Web Mercator vs True Mercator (EPSG:3857 vs EPSG:3395)

Almost every web map you've ever used is in EPSG:3857 — Web Mercator. Almost every nautical chart is in EPSG:3395 — True Mercator. They look identical at a glance and use the same word. They're not the same projection.

What True Mercator is

EPSG:3395 (WGS 84 / World Mercator) is the mathematically rigorous Mercator projection on the WGS 84 ellipsoid. The conformal property — preserving local angles — was Mercator's original innovation in 1569 for nautical navigation: a constant compass bearing draws as a straight line.

True Mercator on the ellipsoid is what the International Maritime Organization standardised for nautical charts (S-57, S-101). For mariners, the angle-preserving property is non-negotiable.

What Web Mercator is

EPSG:3857 — also called Pseudo-Mercator, Spherical Mercator or "Google Mercator" — applies the Mercator formula on the WGS 84 datum but treats the Earth as a sphere with radius equal to the WGS 84 semi-major axis (6,378,137 m). The same lat/lon goes through a slightly simplified mathematics, producing slightly different metres on the projected plane.

Google adopted this in 2005 for Maps. Bing and OpenStreetMap followed. The reason: square pixels at every zoom level (the spherical formula gives integer-pixel tiles where the ellipsoidal one doesn't), simpler tile-server math, and faster server-side rendering at the time.

How big is the difference?

At the equator: zero. At 45° latitude: about 0.7%, or roughly 50 m for a 7-km tile. At 75° latitude: about 5%, or kilometres of offset.

In numerical terms, a WGS 84 point at 45°N converts to:

• EPSG:3857 (Web Mercator): northing ≈ 5,621,521 m
• EPSG:3395 (True Mercator): northing ≈ 5,591,295 m

That 30 km difference is the cost of treating the ellipsoid as a sphere.

When each is the right choice

Use Web Mercator (EPSG:3857) when: - Building tile-based web maps. - Aligning data with Google, Bing, or OpenStreetMap basemaps. - Producing XYZ raster or vector tiles. - The audience is web/mobile and the application is visualisation.

Use True Mercator (EPSG:3395) when: - Producing IHO-compliant nautical charts. - Marine navigation systems where conformality on the actual ellipsoid matters. - Any application that explicitly demands ellipsoidal Mercator.

Common gotchas

• Don't measure distance in Web Mercator metres. A "metre" in EPSG:3857 only equals a real metre at the equator. At 60°N it's about 0.5 m on the ground. For distance calculations, compute on the ellipsoid (geodesic) or reproject to a local equal-distance CRS.
• Both projections are undefined above ±85.05°. Web Mercator clips the world to ±85.05112878° so the world fits a square tile pyramid; True Mercator scales to infinity at the poles.
• EPSG:900913 is a deprecated alias for EPSG:3857. It was originally an unofficial code (ESRI used it) before EPSG formally registered the projection. They're identical.

A worked check

Convert (45°N, 0°E):

• EPSG:3857 → (0, 5621521.49)
• EPSG:3395 → (0, 5591295.92)

For tile alignment, you want the first. For a paper nautical chart, you want the second. Don't mix them.