The ACT standard grid co-ordinates system applies to all ACT spatial data sets. If needed, however, they can be converted to the Australian coordinate reference system Geocentric Datum of Australia 1994  (GDA94) using the methods described. (Refer to the GDA project for information on future changes).

Coordinates

The ACT grid is made up of three separate zones: SGC/AGC, SGC/PGC and SGC/CC.  The cadastre in these zones, as well as the survey marks, is one of SGC/AGC, SGC/PGC or SGC/CC.  The zones are clearly defined and dividing lines between zones have dual coordinates.   Many marks have dual or triple sets of coordinate values.

ACT grid

The ACT grid is a Transverse Mercator map projection that uses the longitude of Mt Stromlo Trig Station as its central meridian.  It is based on the Australian Geodetic Datum 66 (AGD66), which is modified to take advantage of the ACT’s limited east-west dimension and account for scale differences caused by the ACT’s height above sea level.  The resulting ACT grid effectively can be treated as a plane (rather than geodetic) system of coordinates using the formulae of plane trigonometry, without the need to apply scale factors, grid convergence, arc-to-chord, or sea level corrections. As a result, for all but the most accurate work in the ACT, terrestrial, grid and plane measurements can be taken as being identical.  Follow this link for further information on the ACT Grid.

Conversion accuracy

It is possible to convert ACT grid coordinates to Australian Map Grid (AMG) or the Map Grid of Australia (MGA) coordinates and vice versa.
When ACT grid coordinates are converted to AMG, no distinction is made between coordinates of different zones. (Differences from this source are rarely more than a few centimetres but can be up to 0.3 metres in parts of the City Co-ordinate (SGC/CC) zone).

In the GDA9494 adjustment (based on GDA94 coordinates throughout Australia) ACT control was included and adjusted with surrounding New South Wales control. In New South Wales, the MGA94 – a coordinate system based on an “astronomic” datum, the GDA94 – is used.
GDA94 coordinates can be established by several methods of conversion, giving a varying range of accuracy:

• grid co-location incorporating distortion modelling;
• 7-Parameter similarity transformation (Moldensky-Badekas, Bursa-Wolff, 3D Helmert);
• Molodensky’s Formulas; or
• block shifts.

However, NSW-wide 7-parameter methods will not be as precise as those developed from ACT and surrounding area data when used in the ACT (ie methods one and two given below).
Published 1:10,000 and 1:2500 series maps are available showing ACT grid, AMG and AGD geographical coordinates.  In the future these maps will be annotated to the effect that they are not on GDA94 and information will be available to show how equivalent GDA94 coordinates can be determined.

Transformation methods

Both of the following methods transform ACT horizontal coordinates to accuracies appropriate to requirements. Please note:

• the quality of the transformed mark is always determined by its current Class/Order (C/O) statement, for example, if a mark has a C/O of B/2, the quality of the values from transformation are assigned B/U; and
• AHD height + 19.5 metres can be used for any AMG/AGD to MGA94/GDA94 transformation software requiring an ellipsoid height input. If there is no published height for a mark, an interpolated one from the largest-available topographic map is sufficient.  For transforming corners of blocks of the ACT cadastre always use a height of 610 metres.

Method 1 for GIS-type data sets, not including the ACT cadastre

The ICSM Geodesy Technical Sub-Committee has produced transformation grids using a Least-Squares Co-location method (NTv2).  When used with readily available software packages, this can be used to convert AMG values in the ACT and New South Wales to MGA94 (GDA94) values.  Using two steps – ACT to AMG then AMG to MGA94 – ACT grid coordinates can be transformed to MGA94 (GDA94) coordinates.  This process develops MGA94 (or ACT Standard Grid) co-ordinates to about ±0.2 metres of the published values.

Method 2 to transform survey control and corners of the ACT cadastre

An upgraded 7-parameter set has been derived from GDA94 surveys carried out in non-SGC/AGC zones in the ACT at a much closer spacing than has been used for the national survey, where a 7- parameter set of values derived from using the GDA94 values and the old ACT grid values of 40 control points was applied to ACT grid coordinates to obtain corresponding GDA94 coordinates.

Although the absolute accuracy given for 7-parameter methods is about 1 metre, within this GDA94 survey zone these parameters will give accuracies commensurate with grid co-location methods.  See ACT Government Survey Office Plan RM3609.

Using the same parameter set for the rest of the ACT (extrapolation), differences between published and transformed values seldom exceed 0.1 metre.

The ACT Planning and Land Authority will supply transformation parameters free of charge and carry out transformations when the number of points involved is less than ten.

You can also download a calculator to convert coordinates of single points from one system to another using method two.