Understanding coordinate systems
This tutorial presents the various geographic and projection coordinate systems used in Australia. It explores the latest datum, known as the Geocentric Datum of Australia 2020 (GDA2020), and how it differs to its predecessor. Finally, it will discuss the concept of a Project Local coordinate system to eliminate rounding errors, and graphical limitations in non-GIS enabled software, such as Revit.
Geographic Coordinate System
A Geographic Coordinate System (GCS) uses a three-dimensional spherical surface to define locations on the earth. It includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid). In a geographic coordinate system, a point is referenced by its latitude and longitude values (reference). Sydney, for example, has a latitude of 33.877° S and a longitude of 151.215° E.
While a spheroid approximates the shape of the earth, a datum defines the position of the spheroid relative to the centre of the earth. In the last 15 years, satellite data has provided geodesists with new measurements to define the best earth-fitting spheroid, which relates coordinates to the earth’s centre of mass. An earth-centred, or geocentric, datum uses the earth’s centre of mass as the origin (reference). The most recently developed and widely used datum is the World Geodetic System 1984 (WGS84). WGS84 is used for GPS, while WGS 84/Pseudo-Mercator is a variant and used by web mapping tools such as Open Street Map.
A local datum aligns its spheroid to fit the earth’s surface in a particular area closely. A point on the surface of the spheroid is matched to a specific position on the surface of the earth. This point is known as the origin point of the datum. The coordinates of the origin point are fixed, and all other points are calculated from it (reference).
Since 2000, all Australian states and territories have adopted the Geocentric Datum of Australia 1994 (GDA94). However, it is essential to note that nothing on the surface of the earth is fixed. The earth’s tectonic plates are constantly moving in different directions and at different speeds. The Australian plate is moving in a north-easterly direction at a rate of about 7 cm per year. This movement means that the latitude and longitude of points in Australia are changing.
Since the implementation of the GDA94, the Australian plate has moved approximately 1.6 m. Australia’s coordinates are therefore no longer aligned with Global Navigation Satellite Systems (GNSS) such as GPS. To address this issue, as of 15 December 2017, Australia has implemented a new datum known as the Geocentric Datum of Australia 2020 (GDA2020). On 1 January 2020, NSW and VIC adopted GDA2020 with other states progressively adopting the new datum.
Projected Coordinate System
A projected coordinate system is defined on a flat, two-dimensional surface. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. Locations are identified by x,y coordinates on a grid, with the origin at the centre of the grid. Each position has two values that reference it to that central location. One specifies its horizontal position and the other its vertical position. The two values are called the x-coordinate and y-coordinate (reference).
The projected coordinate system associated with GDA94 is the Map Grid of Australia 1994 (MGA94). While the map projection associated with GDA2020 is the Map Grid of Australia 2020 (MGA2020). Both projections are divided into eight equal zones across Australia. Sydney, for example, falls with MGA zone 56.
Map Grid of Australia showing grid zones
Depending on the coordinate system adopted, Sydney, for example, could be classified as either GDA94 / MGA zone 56 (EPSG: 28356) or GDA2020 / MGA zone 56 (EPSG: 7856). Note that the EPSG code shown in brackets is a public registry of the various coordinate systems used throughout the world.
Project Local Coordinate System
Many frequently used CAD/BIM software have inferior GIS capabilities. This can make geo-referencing models difficult. Revit, for example, has a maximum distance limit from its internal origin of 10 miles or 16 kilometres. Beyond this distance, the graphical representation of elements becomes less reliable and less accurate. This limitation applies to geometry created in Revit as well as incoming geometry from an import or a link.
Coordinate system diagram
Since surveys, city models and planning overlays are authored in GIS-enabled software, such as ArcGIS or QGIS, they are often geo-referenced. This means, in general, they are located very away from the origin. Directly importing this information into Revit will cause an error, “Geometry in the file has extents greater than 20 miles (33km)”. To avoid this issue, it is recommended to set up a ‘Project Local’ coordinate system. This is merely repositioning the model closer to the origin point and falsifying its coordinate values. To understand how to achieve this, check out this tutorial.
While Australia undergoes the slow process of switching from GDA94 to GDA2020, it is prudent to review all data sets your organisation use to ensure they still meet the relevant legislation’s requirements. Sydney Local Environment Plan 2012 (version 27 March 2020) for example, is still referencing MGA94 despite NSW switching over on 1 January 2020. We therefore recommend organisations set-up two libraries, one for each coordinate system. Employees should be made aware of when to use GDA94 and when to use GDA2020. Diligence is of the utmost importance when setting-up coordinates. Even a small mistake can have devastating consequences further down the track.