OmniSTAR Network Datum

This information gives some background to OmniSTAR’s use of ITRF2000 (Current Epoch) as the network datum for Australia, and the datum shifts required to transform from ITRF coordinates back to GDA94.

	Background
	Fixed and Dynamic Datums
	Reference Station Coordinates
	Obtaining GDA94 Coordinates
	Current ITRF to GDA94 Datum Shifts

Background

The continuous improvement of Fugro’s DGNSS (Differential Global Navigation Satellite Systems) means that greater accuracies are now possible at longer distances from reference stations than ever before. Fugro operates a glbal network of reference stations; to maintain consistancy throughout all the stations in the network are coordinated with reference to the current International Terrestrial Reference Frame (ITRF).

The current datum for the OmniSTAR reference station network on the Australian continent is  ITRF2005 which was introduced in February 2008, an incermental change from ITRF2000. The move to an ITRF based refesince frame from the Geocentric Datum of Australia 1994 GDA94

There were 3 main reasons for making this change:

1. In Australia, GDA94 coordinates are now offset by some 80 cm from current ITRF2000/ITRF2005 coordinates. This is due to a high drift rate of the Australian tectonic plate (around 7 cm per year to the NNE).
2. When Australian reference stations were still on GDA94 this impacted on high accuracy, long-range solutions such as OmniSTAR VBS and particularly OmniSTAR-HP which often combine data from reference stations in both Australia and the wider Asia-Pacific region.
3. The increased use of "Space State" or Orbit and Clock-based solutions such as OmniSTAR XP, which are inherently based on ITRF current epoch, means that there will be an offset between an XP position and one derived from reference stations coordinated on GDA94 of the same 70 cm magnitude.

Fixed and Dynamic Datums

ITRF  is an Earth Centred datum (ie, with origin at the earth’s centre) and tied to the earth’s polar axis. Hence coordinates of all points on the earth’s surface slowly change over time as the continents drift in different directions. The ‘Epoch’ or time attached to an ITRF coordinate means these were the coordinates of the point on this specific day. ITRF is the standard datum used by the world’s scientific community, as it is the only system that ties together coordinates in different countries into one unified system.

The WGS-84 datum used by the GPS system itself is also regularly updated in order to minimise the difference between it and ITRF. This was last done in early 2002. The current offset between WGS-84 and ITRF2000 (Current Epoch) is around a couple of centimetres, and therefore WGS-84 coordinates from a GPS receiver are also some 80cm different to GDA94. Generally, for most non precision applications using GPS this error is of no concequence.

Most mapping in Australia has now moved from previous national datums such as AGD66 and AGD84 to GDA94. GDA94 is a fixed datum, it was arrived at by tying and fixing GDA94 to the ITRF reference frame on a specific date, as the datum is fixed or static a point coordinated with respect to GDA94 will continue to have the same coordinates over time irrespective of how the Australian land mass moves with respect to othe continents.

For further details see http://www.ga.gov.au/geodesy/datums/cosys.jsp

Reference Station Coordinates

In any differential system the coordinates of the mobile or rover are in the same coordinate  reference frame as that of the reference site(s), whether these are fixed sites on land or satellites in orbit. Because we use the ITRF, we need to regularly update the coordinates of the reference stations to compensate for the centimetre-level movements of the various tectonic plates on which they lie.

Although the reference stations move continually, OmniSTAR have chosen to update the coordinates within Australia only every six months to minimise the impact on users. This frequency keeps the difference between current and true coordinates in Australia within some 35mm and hence within the accuracy of OmniSTAR HP and XP systems. Station coordinates in use for this 6 month period are predicted forwards by 3 months to the mid-point, thereby making the maximum error in this period only +/-18 mm. Forward prediction uses the average of six international plate motion calculators.

Obtaining GDA94 Coordinates 

As with Australia's previous mapping datums, a datum shift has to be applied to convert the position output by the GPS receiver into GDA94. This is generally not handled within the GPS receiver, and for most OmniSTAR users it is easiest done within their mapping or guidance system software (please consult your dealer or sales support representative for further information). If the appropriate datum shift is applied to the position from a GPS receiver using OmniSTAR corrections to convert to GDA94 coordinates, the user can return to exactly th e same point year after year because the computed GDA94 coordinates will remain the same.

OmniSTAR follow the Australian Intergovernmental Committee on Surveying and Mapping (ICSM) recommendation that the ITRF2000 to GDA94 datum shifts published by Geoscience Australia (GA) are used to calculate positions in GDA94.

It should be noted that 'datum shift' parametrs provided below relate to ITRF2000 and not the current ITRF2005 datum in use, as soon as Geoscience Australia release a revised shift we will revise the shifts provided. The real-world difference between ITRF200 and ITRF2005 is in the order of 10 -15mm.

For details relating to the shits between ITRF200 and GDA94 see:  http://www.ga.gov.au/image_cache/GA3795.pdf

Current ITRF to GDA94 Datum Shifts for 2009

1. Spheroid details

2. Current annual datum shifts for 2009

Note: Whilst every attempt has been made to ensure that the datum shift information provided above is accurate users are encouraged to check their results against a known location. OmniSTAR Pty Ltd provides this information for guidance only and as such does not provide warranty or accept any liability resulting from of these parameter shifts.