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Geodesy and GDA

Whether you are carefully unfolding the remnants of your favourite bushwalking map, obeying the commands of your car navigation system or creating a unique digital view of the world using your computer - all of this exists thanks to geodesy!

Geodesy is the scientific aspect of surveying where the 'shape and size of the earth' is taken into account. As we know, the earth is not flat, therefore geodesy is fundamental to everything that uses position or location.

Spatial Services participates in the following geodetic activities in NSW, many of which support national initiatives:

  • Geodetic Survey Network
  • Geocentric Datum of Australia (GDA)
  • Australian Height Datum (AHD)
  • Geoid Determination and AUSGeoid
  • Global Navigation Satellite Systems (GNSS)
  • Continuously Operating Reference Stations (CORS)
  • Positional and Local Uncertainty

Geodetic Survey Network

The science of geodesy is based on the creation of mathematical models which best represent the shape and size of the earth. Historically, this was achieved with the aid of astronomical observations, whereas satellite-based positioning (GNSS) and other slightly more obscure techniques such as SLR (Satellite Laser Ranging) and VLBI (Very Long Baseline Interferometry) are currently used, see Geoscience Australia for more detail.

The physical part of geodesy is seen by the network of trigonometrical (trig) stations, which are located on many hilltops across the country. Most trig stations are linked together by precise survey observations taken with theodolites, Electronic Distance Measurement (EDM) devices and more recently Global Navigation Satellite System (GNSS) technology. The whole geodetic network is now underpinned by the Australian Fiducial Network (AFN) and is fundamental to all spatial applications.

Geocentric Datum of Australia (GDA)

GDA is the Australian datum for horizontal coordinates or position and is compatible with global reference frames such as those used by GNSS. Spatial Services is responsible for GDA in NSW and more detailed information can be found on our GDA page.

Australian Height Datum (AHD)

Height above sea level in Australia was defined in 1971 by mean sea level observations taken at thirty tide gauges around the Australian coast, linked together by levelling through a network of survey bench marks along every major road across Australia. In NSW, these marks were often placed adjacent to the old mile posts, many of which have now disappeared. More information can be found on the ICSM website and the Geoscience Australia website.

Spatial Services has extended this original survey to approximately 123,000 marks throughout NSW, providing a valuable reference datum where height is required.

Geoid Determination and AUSGeoid

The geoid is a derived surface that closely approximates mean sea level. However, rather than being based on tide gauge readings like AHD, the geoid is an irregular equipotential surface based on gravity. A further complication is that technology such as GNSS provides coordinates and height with reference to a mathematical model of the earth called an ellipsoid. The difference (separation) between the geoid and the ellipsoid is therefore critical if accurate height is required. The current model for geoid/ellipsoid separation in Australia is AUSGeoid2020.

Current activity in this area involves the refinement of AUSGeoid and the development of an Australian Vertical Working Surface (AVWS).

Global Navigation Satellite Systems (GNSS)

Many are now familiar with the United States’ Global Positioning System (GPS), which has revolutionised all location-based applications. GNSS describes not only GPS, but the whole range of existing and emerging satellite-positioning constallations, including:

  • Russia’s GLONASS
  • Japan’s QZSS
  • China’s BeiDou
  • the European Union’s Galileo

Spatial Services has a 25-year history with GNSS activity and continiues to provide expertise in a wide range of related services and applications.

Continuously Operating Reference Stations (CORS)

CORS networks are comprised of permanent ground-based GNSS receivers at known locations, where the observation data is sent via high-speed communication links to a network control centre for archiving, distribution or processing. These networks are established for many reasons:

  • Crustal motion studies, such as in Japan, New Zealand and other Pacific nations.
  • Reference datum research and precise orbit determinations, such as the International GNSS Service (IGS).
  • Augmentation services to enhance the accuracy of GNSS for navigation, such as those used by AMSA and CASA.

Spatial Services has established CORSnet-NSW, which provides significant infrastructure support for GNSS activities across the State and compliments similar endeavours in other jurisdictions.

Positional & local uncertainty

Positional and local uncertainty are simple indicators of the quality of position. For horizontal coordinates of features or data, the uncertainty is the radius of a ‘circle of error’ derived using a 95% confidence level. In other words, we are 95% confident that the ‘true’ value lies within that circle. For height, the uncertainity is a linear interval, again expressed at the 95% confidence level.

Positional Uncertainty (PU) is global or absolute, computed with respect to the reference frame or datum.

Local Uncertainty (LU) is local or relative, computed with respect to adjacent features within the same dataset or source.

If we consider the GDA coordinates of a State Survey Mark (SSM) on a street corner in Sydney, the PU may be as large as 0.19 m, while the LU with respect to the SSM on the next street corner may only be 0.02 m.

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