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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 of the world' 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 – Department of Finance, Services and Innovation 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)
  • The Geoid
  • 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 of the world. Historically this was achieved with the aid of astronomical observations, whereas satellite positioning (GPS)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 virtually every hilltop across the country. Most trig stations are linked together by precise survey observations taken with theodolites, electronic distance measurement (EDM) devices and more recently GPS. 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 GPS. 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 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). For more information see 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.

The Geoid

The geoid is a derived surface which closely approximates mean sea level. However rather than tide readings like AHD, the geoid is an irregular equipotential surface based on gravity. A further complication is that technology such as GPS provides coordinates and height with reference to a mathematical model of the world 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 AUSGeoid09.

Current activity in this area involves the computation of a new AUSGeoid, as well as linking high accuracy GPS sites to the levelling network in order to model the difference between AHD and the geoid, which is up to one metre is some parts of Australia.

Global Navigation Satellite Systems (GNSS)

Many are now familiar with the Global Positioning System (GPS) which has revolutionised all location based applications. GNSS is relatively new terminology which describes not only GPS, but the whole range of emerging satellite positioning technologies such as:

Spatial Services has a twenty year history with GNSS activity and continiues to provide expertise in a wide range of 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 center for archiving, distribution or processing. The 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 now established CORSnet-NSW, which provides significant infrastructure support for GNSS activity across the State, and compliment similar endeavours in other jurisdictions.

Positional and Local Uncertainty

Positional and local uncertainty are new, 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 per cent 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 using 95% confidence levels.

Positional uncertainty is global or absolute, computed with respect to the reference frame or datum.

Local uncertainty is computed with respect to adjacent features within the same data set or source.

If we consider the GDA coordinates of a State Survey Mark (SSM) on a street corner in Sydney, the positional uncertainty may be as large as 0.19 metres, however the local uncertainty with respect to the SSM on the next street corner is only 0.02 metres.

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