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
	Introduction of ITFR2005

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 reference stations are therefore coordinated on the International Terrestrial Reference Frame (ITRF) as a means to overcome the errors between regional coordinate adjustments.

The datum for the OmniSTAR reference station network on the Australian continent has been ITRF2000 since 1st September 2004, when it was changed from the Geocentric Datum of Australia (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 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

The ITRF ties all the continents together worldwide. It 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. However, the accuracy of the stand-alone position (ie, non differential) is in many cases too low to notice the offset.

Most mapping in Australia has now moved from previous national datums such as AGD66 and AGD84 onto GDA94. GDA94 is a fixed datum, defined as the coordinates being the same in GDA94 and ITRF on a specific date, meaning that a point coordinated with respect to GDA94 will have the same coordinates in the future. Whilst not an issue for comparing with other GDA94 positions, this does mean that over time the ‘real world’ coordinates of a point within Australia have become different, as the landmass moves to the NNE.

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 frame as that of the reference sites, 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.

Currently the network is referenced to ITRF2000 at 1st October 2006 (Epoch 2006.75). These coordinates were updated on 1st July 2006 (Epoch 2006.5). The next coordinate updates are scheduled for 1st Jan 2007 and will be the values for 1st April 2007.

Obtaining GDA94 Coordinates

If GDA94 coordinates are required, we recommend that the datum shifts published by OmniSTAR for the current 6 month period should be applied to ITRF2000 (Current Epoch) positions output by your GPS receiver to obtain positions on the Geodetic Datum of Australia (GDA).

As outlined above, new datum shifts are applied biannually, and are listed below in both 7-parameter and dE, dN format for this year. A single set of shifts is also listed that apply for the complete calendar year. They relate to the offset between ITRF 2000 at 1st July (Epoch xxxx.5) and any calendar date in that year. As such, the maximum difference between GDA94 and ITRF coordinates will be twice as much, or some 70mm, for the period 1st January to 31st December.

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.

Current ITRF to GDA94 Datum Shifts for 2006

1. Spheroid details

2. Current datum shifts July - December 2006

These shifts are in use for the 6-month period 1st July 2006 to 31st December 2006 and represent the exact values at Epoch 2006.75 (1st October – midway through the period) OmniSTAR recommends using this set of datum shifts to achieve highest position accuracy in GDA94.

3. Datum shifts January - June 2007

These shifts are in use for the next 6-month period 1st January 2007 to 30th June 2007 and represent the exact values at Epoch 2007.25 (1st April – midway through the period). OmniSTAR recommends using this set of datum shifts to achieve highest position accuracy in GDA94.

4. Current datum shifts for whole year 2006

If you wish to use a single datum shift for the whole year, the following values are for the period 1st January 2006 – 31st December 2006 and represent the exact values at Epoch 2006.50 (1st July – midway through the period):
This set of datum shifts gives reasonable accuracy without the need for updating every 6 months.

5. ‘Whole year’ datum shifts for 2007 are as follows:

Full information on how to apply these shifts may be found in the GA paper “ International Terrestrial Reference Frame (ITRF) to GDA94 Coordinate Transformations” by Dawson & Steed.

See http://www.ga.gov.au/image_cache/GA3795.pdf

Introduction of ITRF2005

An update of the current ITRF2000 reference frame has just been announced (October 2006) by the International GNSS Service (IGS). Various organisations plan to introduce this from the start of 2007, and in due course positions on the OmniSTAR system will move to this new network datum.

As a further refinement of the current reference frame, the differences are in the order of 5 to 15mm maximum and are small compared to other error sources. In practice we believe the change is unlikely to be noticeable in most applications.

Should you have any questions relating to these changes please do not hesitate to contact your local OmniSTAR dealer or OmniSTAR direct on 1800 062 221.