Transformation of Coordinates

​​​Three methods of transforming coordinates from the Australian Geodetic Datum 1966 to the Geocentric Datum of Australia 1994 are detailed below and also in Chapter 7 of the ICSM GDA Technical Manual. In order to obtain the best possible spatial accuracy for any type of data, it is recommended the high accuracy grid transformation process be used if possible.

Low Accuracy Transformation

The application of a block shift, determined from one or more common points can be a convenient way of transforming coordinates where only a low order of accuracy is required. The accuracy of this method depends on the accuracy of the common point coordinates and distortions that may be present over the area in which they are being applied.

An average block shift based on the coordinates of 163 survey control marks distributed over the whole of the state enables a conversion to be made with an estimated maximum error of 3m.

TO CONVERT
AGD66 to GDA94
Eastings
Add 112m
Northings
Add 183m
Latitude
Subtract 5.4 sec
Longitude
Add 4.8 sec

Table 1: Block shift values for Tasmania - AGD66 to GDA94


Medium Accuracy Transformation

Similarity transformation parameters to convert between AGD and GDA have been developed for Tasmania, these have an estimated maximum error of 0.5m.

PARAMETER
AGD66 to GDA94
Shift dx (m)
-120.271
Shift dy (m)
-64.543
Shift dz (m)
161.632
Rotation about x (sec)
-0.2175
Rotation about y (sec)
0.0672
Rotation about z (sec)
0.1291
Scale (ppm)
2.4985

Table 2: Similarity transformation parameters for Tasmania - AGD66 to GDA94


A spreadsheet to compute similarity transformations, with sample input and output, is available from the ICSM GDA Technical Manual.

High Accuracy Grid Based Transformation

A process has been developed that enables high accuracy transformations to be carried out over the whole of Australia. This involves a similarity transformation, which is then refined further by distortion modelling. This modelling involves interpolation within a grid file of coordinate shifts that compensates for distortions in a particular area. Tests on the accuracy of this process by applying it to 469 survey control marks in Tasmania with survey accurate AGD66 and GDA94 coordinates resulted in a maximum transformation error of 0.3m. 72% of these marks had a transformation error of less than 0.05m.

The following software supports high accuracy grid based transformation.

PROGRAM
SOURCE
NOTES
GDAit​
Vic Dept of Transport, Planning and Local Infrastructure
Developed by Surveyor-General Victoria
GDAy​
QLD Dept Natural Resources & Mines
Supports coordinate conversion within a datum. ie convert grid coordinates to geographic on the same datum.
GEOD
NSW Land and Property Information
Supports coordinate conversion within a datum. Will convert a file of points in .csv or space separated (free) format.
DatumTran
NSW Land and Property Information
Supports many ASCII data file formats, also ESRI binary "shape" files.

These programs are freely available from the ICSM GDA Software web page.

The first time they are run they will prompt for a grid file. Either of the following can be used:

National transformation grid file (3.2MB)national66.zip
Grid File covering Tasmania only (466KB)Tas66.zip
A spreadsheet containing sample input and output for transformations carried out using GDAit is available from the ICSM GDA Technical Manual​​.

Contact

Program Manager Geodetic System
Scott Strong
134 Macquarie Street
HOBART TAS 7000
Phone: 03 6165 4175
Email: scott.strong@dpipwe.tas.gov.au

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