Coordinate systems

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.

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.

However, in the SGC/CC (City Zone), most permanent marks (Major Control, Sectional Control, SRs, CRMs, etc) placed during the previous 25 years have been connected and adjusted to marks having SGC/AGC values.

This was done (and is still being done) as part of a project to upgrade the ACT cadastre to SGC/AGC.

Before starting any surveys in the City Zone using these marks, it is essential that you confirm their status and reliability (coordinate zone, Class/Order, etc). This is especially the case for boundary definition surveys or setting-out for works on or near boundaries.

In these situations, you must set out from reliable original reference marks (eg, concrete blocks, galvanised iron pipes, drill-holes and wings, etc) and then connect the survey to the SRs, CRMs, etc and show the connections to the permanent marks on your plan.

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.

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 GDA94 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 read off a topographic map at the largest available scale is sufficient. A height is required, as it is a necessary input for the transformation process. However, heights resulting from the transformation must NOT be used as being a reduced level on the ellipsoid being transformed to. The official height datum for Australia is currently the Australian Height Datum of 1971 (AHD71).

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.

If you have a file of 500 points or less, download the Coordinates transformation for single or multiple points and input files calculator.

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