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Geodetic

Geodetic surveying is a science that involves the determination of positional information, such as latitude, longitude and height, for spatial data with respect to a scientific definition or reference system of the earth's surface.

The Geodetic Unit provides the geodetic datums and global spatial data infrastructure required to support land development and geoscience activities in the Territory.

The Unit :

  • Maintains the integrity of the NT Geodetic Survey System (NTGESS) and Geospatial Reference System, which has a framework based on the Geocentric Datum of Australia 94 (GDA94).
  • Enables effective and efficient access to the co-ordinated survey control points stored in NTGESS.
  • Provides a survey - accurate geospatial referencing system for the NT Land Information System (NTLIS) and the surveying and mapping industry throughout the Territory;
  • Enhances the cadastral framework and subsequent spatial databases by co-ordination; and
  • Offers professional geodetic advice and support to Government, industry and the public on spatial data issues such as:
  • Adoption of the Geocentric Datum of Australia (GDA) for the survey, mapping, LIS and GIS community.
  • Standards and specifications for geodetic data capture, manipulation, storage and dissemination.
  • Use of GPS technology for spatial data capture.

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Geodetic or survey control information (NTGESS)

The Survey Branch is currently redeveloping a web based inquiry system that will enable a user to search graphically and textually for co-ordinated survey marks in the NT.

Until this system is complete, all queries regarding survey mark information should be directed to the contacts. To assist the inquiry, please have available the approximate location and extent of search; the datum and accuracy level of the information being sought; and contact particulars. Also requests for survey mark information should be placed at least 2 days prior to the date of requirement.

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Geocentric Datum of Australia

A datum is a mathematical surface on which a mapping and co-ordinate system is based. A geocentric datum has the centre of the Earth as its origin. The advantage of the geocentric datum is that it is directly compatible with satellite-based navigation systems, such as GPS.

GDA is the new geocentric datum adopted by Australia and has been adopted by the Northern Territory.

GDA94 is the geodetic co-ordinate (latitude and longitude) computed in terms of the datum at 1 January 1994. Likewise, the Universal Transverse Mercator co-ordinates (east, north and zone) generated from GDA94 is known as MGA94 (Map Grid of Australia 1994) co-ordinates.

Other commonly used datums are AGD and WGS84.

AGD was based on a mathematical surface of the earth, which was designed to fit the Australian region, and is not a geocentric datum. In fact its origin is approximately 200 m offset from the Earth's centre of mass.

WGS84 is the datum used in GPS receivers and is an Earth-centred datum used and developed by the US Department of Defence.

For all practical purposes GDA and WGS84 co-ordinates are the same.

Generally, the difference between GDA and AGD co-ordinates in the NT is 207 m in a north easterly direction, but this can also vary slightly from place to place. A more accurate transformation between these datums, the technical transformation parameters and / or access to conversion software (GDAit) is available upon request.

Further information on GDA can be found in the Inter-government Committee on Surveying and Mapping (ICSM) publication "GDA Technical Manual" website.

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Geodetic calculations

The Geodetic Unit will perform the following calculations for clients if sufficient details are supplied :

  • The conversion between geographic and UTM co-ordinates
  • The determination of azimuth (bearing) and distance between points on the earth's surface in a specified datum
  • The transformation of grid and / or geographic co-ordinates from one datum to another.
  • The geoid / spheroid height separation (N-Value)
  • The determination of adjusted GDA 94 co-ordinates from RINEXED GPS data.

A service fee may apply for some calculations.

Alternatively, formulae and / or excel spreadsheets are available for most calculations from either the Inter-government Committee on Surveying and Mapping (ICSM) publication "GDA Technical Manual" website OR the “Geodesy and GPS” website hosted by Geoscience Australia.

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GPS test network

The Survey Branch has established pillars in the Darwin region to verify hardware, software / firmware and the operation of GPS equipment / systems or similar global navigation satellite systems. The test network also incorporates the existing EDME baseline pillars at Howard Springs and a pillar on Sattler Airstrip along the Stuart Highway.

This facility is not intended to be used as a means of obtaining legal traceability of GPS measurements / results.