Coordinate transformation is the process of changing coordinates from one reference frame or datum to another: for example, from GDA94 to GDA2020 or from GDA2020 to ITRF 2014. In contrast, a coordinate conversion changes coordinates from one coordinate reference system to another coordinate reference system on the same datum or reference frame: for example, from GDA2020 latitude and longitude to MGA2020 Easting, Northing and Zone.
ICSM, in conjunction with Geoscience Australia, has arranged for the following GDA94 ⇔ GDA2020 transformation products:
- a 2D transformation and distortion grid files; and
- a 7 parameter similarity (Helmert) transformation.
Values supporting a plate motion model to transform coordinates between GDA2020 and ITRF2014 have also been defined.
The Geocentric Datum of Australia 2020 Technical Manual defines and describes the preferred methods to transform coordinates between GDA94 and GDA2020, and GDA94 to AGD84 and AGD66, as well as how to perform coordinate conversions and apply the appropriate AUSGeoid. While this document will satisfy the information requirements of most users, the GDA94 Technical Manual v2.4 has further background on how and why GDA94 was established and additional information about its relationship to AGD66 and AGD84.
ICSM has also organised the development of a number of transformation tools to apply the GDA94 ⇔ GDA2020 transformation products which, in conjunction with simple test datasets also provided, act as a validation service for testing open source and proprietary applications of the transformation products.
ICSM has not defined a set of parameters that directly transform between AGD66 ⁄ AGD84 and GDA2020. It is recommended to first transform to GDA94 and then to GDA2020. ICSM recommends that the Canadian National Transformations version 2 (NTv2) grids be used for transformations wherever possible.
“Know your data know your datum”
Whichever transformation is selected, the metadata for the transformed dataset should always contain the method and parameters used. This ensures other users of the data are aware of the accuracy and lineage of the data.