The information below is intended to reflect the preferred practice of Main Roads Western Australia ("Main Roads"). Main Roads reserves the right to update this information at any time without notice. If you have any questions or comments please contact Ray Watson by e-mail or on (08) 9323-4152.
To the extent permitted by law, Main Roads, its employees, agents, authors and contributors are not liable for any loss resulting from any action taken or reliance made by you on the information herein displayed.
The densification of the Geodetic network across Australia has provided the framework for this common base, the Geocentric Datum of Australia 1994 (GDA 94) or Australian Geodetic Datum 1984 (AGD 84).
Engineering projects are predominantly designed using the modelling techniques associated with Computer Aided Design which use a vast quantity of digital data obtained from a number of sources eg survey, photogrammetric mapping, cadastral and utilities. The GDA 94/AGD 84 is ideally suited to mapping applications, however there are serious limitations when adopted for engineering applications. This is because GDA 94/ AGD 84 rarely maintains the ground measurement to plan measurement relationship. In other words designing on the GDA 94/AGD 84 would mean designing on a spherical plan and not on the 'flat ground'. This situation would further complicate the construction set out phase of the project by introducing scale factors to all measured distances and more importantly the design dimensions.
In order to overcome this situation Plane Project Coordinate Systems are generally adopted by most engineering agencies. Many types of plane coordinate systems have been utilised which are generally very localised and use approximations requiring the least complicated computing solution. The proliferation of project coordinate systems, even on occasions by different organisations operating within the same project has continued to be a problem. Data interchange between agencies has been unnecessarily complex with little data ever being transformed back to the standard GDA 94/AGD 84 base, if at all possible.
It is important to understand that Project Zones created on GDA 94 and AGD 84 are different and should not be confused. All GDA 94 project zones have "94" at the end of their name. eg PCG 94 is the GDA 94 project zones where as PCG is the AGD 84 project zone.
Central MeridianThe longitude of the project zone is generally centrally located within the bounds of the project zone.
False Easting In most instances a standard false easting of 50,000m is used to avoid using negative numbers.
False NorthingIs calculated by the latitude at the centre of the project zone. The number selected will avoid negative numbers.
Central Scale FactorIs the relationship between the height above spheroid (HAS) and the radius of the Earth.
Radius of the EarthGDA 94 Radius of Earth - 6,364, 400 m
AGD 84 Radius of Earth - 6,366,000 m
Example of an AGD 84 determined Central Scale Factor
From studying 1:100 000 map sheets over a project area the average height (h) is determined to be 500 m (this includes the geoid-spheroid separation).
Using R=6,366,000m in the formula R+h the uncorrected CSF is 1.000078542. R
The corrected CSF is 1.000079, which using the formula in reverse, gives a HAS value of 502.914m. This latter value is adopted as HAS for the Project Zone.
As Main Roads projects regularly cover large areas the accuracy of the GDA 94/ AGD 84 - Project coordinate transformation is all important. In selecting a transformation method the following features are desirable:
The transformation accuracy is the difference between the plane distance calculated from project coordinates and the true ground distance. As a general rule 1:50 000 or 20 ppm has been set as desirable to avoid measurable differences in survey control.
The accuracy of the project coordinate transformation is a function of both distance from the project Central Meridian and deviation from the chosen mean terrain height.
Sample GDA 94 Project Zone Summary SheetSample AGD 84 Project Zone Summary SheetProject Zone Presentation (PDF 420KB)