Damon Mcknight
The Australian Height Datum (AHD) is Australia's first and only national height reference system. It was adopted by the National Mapping Council in 1971 as the official height datum for Australia. The AHD is based on mean sea level observations from 1966 to 1968 at 30 tide gauges around the Australian coastline. The AHD was determined through a process called linear adjustment, which involved consolidating levelling and tide gauge data from various surveying operations. This data formed a single homogenous network called the Australian National Levelling Network (ANLN). The AHD serves as the benchmark for all height measurements and vertical control mapping in Australia.
| Characteristics | Values |
|---|---|
| Official name | Australian Height Datum |
| Year of introduction | 1971 |
| Official height datum for | Australia |
| Basis | Mean sea level observation from 1966-1968 at 30 tide gauges around the Australian coast |
| Adopted by | National Mapping Council |
| Date of adoption | May 1971 |
| Datum surface | Based on a least squares adjustment of 97,320 km of 'primary' levelling |
| Tasmania datum | Australian Height Datum (Tasmania) 1983 (AHD-TAS83) |
| Tasmania datum basis | Mean sea level in 1972 at tide gauges in Hobart and Burnie |
| AUSGeoid models | AUSGeoid09, AUSGeoid2020 |
| AUSGeoid09 use | With GDA94 ellipsoidal heights |
| AUSGeoid2020 use | With GDA2020 ellipsoidal heights |
| Alternative reference surface | Australian Vertical Working Surface (AVWS) |
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What You'll Learn
The Australian Height Datum (Tasmania) 1983 (AHD-TAS83)
The AHD-TAS83 is based on the mean sea level (MSL) in 1972 at tide gauges in Hobart and Burnie. It was computed using third-order differential levelling over 72 sections between 57 junction points and was finalised on 17 October 1983. The datum surface passes through the approximate MSL realised between 1966 and 1968 at 30 tide gauges around the Australian coastline.
The AHD-TAS83 is an important reference system for height measurements in Tasmania. It serves as the benchmark to which all height measurements in the state are referred. Prior to the adoption of the AHD-TAS83, Tasmania, like other states and territories, used its own local height datum. The shift to a standardised national height datum has improved consistency and accuracy in height measurements across the country.
The AHD-TAS83 is closely related to the AHD71, which covers the Australian mainland. However, it is important to note that there is a vertical offset between the two datums due to differences in the time and location of the MSL observations. This offset has been estimated to be approximately 10 cm to 26 cm. As a result, it is crucial to use the correct datum when referencing heights in Tasmania to ensure accuracy and consistency in measurements.
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The Australian Height Datum 1971 (AHD71)
The Australian Height Datum is an amalgamation of decades of spirit levelling work conducted by state and territory authorities across the country. It was corrected to align with the mean sea level observations of the tide gauges. The distances between each successive tidal gauge varied significantly, with the smallest interval between Port Kembla to Camp Cove (128 km) and the largest interval between Point Lonsdale to Port Kembla (1,150 km). Sea level observations were conducted hourly from 1 January 1966 to 31 December 1968 for 29 of the designated tidal gauges. The Port MacDonnell tidal gauge station was selected as the "nominal origin", with its mean sea level assigned a value of 0.000 m.
The AHD71 was primarily realised through the national adjustment of traditional levelling survey measurements to derive heights for an integrated network of survey control marks across the country. This network of survey control marks is managed by the Surveyor-General of Victoria (SGV) Geodesy division, which involves maintaining the physical survey control marks and levelling observations.
AUSGeoid09 and AUSGeoid2020 were developed to support the determination of AHD height estimates from Global Navigation Satellite Systems (GNSS) positioning. AUSGeoid09 must be used with GDA94 ellipsoidal heights, while AUSGeoid2020 must be used with GDA2020 ellipsoidal heights. The accuracy of AHD heights depends on the quality of the GNSS ellipsoidal height and the fit of the AUSGeoid model to the local AHD network.
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AUSGeoid09 and AUSGeoid2020
AUSGeoid09 is a model used to determine the Australian Height Datum (AHD) by converting ellipsoidal heights to AHD heights. The ellipsoidal model is a simplified mathematical representation of the Earth, which sometimes sits inside the Earth's surface and sometimes above it. The difference between ellipsoid and AHD heights can vary between -30 and +70 meters across Australia. Thus, ellipsoidal height measurements need to be converted to AHD heights. The current AUSGeoid09 model provides a blanket uncertainty value of 30-50mm.
AUSGeoid2020 is an updated model that provides the separation between the Geocentric Datum of Australia 2020 (GDA2020) ellipsoid and the AHD. It is a combined gravimetric-geometric model, sometimes referred to as a "hybrid quasigeoid model". This model is designed to support improved determination of AHD height estimates from GNSS observations. It provides ellipsoid to AHD separation values onshore with uncertainty. The gravimetric component is a grid of ellipsoid-quasigeoid separation values created using data from gravity satellite missions, re-tracked satellite altimetry, airborne and land gravity data, and a Digital Elevation Model for terrain corrections.
AUSGeoid2020 offers location-specific uncertainty, which is computed from a combination of levelling, GPS ellipsoidal height, and gravimetric quasigeoid data errors using least squares prediction. This model is more accurate than traditional third-order levelling in Australia for distances beyond 3 km. It is important to note that since AHD is an onshore datum, AUSGeoid2020 should only be used onshore or near the shore.
The development of these models ensures that Australia remains at the forefront of mapping, surveying, and positioning technologies, which are crucial for various applications, including precision agriculture.
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The Australian Height Datum's deficiencies
The Australian Height Datum (AHD), introduced in 1971, is the official vertical datum for Australia. It serves as the benchmark for all height measurements and vertical control mapping in the country. However, despite its widespread adoption, the AHD has been found to have several deficiencies, offsets, and distortions.
The AHD was established by taking the mean sea level of 30 tide gauges along the Australian coastline during 1966-1968, resulting in a vertical height of 0.000m. This process involved a massive undertaking of levelling work by state and territory authorities, covering 97,320 km of primary levelling data and an additional 80,000 km of supplementary data. However, due to the urgency to meet Australia's surveying needs in the 1970s, especially for mining exploration, the AHD was constructed using lower-quality equipment and techniques.
One of the significant deficiencies identified in the AHD is the presence of a distinct 1.5m north-south slope. This slope is a result of spatial variation in sea level change, freshwater outflow in harbour areas, and ocean dynamics. To address this issue, the AUSGeoid09 gravimetric quasigeoid model was introduced to improve height estimates from Global Navigation Satellite Systems (GNSS). Additionally, there is an offset of 26 cm between mainland Australia and Tasmania, along with regional distortions in Western Australia.
The accuracy of AHD heights is dependent on the quality of GNSS ellipsoidal heights and the fit of the AUSGeoid models to the local AHD network. The AUSGeoid models, including AUSGeoid09 and AUSGeoid2020, were designed to align with adjusted AHD heights on survey control marks. These models help account for offsets and improve the accuracy of height measurements.
Overall, while the AHD has been a crucial standard for height measurements in Australia, its deficiencies have prompted discussions about defining a new Australian vertical datum that can better meet the country's evolving needs.
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The Australian Height Datum's history
The Australian Height Datum (AHD) is the official national vertical datum for Australia. It was adopted by the National Mapping Council in May 1971 as the benchmark to which all vertical control for mapping was to be referred.
Prior to the creation of the AHD, levelling surveys were carried out by professional surveyors for construction and mapping purposes using only local or state-based vertical datums. By 1960, 20,800 km of controlled levelling had been completed. To assist mining companies with the search for oil during the 1960s, the federal government provided additional funds to speed up the surveying effort.
By 1970, the consolidation of levelling and tide gauge data from these collective surveying operations resulted in the formation of a 97,320 km "single homogenous network" of primary levelling spanning the entire country, named the Australian National Levelling Network. The AHD was determined through the linear adjustment of 757 primary levelling sections across this network, while simultaneously holding 30 tide gauges fixed at their mean sea level values. The datum surface passes through the approximate mean sea level (MSL) realised between 1966 and 1968 at tide gauges around the coastline.
The AHD is primarily realised through a network of survey control marks, maintained by the Surveyor-General of Victoria. The Victorian levelling observations are combined in one contiguous, state-wide levelling network adjustment dataset, supporting rigorous propagation and determination of adjusted AHD heights across the state.
Since its inception, numerous deficiencies, distortions, offsets and other errors have been identified and examined using newer technologies. This has led to discussions about defining a new Australian vertical datum.
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Frequently asked questions
The Australian Height Datum (AHD) is the official national vertical datum for Australia. It is the benchmark to which all height measurements in the country are referred.
The Australian Height Datum was introduced in 1971. It was derived through the consolidation of levelling and tide gauge data from various surveying operations.
The Australian Height Datum was determined by taking the mean sea level of 30 tide gauges along the Australian coastline during 1966-1968. This data was combined with levelling observations to create an extensive network of level sections and junction points.
While the Australian Height Datum remains the published vertical datum for Australia, newer technologies have revealed deficiencies, offsets, and distortions within the datum. These issues impact users requiring consistent heights over large distances for engineering and scientific purposes.
