Introduction
You collect a point with your GPS. Later, you try to plot it on a map, and it's in the wrong place, sometimes by hundreds of metres. The problem isn't your GPS. It's the datum.
What Is a Datum?
A datum is a mathematical model of the Earth used to calculate coordinates. Think of it as a coordinate system's "starting point", the reference surface, origin point, and orientation. The problem: there are many datums. Coordinates that represent the same physical location will have different numbers in different datums.
The Earth Isn't a Perfect Sphere
- Geoid: Actual mean sea level surface, bumpy and irregular
- Ellipsoid: Mathematical approximation, smooth and computable
GNSS uses ellipsoids because they're mathematically simple. Different datums use different ellipsoids, positioned differently relative to the Earth's centre.
Common Datums
- WGS84, Used by GPS; global standard; most devices output WGS84 by default
- NAD83, Used in North America; intended to match WGS84 initially; now diverges slightly (1–2 metres)
- NAD27, Older North American datum; based on Clarke 1866 ellipsoid; can be 200+ metres different from WGS84
- ETRS89, European standard; fixed to Eurasian plate; diverges from WGS84 by ~2.5 cm/year
- GDA2020, Australia's current datum; replaces GDA94 (1.8 m difference)
Why Datums Matter
Example: a point in Denver, Colorado.
| Datum | Latitude | Longitude | Difference from WGS84 |
|---|---|---|---|
| WGS84 | 39° 44′ 55.123″ N | 104° 59′ 33.456″ W | 0 m |
| NAD83 | 39° 44′ 55.127″ N | 104° 59′ 33.461″ W | ~0.5 m |
| NAD27 | 39° 44′ 52.891″ N | 104° 59′ 41.234″ W | ~200 m |
Plate Tectonics: The Moving Problem
Continents move, and this means coordinates shift over time.
| Plate | Movement |
|---|---|
| North American | ~2 cm/year west |
| Pacific | ~8 cm/year northwest |
| Australian | ~7 cm/year north |
| Eurasian | ~2 cm/year east |
Implication: coordinates fixed to a plate drift relative to global systems. Australia's GDA94 coordinates are now ~1.8 metres different from when they were established.
Modern Solutions: Dynamic Datums
Countries are adopting dynamic datums that account for plate motion, Australia (GDA2020), New Zealand (NZGD2000 with deformation model), and North America is likely moving to a time-dependent system.
Coordinate Systems vs. Datums
Don't confuse datum with coordinate system. Geographic coordinates (latitude, longitude) sit on an ellipsoid. Projected coordinates (Easting, Northing) exist on a flat map. You can have the same datum but different projections (UTM, State Plane, etc.).
Practical Advice
Always know: what datum your receiver is using, what datum your map/software expects, and what datum your final deliverable requires. Most receivers default to WGS84, check settings before starting and log datum information with your data. In the office, use transformation software; modern GIS handles transformations automatically.
Transformation Tools
| Tool | Use |
|---|---|
| EPSG.io | Find coordinate system codes |
| PROJ | Open-source transformation library |
| NOAA NCAT | NADCON coordinate converter |
| Geotrans | Military standard converter |
| GIS software | Built-in transformations |
Vital Points
- A datum is a reference frame for coordinates
- Different datums give different numbers for the same place
- WGS84 is GPS native, but not always what you need
- Plate tectonics means coordinates drift over time
- Always know your datums, input and output
- Metadata matters, record your datum