What is EPSG 3857? Web Mercator Projection

EPSG 3857 (also called Web Mercator or Pseudo-Mercator) is the projected coordinate system that powers nearly every interactive web map. Google Maps, OpenStreetMap, Bing Maps, and Mapbox all use it to display map tiles.

How Web Mercator Works

The Mercator projection wraps a cylinder around the globe at the equator and projects Earth's surface onto it. EPSG 3857 treats the Earth as a perfect sphere (not an ellipsoid) for faster tile rendering. This means:

• The map is a perfect square (the world fits in one tile at zoom level 0)
• North is always up
• Lines of latitude become horizontal lines
• Lines of longitude become vertical lines
• Shapes are preserved near the equator but distorted near the poles

Why Web Mercator Distorts Reality

The price of making a square, scrollable map: areas near the poles appear much larger than they really are.

EPSG 3857 Coordinates

In EPSG 3857, coordinates are expressed in meters from the intersection of the equator and the Prime Meridian (0,0):

• X (Easting): meters east or west of the Prime Meridian
• Y (Northing): meters north or south of the equator

Example: Tokyo (35.6762, 139.6503 in EPSG 4326) becomes approximately X: 15540587, Y: 4257066 in EPSG 3857.

EPSG 3857 vs EPSG 4326

PROJ4 Definition

+proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0
+x_0=0 +y_0=0 +k=1 +units=m +nadgrids=@null
+wktext +no_defs +type=crs

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