How are GDA2020 co-ordinates realised

How are GDA2020 co-ordinates realised?

On 11^th October 2017, the National Measurement Institute signed off and gazetted the Geocentric Datum of Australia (GDA2020) as the new Recognised-Value Standard of Measurement of Position. Here is a link to the Legislative Instrument.

GDA2020 is based on the most recent realisation of the International Terrestrial Reference Frame 2014 (ITRF2014) at epoch 2020.0. The Geocentric Datum of Australia 1994 was based on the realisation of ITRF1992 at epoch 1994.0. Since then:

due to plate tectonic motion, GDA94 coordinates have continued to diverge from ITRF92 coordinates. By 2020, the difference will be approximately 1.8 m;
there have been many improvements and realisations of the ITRFs which better define the shape of the earth. For example, differences between ITRF1992 and ITRF2014 (on which GDA2020 is based) causes a ~9 cm change in ellipsoidal heights in Australia (GDA2020 heights are ~9 cm less than GDA94 ellipsoidal heights); and
parts of the Australian crust have deformed (e.g. subsidence and surface heave).

GDA2020, like GDA94, is a static datum. Due to the ellipsoidal height difference, the new geoid model AusGeoid2020, should only be used with GDA2020.

For more technical details, refer to ISCM website: GDA2020 fact sheets, technical manual and GDA products and tools.

What ellipsoid does GDA2020 use?

GDA2020 uses the GRS80 ellipsoid. This is the same ellipsoid as GDA94. Therefore, the semi-major axis and semi-minor axis of the earth's defined shape are the same. There is no change to the parameters used for calculations on the spheroid.

What geoid should be used with GDA2020?

Improved realisation of the reference frame has resulted in a minor difference between ellipsoidal heights in GDA94 and those defined in GDA2020. AusGeoid2020 has been computed and published for use with the GDA2020 datum. Previous versions of AusGeoid should continue to be used with GDA94 coordinates.

What has changed with Horizontal Accuracy statements?

The Horizontal Accuracy for GDA94 were determined based on SP1 v1.7 and were based on interpretation of class and order. However, this standard has been updated to SP1 v2.1 and GDA2020 coordinates were computed using a fully rigorous adjustment. This fully rigorous adjustment enables the determination of the computation of positional uncertainty and relative uncertainty between any survey control marks.

For the publishing of GDA2020 coordinates with GOLA, the term ‘Horizontal Accuracy’ has been changed to ‘Relative Horizontal Accuracy’. Please note, the GDA2020 values have been based on the full Variance Covariance (VCV) matrix of the adjustment and may not be comparable to the GDA94 reported values.

The rigorous relative horizontal accuracy can only be provided for a pair of survey marks. To assign an estimated relative horizontal accuracy to individual marks, an average value of the relative accuracies between the mark and its closest neighbours has been provided in parts per million (ppm).

Please note, to determine a mark’s quality, relative horizontal accuracy should always be used in conjunction with its absolute horizontal accuracy as defined by Positional Uncertainty (PU).

How have published ellipsoidal heights been calculated?

Recent modifications to the GOLA report now allow the publishing of observed ellipsoidal heights as well as heights determined from an interpolation of the geoid and the orthometric height. Most of the reported ellipsoidal heights have been calculated from the national GDA2020 adjustment. These heights are reported if the mark has a GNSS network connection to either an Asia-Pacific Reference Frame (APREF) or National GNSS Campaign Archive (NGCA) calculated ellipsoidal height.

What is the difference between the new project grid parameters computed for GDA2020 and the parameters used with GDA94?

Same as GDA94, GDA2020 uses Universal Transverse Mercator (UTM) projection named MGA2020. As the UTM predefined parameters include false easting and northing, MGA2020 will differ by approximately 1.8m from MGA94.

However, the project grids are rigorously defined on the Transverse Mercator (TM) projection where some of the standard projection parameters of the UTM have been modified to suit the region concerned. In particular, a central scale factor (CSF) is selected which compensates the projection enlargement against the ground to ellipsoid reduction.

All project grids have been redefined for GDA2020. To avoid confusion 100,000m has been added to the false northing. For more information and project grids parameters, refer to Project grids and calculations on Landgate's website.