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15 changes: 9 additions & 6 deletions README.md
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Expand Up @@ -10,14 +10,14 @@ To change a coordinate from one datum to another, a mathematical process that mo

Transformation grids provide users of spatial data with a simple and nationally consistent method to transform coordinates between datums and potentially allow for a large number of common points to be used in developing the relationship model.

Australian grids of 2D coordinate shifts are provided in the Canadian National Transformation version 2 (NTv2) format as binary files (.gsb) but software provided by ICSM jurisdictions can readily convert them to ASCII format (.gsa). An in-depth explanation of the format can be found in Appendix C of the "GDAit" User Guide and the GDAit Software Documentation available from [www.dtpli.vic.gov.au/geodesy](www.dtpli.vic.gov.au/geodesy) > Geocentric Datum of Australia 1994 (GDA94) > GDA94 useful tools.
Australian grids of 2D coordinate shifts are provided in the Canadian National Transformation version 2 (NTv2) format as binary files (.gsb) but software provided by ICSM jurisdictions can readily convert them to ASCII format (.gsa). An in-depth explanation of the format can be found in Appendix C of the [GDAit] User Guide and the GDAit Software Documentation.

Detailed information on Australian datums and transformations between them is available in the GDA2020 Technical Manual available at [http://www.icsm.gov.au/gda/tech.html](http://www.icsm.gov.au/gda/tech.html) - in particular Chapter 3 and Appendix B describe the transformation methods including background to and area of application of the transformation files.
Detailed information on Australian datums and transformations between them is available in the [GDA2020 Technical Manual] – in particular Chapter 3 and Appendix B describe the transformation methods including background to and area of application of the transformation files.

## Description of the Transformation Grids
### AGD66 <> GDA94

Full national coverage two-way transformation file – see Appendix B in the GDA2020 Technical Manual available at [http://www.icsm.gov.au/gda/tech.html](http://www.icsm.gov.au/gda/tech.html) for details.
Full national coverage two-way transformation file – see Appendix B in the [GDA2020 Technical Manual] for details.

| File | EPSG Transformation Code
|-|-
Expand All @@ -26,15 +26,15 @@ Full national coverage two-way transformation file – see Appendix B in the GDA

### AGD84 <> GDA94

Full coverage two-way transformation file in those states that originally adopted AGD84 - West Australia, South Australia, Queensland - see Appendix B in the GDA2020 Technical Manual available at [http://www.icsm.gov.au/gda/tech.html](http://www.icsm.gov.au/gda/tech.html) for details.
Full coverage two-way transformation file in those states that originally adopted AGD84 - West Australia, South Australia, Queensland - see Appendix B in the [GDA2020 Technical Manual] for details.

| File | EPSG Transformation Code
|-|-
|`National_84_02_07_01.gsb`|1804 [AGD66 to GDA94 (5)]


### GDA94 <> GDA2020
Full national coverage two-way transformation files – See Sections 3.2 and 3.7 in the GDA2020 Technical Manual available at [http://www.icsm.gov.au/gda/tech.html](http://www.icsm.gov.au/gda/tech.html) for details.
Full national coverage two-way transformation files – See Sections 3.2 and 3.7 in the [GDA2020 Technical Manual] for details.
Two types of GDA94 – GDA2020 transformation grids have been developed:

• Conformal: predominantly plate tectonic motion (~1.8 m NNE)
Expand All @@ -43,7 +43,7 @@ Two types of GDA94 – GDA2020 transformation grids have been developed:

The difference between GDA94 and GDA2020 coordinates is comprised of a conformal transformation component primarily due to plate tectonic motion and an irregular (non-conformal) distortion component. The conformal component accounts for the primary coordinate movement between GDA94 and GDA2020 and replicates a seven-parameter similarity transformation. Conformal grids are also provided for Christmas Island and Cocos (Keeling) Isalnds.
The distortion component is attributable to several second-order effects, such as an improved realisation of the global reference frame over time; irregular ground movement since GDA94 was established; and improvements in computation methods since GDA94. These effects vary in magnitude and direction around the country and can be as large as ~0.5 m.
Advice on choosing which grid to use is included in Table 3.4 in in the GDA2020 Technical Manual available at [http://www.icsm.gov.au/gda/tech.html](http://www.icsm.gov.au/gda/tech.html).
Advice on choosing which grid to use is included in Table 3.4 in in the [GDA2020 Technical Manual].

| File | EPSG Transformation Code
|-|-
Expand All @@ -66,3 +66,6 @@ The transformation grids can be accessed by downloading from this project on Git

# License
[![License: CC BY 4.0](https://img.shields.io/badge/License-CC%20BY%204.0-lightgrey.svg)](https://creativecommons.org/licenses/by/4.0/)

[GDA2020 Technical Manual]: http://www.icsm.gov.au/datum/gda2020-and-gda94-technical-manuals
[GDAit]: https://www.propertyandlandtitles.vic.gov.au/surveying/geodetic-survey/geocentric-datum-of-australia