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Guideline Transformations from GDA2020 to GDA94 Datum Control October 2019
Guideline Transformations from GDA2020 to GDA94 Datum Control October 2019
Guideline, Transport and Main Roads, October 2019
Copyright
© The State of Queensland (Department of Transport and Main Roads) 2019. Licence
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Contents
1 Datum Transformation from GDA2020 to GDA94 .......................................................................1
1.1 Background ..................................................................................................................................... 1
1.2 Using 12d Model for a 3D transformation from GDA2020 to GDA94 ............................................. 1 1.2.1 Create .csv file of GDA2020 longitudes, latitudes and ellipsoidal heights .....................1 1.2.2 Input .csv file in 12d Model (Survey Customisation Version 12.1.2) ..............................2
1.3 Data Checks ................................................................................................................................. 12 1.3.1 Transform back from GDA94 to GDA2020 as a 3D transformation check ................. 12 1.3.2 Transform back from GDA94 to GDA2020 as a 2D transformation check ................. 14
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1 Datum Transformation from GDA2020 to GDA94
1.1 Background
Geoscience Australia is the appointed Verifying Authority for Position. Geoscience Australia issues certificates of verification under Regulation 13 of the National Measurement Regulation 1999, commonly known as Regulation 13 Certificates. With the implementation of Geocentric Datum of Australia 2020 (GDA20200), Geoscience Australia is updating Regulation 13 Certificates on Continually Operating Reference Stations (CORS) survey marks to be published as GDA2020 coordinate values.
As many current projects are based on GDA94, it will be necessary to undertake a coordinate transformation from GDA2020 to GDA94 from the GDA2020 published values. The Intergovernmental Committee on Surveying and Mapping (ICSM) has published a technical manual (Geocentric Datum of Australia 2020 Technical Manual) from the ISCM website https://www.icsm.gov.au/sites/default/files/GDA2020TechnicalManualV1.1.1.pdf that provides descriptions and transformation parameters to assist with datum transformation between GDA2020 to GDA94.
Transport and Main Roads has used these transformation parameters within 12d Model to enable a conversion process.
1.2 Using 12d Model for a 3D transformation from GDA2020 to GDA94
1.2.1 Create .csv file of GDA2020 longitudes, latitudes and ellipsoidal heights
The simplest method to input the longitudes, latitudes and ellipsoidal heights from the Reg 13 certificates is to create a .csv file using the published values.
Using a text editor, enter the values in Point ID, longitude, latitude and ellipsoidal height. Enter the longitude and latitude values in degrees, minutes and seconds format. NOTE: the South values of latitude will be entered as negative values. For example, the following coordinate values from a Reg 13 Certificate GDA2020 values of:
Station ID RSBY, East Longitude 150° 47’ 24.30663”, South Latitude 23° 09’ 39.54236”. Elevation above ellipsoid 58.130 will be entered into a text editor as below:
NOTE: DO NOT use Excel to create a .csv file. It can round off the 9 decimals to 7 decimals.
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1.2.2 Input .csv file in 12d Model (Survey Customisation Version 12.1.2)
In 12d Model, User > TMR Survey > File INPUT > Read pn,x,y,z
or select “Read pn,x,y,z (Comma separated)” from the TMR Survey Customisation Toolbar
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Check that format on is as follows:
Create a Default model, for example “GDA2020 long lat ellipsoidal ht”
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Read in the .csv file
Now undertake the conversion. Survey > Conversions > General transformations
The transformation process will be a 3D transformation from GDA2020 longitude latitude and ellipsoidal height to GDA94 longitude latitude and ellipsoidal height
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Key Point
There are two methods of transforming coordinates between GDA2020 and GDA94. One is two-dimensional (2D) and the other is three-dimensional (3D).
The 2D transformation uses the National Transformation version 2 (NTv2) transformation grid to transform between Australian Datums. However, the NTv2 format does not store ellipsoidal height information and therefore cannot be used to transform heights.
The 3D transformation uses a 7-parameter similarity transformation that can be used to transform between datums in three-dimensions. It is important to realise that the Geocentric Datum of Australia 2020 (GDA2020) and the Geocentric Datum of Australia 1944 (GDA94) are both ellipsoidal surfaces expressed as latitudes and longitudes. And both these ellipsoidal surfaces have slightly different shapes from each other and with an approximate height difference of 0.1 m. Due to this ellipsoidal surface difference, it is important to undertake a 3D transformation and not a 2D one. The transformation 7-parameters for GDA94 to GDA2020 are listed on Table 3.2.
http://www.icsm.gov.au/sites/default/files/GDA2020TechnicalManualV1.1.1.pdf
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Select the following:
• Transformation Settings tab, then:
• Datum A < Datum B.
• Medium Accuracy: 7 parameters similarity transf. This uses 3D transformation parameters. Whereas, the High Accuracy: Grid shift file based transformation is for 2D only.
• Seven Parameters GDA94 to GDA2020 from the drop down menu
Select model for the Data source. In this example, the previously created “GDA2020 long lat ellipsoidal ht”
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Select the Target Model (for example, GDA94 long lat ellipsoidal ht) to copy to:
Select the Datum B settings. Select Long Lat – degrees (dms) and the relevant MGA2020 Zone
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Select the Datum A settings. Select the Data Format Long Lat – degrees (dms) and the relevant MGA94 Zone and Process
Now write an output file to create a comma separated file for longitude, latitude and ellipsoidal height in GDA94
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Or select “Write pn,x,y,z (Comma separated)” from the TMR Survey Customisation Toolbar
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Select the following:
• Model GDA94 long lat ellipsoidal ht
• Leave ‘X, Y, Z, Pt id and Delimiter default settings as is
• Change Number of decimal places to 12.
• Create a File name (for example, GDA94 long lat ellipsoid ht.csv)
• Select the Folder format to Ansi format (System codepage)
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File will highlight red. Ignore this and select Write.
Open the output file using Explore
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1.3 Data Checks
1.3.1 Transform back from GDA94 to GDA2020 as a 3D transformation check
Use a datum spreadsheet transformer from the following link:
https://github.com/icsm-au/DatumSpreadsheets/blob/master/Transformation_Conversion.xlsx
You will need to use Google Chrome and copy the website’s URL (link) to the browser’s address bar.
Select Download and open the Transformation spreadsheet in the bottom left corner
Select the Geographic to Cartesian tab. Then select Enable Editing
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Now you can edit the data in the grey boxes. Enter the GDA94 values previously obtained from the GDA2020 translation with 12d Model.
Now select the Cartesian to Geographic tab to view the GDA output results
Original values: Station ID RSBY, East Longitude 150° 47’ 24.30663”, South Latitude 23° 09’ 39.54236”. Ellipsoid 58.130
The difference is 0.00002” in longitude and 0.00002” in latitude which is approximately 0.5 mm difference in distance.
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1.3.2 Transform back from GDA94 to GDA2020 as a 2D transformation check
Use GDay Ver. 3.0 as a 2D transformation software
NOTE: GDAy does not use height values
Double click on the GDAy icon
Set Coordinate System
Input Coordinates:
• Datum: Geocentric Datum of Australia (GDA 1994)
• Coordinates: Geographic
− Format: d.mmssss
Output Coordinates:
• Datum: Geocentric Datum of Australia (GDA 2020)
• Coordinates: Geographic
− Format: d.mmssss
Then select OK
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Now Select the Transformation. GDA94 to GDA2020
When this message appears, select OK as we want to proceed with using the 7-parameter transformation.
Type in the GDA94 Input Coordinates
• Longitude and Latitude values previously calculated from 12d Model. Once entered, the degrees, minutes and second values will be displayed
− Note: Use negative sign (-) for the latitude value. Once entered, the S value (for South) values will be displayed
• and select Compute
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Original GDA2020 values: Station ID RSBY, East Longitude 150° 47’ 24.30663”, South Latitude 23° 09’ 39.54236”.
The difference is 0.000024” in longitude and 0.000024” in latitude which is approximately 0.7 mm difference in distance.
