Why are they bothering about a 1.8 meter shift ?
It’s a good question - why should we care about a 1.8 meter shift in coordinates ?
It comes back to plate tectonics - the Australian plate has been moving North and slightly East for millions of years. Remember that all the continents look like jigsaw pieces that fitted together at some stage…. well they did & they’ve been moving apart very slowly ever since. That rate of plate tectonic movements is what caused mountain ranges to form, volcanoes, earthquakes and deep sea trenches…. and it also causes every continent to move.
In the past it hasn’t mattered that we’re moving. That’s because everything was measured relative to something on the same tectonic plate. Historically, when you buy a block of land it is next to both your neighbors, and will always be in exactly the same spot relative to them… your fences will always move with you all.
It’s a bit like adjacent cabins in a cruise ship. It doesn’t matter that you’re heading across the Bass Straight, as long as the cabin walls all move at the same pace. You’ll wake up exactly the same 3m away from your neighboring cabin, even though you’re now both in Hobart.
What’s Changed ?
It’s only very recently that we’ve changed the way we measure things. That’s because instead of using maps and compasses and distances between objects (like houses & roads), we’ve changed to using the GPS. Or more accurately the GNSS (Global Navigation Satellite System) - there used to be just one of them - the US GPS system. Now you phone will pick up satellites from up to 7 GNSSs; operated by Russia, China, Japan, Eurozone, USA and India.
The GNSSs all operate in space, they don’t know (or care) that the tectonic plates on earth are moving around. All they know is that relative to the centre of the earth they can pinpoint any point on the Earths’ surface to within an accuracy of 5 meters. That was all fine back in 1994 when everything was aligned perfectly, but since then, the Australian plate has moved 1.5 meters. So the GDA2020 will get us back in sync with the GNSSs.
But what happens after that ? Will we have to keep updating to a new datum every year when our plate moves a bit further ? Nobody has actually made a decision about that question yet, there are a few different directions each organisation can take depending on several factors. However, the answer is probably not, since we know we’re moving at around 70mm every year, we will be able to automatically update our location to a fairly high degree of accuracy. We will know exactly where we were in the year 2020, and we know we will have moved 1.5mm each week since then, so it should be easy to work out exactly where we are now relative to the satellites. There is a more detailed answer, but you need to get into the details of the ATRF.
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