Australian Height Datum
Official vertical datum for Australian surveying and engineering.
Wikipedia / Wikimedia Commons
The official vertical reference for all height measurements in Australia is the Australian Height Datum (AHD), established in 1971. This datum was built by combining decades of spirit levelling surveys carried out by various state and territory authorities, and it was adjusted to match the mean sea level recorded at thirty tide gauges distributed around the entire coastline. Although the AHD remains the published standard for surveying and engineering across the country, newer technologies have revealed numerous errors, offsets, and distortions, prompting discussions about creating a replacement.
**Background**
Before the AHD existed, surveyors relied on local or state-based vertical datums for construction and mapping. World War II exposed a severe lack of topographic maps of the Australian mainland, leading the Commonwealth Surveyor-General and state surveyors-general, under the Department of the Interior, to establish the National Mapping Council of Australia in 1945. Its primary goal was to plan and manage the nation’s topographic mapping. The council’s first members included the Director of National Mapping, the Commonwealth Surveyor General, and the Surveyors-General from each state. At their initial meeting, these authorities discussed the need for a unified national surveying and mapping scheme. They unanimously agreed that the Director of National Mapping, Frederick Johnston, would coordinate local, state, and Commonwealth efforts to map the entire continent—a project later called the Australian Levelling Survey.
**The Australian National Levelling Network (1945–1971)**
Between 1945 and 1955, state authorities in New South Wales, Victoria, and Western Australia, with help from the Royal Australian Survey Corps, began the Australian Levelling Survey by levelling approximately 4,800 miles (about 7,725 km) of the mainland. By 1960, with the addition of the Northern Territory and Queensland, 20,800 km of controlled levelling had been completed. During the 1960s, the federal government provided extra funds to the Director of National Mapping to hire private contract surveyors, speeding up the work to assist oil exploration by mining companies. By 1970, all these efforts had produced a 97,320 km “single homogenous network” of primary levelling covering the entire country, known as the Australian National Levelling Network. The AHD was determined by li
- introduced
- 1971
- field
- Geodesy, Surveying, Cartography
- country
- Australia
- known_for
- Official vertical datum for all surveying and engineering operations in Australia
- based_on
- Australian National Levelling Network (97,320 km of primary levelling)
- tide_gauges_used
- 30
- nominal_origin
- Port MacDonnell tide gauge station
Lore & Background
Prior to the AHD, levelling surveys used only local or state-based vertical datums. World War II exposed the lack of Australian topographic mapping, leading the Commonwealth Surveyor-General and state surveyors-general, under the Department of the Interior, to establish the National Mapping Council in 1945. The council's first meeting discussed a unified national scheme, and Director of National Mapping Frederick Johnston was tasked with coordinating the Australian Levelling Survey. From 1945 to 1955, New South Wales, Victorian and Western Australian authorities, with the Royal Australian Survey Corps, levelled approximately 4,800 miles (about 7,725 km). By 1960, 20,800 km were completed with the addition of the Northern Territory
Reader's Guide
The Australian Height Datum remains the published vertical datum for all surveying and engineering operations in Australia, but newer technologies have uncovered numerous deficiencies, offsets and distortions. These include a north–south slope of 1.5 m, an offset between mainland Australia and Tasmania of 26 cm, and regional distortions throughout Western Australia. The north–south slope invalidates the assumption that the datum surface is perfectly horizontal, which is significant for large-scale studies spanning long distances. The AHD was constructed using a lower standard of spirit-levelling equipment and techniques to meet the urgent need for vertical control and topographic mapping during the 1970s mining boom; approximately 80% of primary levelling was completed by private contractors between 1960 and 1970. The mean sea level approach, which fixed 30 tide gauges over a two-year period, ignored natural variations in sea-surface topography, leading to these systematic errors. Discussions about defining a new Australian vertical datum have arisen as a result.
Did You Know?
- The Australian Height Datum was derived from 97,320 km of primary levelling and 80,000 km of supplementary levelling.
- Sea level observations for 29 of the 30 tide gauges were conducted hourly from 1 January 1966 to 31 December 1968; the Karumba gauge used data from 1 January 1957 to 31 December 1960.
- The smallest interval between successive tide gauges was 128 km (Port Kembla to Camp Cove), and the largest was 1,150 km (Point Lonsdale to Port Kembla).
- The north–south slope error in the AHD is approximately 1.5 m from the north coast to south coast of Australia.
Frequently Asked Questions
What is the Australian Height Datum?
The Australian Height Datum (AHD) is the official vertical reference system used across Australia for all height measurements in surveying, engineering, and cartography. It defines a single consistent 'zero' level from which elevations are measured nationwide.
When was the Australian Height Datum established and how was it built?
The AHD was formalised in 1971 after state and territory authorities pooled decades of spirit-levelling survey data. That collected network was then adjusted so its zero point aligned with the mean sea level recorded at thirty tide gauges spread around the entire Australian coastline.
What is the Australian Height Datum based on physically?
It rests on the Australian National Levelling Network, a continuous chain of primary levelling lines totalling roughly 97,320 km. Those lines link the thirty coastal tide gauges to inland benchmarks, creating one unified vertical framework for the continent.
Why is the Australian Height Datum important to Australia?
It gives every surveyor, engineer, and cartographer a single shared reference so that a height measured in Perth and one in Darwin are directly comparable. Without it, infrastructure projects, flood modelling, and land registration would lack a consistent nationwide baseline.
Is the Australian Height Datum still accurate, or is it being replaced?
Although the AHD remains the published standard, modern satellite and GPS-based techniques have exposed measurable offsets and distortions across the original levelling network. Those findings have sparked ongoing discussions about developing a newer, more precise vertical datum to eventually supersede it.
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