• Before the Geocentric Datum of Australia 2020 (GDA2020) was adopted in the ACT, the datum was Australian Geodetic Datum 1966 (AGD66).
  • Information on past ACT projection, ACT Standard Grid Coordinates (SGC).
  • Information on past ACT coordinate systems.

Definition of the ACT Standard Grid

Prior to the adoption of GDA2020, the official horizontal datum for the ACT was the Australian Geodetic Datum 1966 (AGD66). The local Transverse Mercator projection of AGD66 is known as ACT Standard Grid, which was introduced in 1972, and the resulting coordinates are Standard Grid Coordinates (SGC).

The ACT Standard Grid projection minimises the difference between plane and ground distances, allowing for the use of plane trigonometry and generally minimising the need to apply combined scale factors and arc-to-chord corrections for many small-to-medium sized survey projects. ACT Standard Grid is commonly referred to as "Stromlo Grid" as the central meridian passes through Stromlo trigonometrical station.

Parameters for the ACT Standard Grid

Ellipsoid is the Australian National Spheroid (ANS)

Major semi-axis (a) = 6,378,160 metres

Flattening (f) = 1/298.25 exactly

Additional Quantities

Minor semi-axis (b) = 6,356,774.719 metres

Eccentricity² (e²) = 0.006 694 541 855

Second eccentricity² (e´²) = 0.006 739 660 796

Meridian of Origin is Stromlo Trig Station: 149° 00' 33".46139 East

Latitude of Origin is the Equator: 0°

Latitude of Stromlo Trig: 35° 19' 03".85060 South

Scale Factor (k0) at Central Meridian (CM) = 1.000 086

Zone Width: As the projection has only one zone, this quantity is discretionary, but preferably should be restricted so as to contain the E-W extents of the Australian Capital Territory.

Meridian Distance

The meridian distance (m) from the equator to Stromlo Trig along the line of longitude at 'sea level' = 3,909,857.2463 metres. It is calculated using the expression given in paragraph 4.3 of the abovementioned SP10 document.

The distance (mk0) from the Equator to the False Origin for Northings, that is, a point 600,000 metres south of Stromlo Trig, is comprised of the true distance from the Equator to Stromlo Trig, multiplied by the above Combined Scale Factor (k0).

(3,909,857.2463 x 1.000 086) + 600,000.000 = 4,510,193.494 metres. This is the quantity that is used as the 'False Northing' for software development.

The False Easting is 200,000.000 metres.

ACT Standard Grid Coordinates of Stromlo Trig

E 200,000.000 metres

N 600,000.000 metres

For further information, refer to:

Wellspring, K.H. (1973). Some aspects of the conversion from imperial to metric co-ordinates in the Australian Capital Territory. Technical Papers of the 16th Australian Survey Congress, Canberra. [1.3 MB]

Local Coordinate Systems

Due to historical reasons, there are three separate coordinate zones within ACT Standard Grid, being:

  • Standard Grid Coordinates/Adjusted Grid Coordinates (SGC/AGC)
  • Standard Grid Coordinates/Preliminary Grid Coordinates (SGC/PGC) and
  • Standard Grid Coordinates/City Coordinates (SGC/CC).
City Coordinate zone (CC)

Early surveys in the ACT were based on NSW trigonometric stations, which were established in the 19th century. As increased density of control was required, further control was established where the future City of Canberra was to be developed. This control was used in the years 1910 to 1917 for radial surveys along the planned main thoroughfares, and on the ACT/NSW border survey.

The radial surveys were later extended and accurately remeasured. They formed the basis for laying out of Civic and the earlier suburbs of Canberra, based on a design of a population of 75,000.

This City Coordinate zone was based on the NSW trigonometric system, which was calculated on the Clarke 1858 spheroid with Stromlo trig station defining the central meridian and origin.

City Coordinates are a plane system and are in feet.

As the furthest point on this plain is only 14km from the central meridian, the maximum error due to the divergence of the earth from the plane is negligible – about 3 parts per million (ppm). However, due to the survey techniques and instrumentation available at the time, significant coordinate discrepancies have since been found within this zone.

Preliminary Grid Coordinate zone (PGC)

Development outside the radial system began in the early 1960s. Control networks based on triangulation and precise traversing were established for the new areas of Woden Valley, Belconnen, and Weston Creek. These were adjusted by more rigorous methods, resulting in a new set of coordinates called Preliminary Grid Coordinates (PGC). Coordinates are in feet and are calculated on a Transverse Mercator projection with Stromlo trig as the origin. Woden Valley and Belconnen PGC are based on the Clarke 1858 spheroid, while Weston Creek PGC is based on the Australian National Spheroid.

Adjusted Grid Coordinate zone (AGC)

When it became apparent that developments might take place in any part of the ACT, a new system was adopted which was designed to give homogeneous coordinates over the whole Territory. For this purpose, the control was extended, strengthened, and rigorously adjusted.

Standard Grid Coordinates (SGC)

At the time of metrication, the opportunity was taken to apply further refinements to the coordinate system. A central scale factor of 0.999 99 was applied to reduce the overall effect of the projection. This factor was combined with a scale correction to take into account a height of 2000 feet (610 metres) above sea level. A false origin was also adopted, ensuring that coordinates are always positive throughout the ACT.

As SGC is a projection, the difference between grid and ground distances becomes significant when working some distance from the central meridian, measuring long lines, at altitudes above the 610m surface, and especially any combination thereof. To obtain a ground distance, apply the combined line scale factor to the grid distance. SGC arc-to-chord corrections to horizontal direction observations are negligible for all but the most precise survey projects.

Note, the terms SGC/AGC, SGC/PGC and SGC/CC refer to the metric conversion of the imperial coordinates. While the SGC/PGC and SGC/AGC areas are homogenous, areas of the older SGC/CC are not. Furthermore, differences of up to 0.15m are present between zones.

For detailed information on the history and development of the various horizontal datums and projections used in the ACT, refer to the following papers:

Kennedy, W.D. (1973). Origin and development of the ACT land surveying system [7.6 MB]. Technical Papers of the 16th Australian Survey Congress, Canberra.

Nicolson, J.K.L. (1973). An offering to those not fully conversant with metric conversion in the ACT [730.7 KB]. Australian Survey Office, Department of Administrative Services, Canberra.

Taylor, J.K. (1977). The cadastre of the Australian Capital Territory. Australian Survey Office, Department of Administrative Services, Canberra.

Wellspring, K.H. (1973). Some aspects of the conversion from imperial to metric co-ordinates in the Australian Capital Territory [1.3 MB]. Technical Papers of the 16th Australian Survey Congress, Canberra.

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