Australian Height Datum

On 5 May 1971, Geoscience Australia, on behalf of the National Mapping Council of Australia, carried out a simultaneous adjustment of 97 230 kilometres of two-way levelling. Mean sea level for 1966-1968 was assigned a value of 0.000m on the Australian Height Datum (AHD) at 30 tide gauges 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 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 30 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 at Hobart and Burnie. Mean sea level at both Hobart and Burnie was assigned the value of zero on the AHD (Tasmania).

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'.

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.