But the Australian Tectonic Plate doesn't move evenly.

To say that the Australian Tectonic Plate moves North East by 70mm every year is an oversimplification for several reasons.

The first is that the plate is also rotating slightly… Western Australia is moving about 10% faster that the Eastern States. As there is a slight rotation it’s not necessarily as simple as applying a simple translation to all coordinates on our tectonic plate.

The second is that the plate is also distorting a little in places. A 4000 kilometer mass of relatively thin crust can’t be expected to float without a little distortion - and that is what happens - some parts move locally at different speeds and compact or stretch. Even heavy rain can distort the ground - the recent hurricane Irma depressed large parts of Southern US by up to 20mm.

A further cause of distortions is man made - mining can cause subsidence and movement of kilometers of the surface; ground water removal has a similar effect - areas around Perth have subsided by 200mm over the past 20 years.

Additionally, a small number of known locations (e.g. Trig Stations and State Survey Marks) were inaccurately surveyed several decades ago; some of these marks are now known to be incorrect - sometimes by several tens of meters. The use of GPSs has identified some of these errors and when these are corrected the location of these marks and anything that is referenced from them will change significantly.

If you need your data to be really accurate - like surveyors, road and rail builders do, and anyone who has every argued with their neighbor about a fence line, then you’ll probably need to know exactly how many millimeters that tiny patch of Australia has moved since 1994. Fortunately, GeoScience Australia knows the answer. They have had a number of fixed GPS reference stations concreted into the bedrock of the Australasian Plate for many years. So they know which bits of dirt have moved how far. These stations are known as CORS….

 

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