GDA94 vs GDA2020: Australia's Moving Datum, Explained
Australia rides the planet's fastest-moving continental plate — about 7 centimetres north-east per year — and its coordinate systems are snapshots of where the continent was parked. GDA94 froze it at 1994; GDA2020 re-froze it 26 years and 1.8 metres later; and the WGS84 your GPS speaks tracks the continent's position now, which by 2026 sits roughly half a metre past GDA2020 and counting. For bushwalking the differences vanish inside GPS error; for GIS layers and surveying they're real, systematic, and worth understanding once.
A continent in motion
The Australian plate is geology's speedster: about 7 cm north-east every year, the fastest sustained drift of any continent. That's harmless until you want coordinates to mean something for decades — a datum has to park the continent at some instant and measure everything from there. Park it in different years and you get different, equally self-consistent systems whose numbers disagree by exactly the drift accumulated between them. Australia has done this three times in the GPS era, which is the entire story of GDA94, GDA2020, and the gap to WGS84.
The 1.8-metre jump: GDA94 to GDA2020
GDA94 fixed Australia where it sat in 1994. By the mid-2010s the continent had physically moved about 1.6–1.8 m away from those coordinates, so GDA2020 re-fixed it at where it would sit at the start of 2020 — a deliberate jump of about 1.8 m to the north-east. Crucially, the ellipsoid (GRS80) and the MGA projection are unchanged: an MGA94 and an MGA2020 coordinate for the same paddock differ by that 1.8 m and nothing else. It's why mixing datum-unlabelled GIS layers produces that characteristic uniform misalignment — every feature shifted the same way.
Where WGS84 fits
The WGS84 your receiver broadcasts effectively tracks the Earth now — it doesn't park the plate. At the start of 2020, GDA2020 and GPS agreed almost exactly (that was the point); since then the continent has kept sliding, so by 2026 raw GPS positions sit roughly 0.4–0.5 m north-east of GDA2020 values, growing 7 cm each year. A consumer fix's own error is still several times that, which is why phones and bushwalkers can treat them as identical — and why our MGA page says so plainly. For work that genuinely needs centimetres over years, Australia publishes ATRF, a time-dependent frame that moves with the plate instead of parking it.
The 200-metre trap: AGD66, AGD84 and AMG
The shift that actually bites in the field is the old one. Maps and datasets from before roughly 2000 used the AGD datums with their AMG grid — and AGD sits about 200 metres from GDA, a product of pre-satellite surveying, not plate drift. A grid reference lifted from a 1980s topo sheet and typed into a GDA-era tool lands you two football fields away. The tell: the map legend says AGD66, AGD84 or AMG. Treat any pre-2000 Australian reference as suspect until the datum is confirmed.
What should you actually use?
Bushwalking, 4WD touring, emergencies: GDA2020/MGA — it's what current maps print, and GPS agrees with it to within noise (convert live here). Exchanging data: label the datum, always — "MGA Zone 55" without a year is the ambiguity that causes the 1.8 m mystery offsets. Surveying and engineering: the formal GDA94↔GDA2020 transformations or ATRF, applied in proper software. And any time coordinates look shifted by "about two hundred metres", you already know the diagnosis: an AGD-era source wearing modern clothes.