Getting the coordinate system right: MGA2020, GDA94 and AHD for QLD and NSW site data
You've downloaded contours and lot boundaries for a site, dropped them into Civil 3D or QGIS, and something's off. The cadastre sits a metre or two from where it should. Or the whole site lands in the middle of the ocean. Or your levels look fine until you check them against a survey mark and they're out by half a metre.
Nine times out of ten, it's a coordinate system problem. Get the datum and projection right at the start and everything lines up. Get it wrong and you'll chase the error through your whole design.
This is a quick, practical guide to setting the correct coordinate system for Queensland and New South Wales site data, before it bites you.
The three things you actually need to pin down
When you import site data, three settings have to agree with the data you're loading:
- The horizontal datum — the reference frame for your eastings and northings. In Australia that's almost always GDA94 or GDA2020.
- The projection (map grid) — how the round earth gets flattened onto a grid. For most of QLD and NSW that's MGA (Map Grid of Australia), in zone 55 or zone 56.
- The vertical datum — what your levels are measured from. On the ground that's almost always AHD (Australian Height Datum).
If any one of these is mismatched between your data and your drawing, things shift.
GDA94 vs GDA2020: the difference is real
This is the one that trips people up most often.
Australia moved from GDA94 to GDA2020 to account for the continent drifting (we move north-east by around 7 centimetres a year). The two datums differ by roughly 1.8 metres at the moment.
So if your contours are in GDA2020 and your cadastre is in GDA94, they'll be offset by nearly two metres. That's enough to put a boundary on the wrong side of a fence, or a building setback over the line.
A metre or two might not matter for a rough desktop review. It matters a great deal for a set-out, a boundary check, or anything you'll hand to a surveyor. Always confirm which datum each dataset is in, and transform one to match the other rather than letting your software guess.
Which MGA zone am I in?
MGA splits the country into north–south zones, each six degrees of longitude wide.
As a rough guide:
- Zone 56 covers most of the populated east coast — Brisbane, the Gold Coast, the Sunshine Coast, Sydney, Newcastle, the Hunter and the Central Coast.
- Zone 55 covers further inland and to the west — Toowoomba, much of central and western QLD, and inland NSW including Wagga Wagga and the Riverina.
Sites near a zone boundary need care: data either side of the line uses different grid coordinates. If your project straddles a boundary, pick one zone and reproject everything into it.
The full coordinate system string you'll usually want looks like GDA2020 / MGA zone 56 (EPSG:7856) or GDA2020 / MGA zone 55 (EPSG:7855). The GDA94 equivalents are EPSG:28356 and EPSG:28355.
Setting it correctly in your software
QGIS reads the coordinate system from a .prj file (with a Shapefile) or stores it inside a GeoPackage. If a layer loads with no CRS, QGIS will ask. Don't just accept the default — set it to the CRS the data was actually exported in. Use Layer > Set CRS to declare it, and Export > Save Features As with a target CRS to reproject. The two are not the same: declaring tells QGIS what the data already is; reprojecting moves the coordinates.
Civil 3D wants you to assign a coordinate system to the drawing in the Drawing Settings before you bring data in. Search the zone code, attach it, then import. If your survey and your imported data are in different systems, Civil 3D can transform on import — but only if both are correctly tagged.
ArcGIS behaves much like QGIS: it stores the spatial reference with the data and will reproject on the fly for display, but you should still project to a common CRS before doing any analysis or area calculations.
Don't forget the vertical
Horizontal coordinates get all the attention, but levels matter just as much for civil work.
Elevation data for QLD and NSW is normally supplied in AHD — the same datum your survey marks reference. That's what you want. The trap is ellipsoidal heights (raw GPS heights), which can differ from AHD by tens of metres. If your DEM or contour heights look wildly wrong, check whether they're AHD or ellipsoidal before you do anything else.
For early earthworks and drainage thinking, AHD contours straight off a published elevation model are usually fine. For detailed design, you'll still verify against a real survey — but starting from clean, correctly-referenced data saves a lot of grief.
A simple workflow that avoids most pain
- Before importing, find out the datum, zone and vertical datum of every dataset.
- Pick one target system for the whole project — for the east coast that's commonly GDA2020 / MGA zone 56, with levels in AHD.
- Reproject anything that doesn't match, rather than relying on on-the-fly display transforms.
- Sanity-check by overlaying cadastre on aerial imagery and comparing a known level to a survey mark.
When you export site data through LayeredGeo, you choose the format and the data comes correctly referenced for QLD and NSW — so contours, cadastre and elevation drop into Civil 3D, QGIS or ArcGIS already lined up, rather than floating two metres off.
Common questions
Should I use GDA94 or GDA2020 for a new project? For new work, GDA2020 is the current national datum and the sensible default. The key thing is consistency: pick one and make sure every layer — contours, cadastre, survey — is in the same datum. Mixing the two introduces a ~1.8 metre offset.
Why does my cadastre line up in QGIS but not in Civil 3D? QGIS reprojects layers on the fly for display, so mismatched data can still look correct on screen. Civil 3D relies on the drawing's assigned coordinate system. If you haven't set the drawing CRS, or the data isn't tagged, it won't transform. Assign the right system to the drawing before importing.
How do I know which MGA zone my site is in? Most of the QLD and NSW east coast (Brisbane to Sydney and surrounds) is zone 56; inland areas are often zone 55. If you're unsure, the EPSG code travels with properly exported data, so you can just match it.
Get your site set up on the right footing from the first import. Export site data for any address or see a sample export.
LayeredGeo lets you download QLD and NSW site data — contours, cadastre and more — in DXF, Shapefile, GeoPackage, KMZ and raster formats, ready for Civil 3D, QGIS and ArcGIS.