Union:

Semi-major axis (a):	6 378 160 metres
Flattening (f):	1/298.25

This spheroid was called the Australian National Spheroid.

In May 1965 a complete recomputation of the geodetic surveys of Australia was begun, emanating from the trigonometrical station Grundy, whose coordinates on both the 165 Central datum and the Australian National Spheroid Central origin were:

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Laplace stations

"By December 1965 the total number of Laplace stations in Australia was 533. From these, 275 stations were selected… no corrections for the topography were applied"… and it was found "that random undulations in the geoid make it impossible to locate a centre for the spheroid with a standard error of less than 0.5 seconds, about 15 metres, even with a very large number of stations". (Bomford, 1967). The image displays a coloured map of Australia showing the numbered 275 Laplace Stations in December 1965.

The Central origin was therefore retained, but is now defined in terms of the Johnston memorial cairn. The Central origin was originally defined in terms of the trigonometrical station Grundy… "The spheroid is oriented by defining the minor axis to be parallel to the earth's mean axis of rotation at the start of 1962 and defining the origin of geodetic longitude to be 149 00'18.855" west of the vertical through the photo zenith tube at Mt Stromlo. The size, shape, position and orientation of the spheroid are thus completely defined, and together define the Australian Geodetic Datum." (Bomford, 1967)

This information is historical and the current datum in Australia is Geocentric Datum of Australia (GDA94).

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References

Bomford, A.G. (1965) Internal Correspondence, Division of Geoscience Australia

Bomford, A.G. (1967) The Geodetic Adjustment of Australia, 1963-1966, Survey Review, No. 144, Vol. XIX, 1967, pp57-58.

Royal Geographic Society, Technical Series No. 4

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