Australia on the move

The entire continent of Australia has officially jumped 1.8 metres to the north-east as satellite-based global positioning systems try and keep pace with one of the fastest-moving tectonic plates on the planet.

Australia this month introduced new official reference points for GPS-based systems that updates the ones last benchmarked in 1994 when the continent was 1.5 metres further south-west compared with where it is today.

The jump to 1.8 metres – mostly northwards, but also a little bit easterly – accounts for the fact that parts of Australia are marching at different speeds; Perth and Darwin have moved 1.8 metres closer to the equator since 1994, while Sydney, Melbourne and Hobart have only crept 1.5 metres closer. Brisbane, meanwhile, sits in the middle with a 1.6-metre shift.

The difference in positions is down to the fact that while Australia is quite literally moving up in the world, and taking its visible landmarks with it. In contrast, satellite-based GPS systems use the unmoving centre of the Earth as a reference point, and not features on its surface.

Improving accuracy
Up until now that hasn’t really mattered, because the old GPS system was partially blind – the accuracy of 1994’s systems was around 10 metres, meaning a small-by-comparison 1.8-metre shift on the ground didn’t register.

However, the update to the latest Geocentric Datum of Australia 2020 as the change is officially known, increases the accuracy of GPS-based systems to around 10 centimetres.

What does that increase in accuracy mean? Under the old system, if someone sent you a GPS location from their smartphone to pick them up off a jetty, your phone’s GPS system would show the jetty and you’d have to look for where they were standing; under the new system, you’ll be able to see that they’re standing right at the end of the jetty.

It also means the saved locations on GPS systems will become more accurate. Instead of slowly drifting to the edge of your favourite fishing bommie over time, with this update you’ll be right over the top of it again.

But there’s a problem. Australia is clipping along at a speed of 70mm a year, so with the new GPS system accurate to 100mm, it won’t be long before accuracy again becomes an issue.

Australia will get around this by also offering what is called the Australian Terrestrial Reference Frame, a moving GPS map that will shift at the same pace as the continental plate.

Another problem that the GPS upgrade fixes is how we calculate height of sea level. The Earth is not perfectly round, as it is slightly squashed in at the poles, where Australian data has always assumed it is a perfect sphere.

In some instances, map data would suggest that water would flow uphill rather than down.