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Coordinate Systems

EPSG:3857 Web Mercator (Pseudo-Mercator)

Updated · GeoCAD Bridge engineering team · 8 min read

Summary

EPSG:3857 (WGS 84 / Pseudo-Mercator) is the projection used by web map tiles from Google, Bing, OpenStreetMap and Esri. It applies spherical Mercator formulas to WGS 84 coordinates, which makes tiles simple but means the projection is not exactly conformal and its scale error grows rapidly with latitude: 1/cos(φ), already 1.101 at Riyadh’s latitude. Use it for display, never for measuring distances or areas.

Definition

Web Mercator: a Mercator projection using spherical equations with radius 6 378 137 m applied to WGS 84 latitude and longitude.

  • Units: metres
  • Limit ±85.0511°
  • Extent ±20 037 508.34 m
  • Scale = 1/cos φ
  • Former code: EPSG:900913

Formulas

x = R · λ y = R · ln(tan(π/4 + φ/2)) R = 6 378 137 m

The latitude limit ±85.0511° makes the world a square from −20 037 508.34 m to +20 037 508.34 m, which is the zoom-0 tile.

Why not to measure in 3857

Scale is 1/cos φ in every direction (on the sphere). At 25° N lengths are exaggerated by 10.1%, at 60° by 100%. Areas are exaggerated by the square of that. Measure in UTM, a national grid, or geodesically on the ellipsoid.

Worked example

Riyadh 46.6753° E, 24.7136° N → EPSG:3857 x = 5,195,870.63 m, y = 2,840,607.63 m scale factor = 1/cos(24.7136°) = 1.1008

FAQ

Is EPSG:3857 the same as EPSG:900913?

Yes. 900913 (“google” in calculator digits) was an unofficial code used before EPSG registered 3857.

Why do Greenland and Africa look the same size on web maps?

Mercator scale grows with latitude, so high-latitude areas are hugely enlarged.

Can I digitize in Web Mercator?

You can trace features, but convert them to a suitable projected CRS before measuring or delivering.

Tools

Articles

References

  1. IOGP. EPSG Geodetic Parameter Dataset, epsg.org.
  2. IOGP. Geomatics Guidance Note 7-2, section on Popular Visualisation Pseudo-Mercator (EPSG method 1024).
  3. Snyder, J. P. Map Projections: A Working Manual, USGS Professional Paper 1395, 1987.

Cite this page

GeoCAD Bridge. “EPSG:3857 Web Mercator (Pseudo-Mercator).” GeoCAD Bridge Knowledge Center, updated 2026-10-05. https://www.geocadbridge.com/knowledge/coordinate-systems-epsg-3857/