WGS 84 vs NAD83: Why They're Not the Same Datum

It's the most common confusion in U.S. GIS work: a developer pulls Census or USGS data tagged "NAD83", overlays it with GPS readings tagged "WGS 84", and finds everything is offset by a couple of metres. Both look like lat/lon. Both use the GRS 80 ellipsoid. So why don't the coordinates match?

Born close together

In 1986 the U.S. National Geodetic Survey released NAD83. Its goal was to provide a continent-wide datum aligned with the global ITRF (International Terrestrial Reference Frame). At its release, NAD83 and WGS 84 — which is also aligned with ITRF — agreed to a few centimetres.

For about a decade you could treat them as identical. Then the world moved.

Why they drifted apart

NAD83 was *fixed* to the North American plate at the 1986 epoch. From that moment on, all NAD83 coordinates remain stable relative to the North American landmass: a point on the ground keeps the same NAD83 lat/lon forever, even as the continent itself drifts.

WGS 84, by contrast, *tracks* the global ITRF. Each new ITRF realisation (and each WGS 84 realisation that follows it) reflects the latest model of Earth's centre of mass and orientation. Because the North American plate is moving south-westward by about 2.5 cm per year, WGS 84 coordinates of a fixed point on the ground change by that much annually.

After ~40 years, the cumulative drift between NAD83 and WGS 84 across the contiguous United States is roughly 1-2 m. In Alaska it's larger because the Pacific plate motion compounds; in Mexico the gap is different again.

When the difference matters

• Sub-metre cartography — placing a building on the right side of a property line.
• Engineering and construction surveys.
• Cadastre — legal property boundaries.
• Aerial photogrammetry at sub-metre ground sampling distance.
• Combining commercial GPS data (often labelled "WGS 84" without specifying realisation) with U.S. federal "NAD83" datasets.

When the difference doesn't matter:

• Web mapping at country to neighbourhood scale.
• Thematic mapping of land cover, demographics, ecology — the offset is invisible at the scale you're displaying.
• Routing and address geocoding.

How to convert correctly

Browser-based tools (including this one) apply a Helmert seven-parameter transformation between WGS 84 and a specific NAD83 realisation. Accuracy is roughly a few centimetres for the modern epoch — adequate for almost all analytical work.

For surveying-grade conversions:

1. Specify the *exact* NAD83 realisation: NAD83(2011), NAD83(NSRS2007), NAD83(CORS96) etc. Each has its own EPSG code (NAD83(2011) is EPSG:6318).
2. Use NGS's NCAT online tool or a desktop GIS with NADCON 5.0 grid files.
3. Treat the time stamp on your GPS data carefully — WGS 84 realisation matters at the centimetre level.

EPSG codes to know

EPSG:4267 (NAD27) is a separate story altogether — it differs from NAD83 by tens of metres in some regions and is converted via NADCON grid shifts, never with a single Helmert.

The takeaway

WGS 84 and NAD83 are similar enough that you can mix them at neighbourhood scale, but different enough that you should always know which one your data is in. Tag every layer with its EPSG code. When integrating data from different sources, reproject through a tool that handles datum shifts properly. And remember that "NAD83" without a realisation tag is fine for casual use but ambiguous for surveying — pick a specific realisation when accuracy matters.