· 5 min read
What Is WGS-84?
WGS-84 is the datum every GPS fix is reported on. What a datum actually is, how far a coordinate moves when the datum is wrong, and why your GPS altitude disagrees with the elevation printed on the map.
A coordinate on its own is incomplete. 36.24660° N, 116.81660° W does not
identify a place until you also know what model of the Earth those numbers were
measured against. That model is the datum, and for anything involving GPS the
answer is almost always WGS-84.
Getting this wrong does not produce a small error. It produces a confident, precise-looking coordinate that is a couple of hundred metres from where you meant.
What a datum actually is
The Earth is not a sphere and it is not a smooth ellipsoid either. It is lumpy, slightly pear-shaped, and its surface moves. To turn that into arithmetic you need three things, and together they make a datum:
- An ellipsoid
- A smooth mathematical shape approximating the Earth
- An origin and orientation
- Where that shape is positioned relative to the real Earth
- A reference frame
- Physical stations whose coordinates define it in practice
Change any of those and every coordinate on the datum shifts. Two datums can use an identically shaped ellipsoid but position it differently, and produce coordinates hundreds of metres apart for the same rock.
WGS-84 specifically
World Geodetic System 1984 is a global, Earth-centred datum. Its origin is the Earth's centre of mass rather than a monument on any particular continent, which is what makes it work everywhere rather than well in one region.
- Semi-major axis
- 6 378 137.0 m
- Inverse flattening
- 298.257223563
- Origin
- Earth's centre of mass
- Maintained by
- US National Geospatial-Intelligence Agency
Every GPS satellite broadcasts its position in WGS-84. That is the single most important practical fact about it: unless your receiver has been told otherwise, the coordinate on your screen is a WGS-84 coordinate. The same is true of virtually every online map, GPX file and GeoJSON feature you will encounter.
WGS-84 is not frozen
It has been refined several times since 1984, in realisations usually written like G2296. Modern versions agree with ITRF to within a couple of centimetres. For navigation this is irrelevant; for survey and geodesy it is not. If a document specifies a realisation, it has a reason to.
How far off is the wrong datum?
Far enough to matter. The comparison that catches people most often in the United States is WGS-84 against NAD 27, the North American Datum of 1927, which is printed on a great many older USGS topographic sheets.
| Region | Typical WGS-84 to NAD 27 shift |
|---|---|
| Eastern United States | Roughly 30 to 50 m |
| Central United States | Roughly 80 to 100 m |
| Western United States | Roughly 150 to 220 m |
| Alaska | Can exceed 300 m |
The shift is not a constant offset you can memorise. It varies continuously with position, because NAD 27 was fitted to North America from a single origin point in Kansas and drifts progressively as you move away from it.
Read the datum off the map before you plot
Every topographic sheet states its datum in the marginal information, usually near the scale bar and the declination diagram. A GPS coordinate plotted on a NAD 27 sheet without conversion is wrong by the amounts in that table, and it will look completely plausible while being wrong.
Other datums you may meet:
- NAD 83, the modern North American datum. Close to WGS-84, originally within about a metre, though tectonic drift has opened that to roughly 1 to 2 m in much of North America.
- ETRS89, the European standard. Fixed to the stable part of the Eurasian plate, so it has drifted from WGS-84 by around 80 cm since 1989 and continues at about 2.5 cm per year.
- OSGB36, used by Ordnance Survey mapping in Great Britain. Differs from WGS-84 by up to about 120 m.
- Pulkovo 1942, widely used across the former Soviet Union and on a lot of older mapping still in circulation.
Why your GPS altitude disagrees with the map
This is the second half of the datum problem and it surprises people more than the horizontal shift does.
The WGS-84 ellipsoid is a smooth mathematical surface. Mean sea level is not: it follows the Earth's gravity field, bulging where the crust is dense and dipping where it is not. That gravity-defined surface is the geoid, and the difference between the two is called the geoid height or geoid separation.
That separation ranges from about -105 m near southern India to about +85 m near Iceland and New Guinea. So a receiver reporting raw ellipsoidal height can disagree with the elevation printed on your map by the better part of a hundred metres, without either being faulty.
Most consumer receivers hide this by applying a geoid model, usually EGM96 or EGM2008, and reporting an approximate mean sea level height instead. That is almost always what you want. But it means altitude has an extra model in the chain that horizontal position does not, and it is one reason GPS altitude is generally less trustworthy than GPS position even before satellite geometry is considered.
Practical rules
Four habits remove almost all datum errors:
- State the datum whenever you record or pass on a coordinate. "WGS-84" is four extra characters and removes an entire class of error.
- Check the map sheet before plotting anything from a GPS. If it says NAD 27 or anything other than WGS-84, convert first.
- Never mix datums inside one route or overlay. Waypoints from two sources on two datums will look consistent and be quietly inconsistent by 100 m or more.
- Treat altitude as a separate question. Confirm whether a stated height is ellipsoidal or mean sea level before comparing it to anything.
The short version
WGS-84 is the global, Earth-centred datum that GPS broadcasts on and that effectively every digital map uses. A coordinate without a datum is not a location, and the wrong datum can put you 200 m off with no visible sign that anything is wrong. Its ellipsoid is smooth, mean sea level is not, and the gap between them is why GPS altitude and map elevation rarely agree.
With the datum settled, the grid systems built on top of it make sense: see MGRS vs UTM for how the same position gets written two ways, and how to read MGRS coordinates for the character-by-character breakdown.