Australian Height Datum [AHD]

(Extracted from "Geocentric Datum of Australia Technical Manual", Chapter 8).

Introduction

On 5 May 1971 the then Division of National Mapping, on behalf of the National Mapping Council of Australia, carried out a simultaneous adjustment of 97230 kilometres of two-way levelling. Mean sea level for 1966-1968 was assigned the value of zero on the Australian Height Datum at thirty tide gauges [gif_6k] around the coast of the Australian continent.

The resulting datum surface, with minor modifications in two metropolitan areas, has been termed the Australian Height Datum (AHD) and was adopted by the National Mapping Council at its twenty-ninth meeting in May 1971 as the datum to which all vertical control for mapping is to be referred. The datum surface is that which passes through mean sea level at the thirty tide gauges and through points at zero AHD height vertically below the other basic junction points.

Further information on the determination of the AHD is given in Division of National Mapping Technical Report No. 12, The Adjustment of the Australian Levelling Survey, 1970-71 (2nd edition, 1975).

The levelling network in Tasmania was adjusted on 17 October 1983 to re-establish heights on the Australian Height Datum (Tasmania). This network, which consists of seventy-two sections between fifty-seven junction points is based on mean sea level for 1972 at the tide gauges [gif_6k] at Hobart and Burnie. Mean sea level at both Hobart and Burnie was assigned the value of zero on the AHD (Tasmania).

Look up AHD (Australian Height Datum) under Glossary of Terms.

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Basic and supplementary levelling

Two-way levelling of third order accuracy or better, used in the original adjustment of 5 May 1971 which formed the AHD, is called "Basic levelling". Levelling subsequently adjusted to the AHD is called "Supplementary levelling".

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Islands

If the levels on islands closely adjacent to the Australian mainland are observed to standard third order accuracy, and are referred to mean sea level at a satisfactory tide gauge, they are deemed to be part of the Australian Height Datum.

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AHD & Ellipsoidal heights

Heights obtained from GPS (observed ellipsoidal heights) do not refer to the AHD, but to the mathematical reference surface (the ellipsoid). The geoid is an equipotental surface closely approximated by sea level and in Australia is represented by AUSGeoid98.

The difference between the ellipsoid and the geoid is known as the geoid-ellipsoid separation.

If a geoid-ellipsoid separation is subtracted from an observed ellipsoidal height, the resulting height above the geoid will approximate an AHD height (typically within ±0.5m) and is referred to as a derived AHD value. A similar problem exists if the geoid-ellipsoid separation is added to a known AHD value to produce a derived ellipsoidal height.

In either case, the discrepancy between derived and observed values may be due to:

• Uncertainty in the observed ellipsoidal height. However, appropriate GPS observations and post-processing techniques allow ellipsoidal heights to be computed with an accuracy of a few centimetres;
• Uncertainty in the geoid-ellipsoid separation. This is difficult to quantify, given the correlation between the other error sources, but maybe be of the order of 10cm or less;
• Uncertainty in the known AHD value due to optical levelling propagation errors (12mm sqrt(km) for the typical 3rd order levelling); and
• The fact that the geoid does not coincide with the AHD, mainly due to sea surface topography. This difference can be up to ±0.5m.

These discrepancies are minimised by applying the N values differentially, rather than in an absolute sense.

Note: Observed ellipsoidal heights are generally referred to the International Terrestrial Reference Frame (ITRF) at a particular point in time. Although the observed ellipsoidal height may have an uncertainty of just a few centimetres in terms of this reference frame, the value can change by a few centimetres depending on the version of ITRF used (eg ITRF97, ITRF2000) and to a much lesser extent because of the epoch of the ellipsoidal height (due to tectonic motion).

GDA94 is the datum used for horizontal positions in Australia. Observed ellipsoidal heights at the same epoch and in terms of the same reference frame as GDA94 (ITRF92 at 1994.0) may sometimes be referred to as GDA94 ellipsoidal heights.

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