OpenStreetMap Projection

When features on the curved surface of the Earth are represented on a flat surface, their shape or size will always be subject to distortion in one way or another. Methods for representing these features on paper are known as map projections.

OpenStreetMap use a Pseudo-Mercator projection for its map images. The Mercator map was developed in 1569 by Gerardus Mercator as a navigation tool based on a rectangular grid with parallel lines of latitude and longitude. The Mercator map was designed as an aid to navigators since straight lines on the Mercator projection are loxodromes or rhumb lines - representing lines of constant compass bearing - perfect for 'true' direction.

The Mercator projection exaggerates the size of areas far from the equator. For example, Greenland is presented as having roughly as much land area as Africa, when in fact Africa's area is approximately 14 times greater than Greenland. OpenStreetMap cannot show the poles as the Mercator projects them at infinity. Instead it cuts off coverage at 85° north and south. Direct measurements of distances on OpenStreetMap will be approximate, except on the Equator, and the aspect ratios on the rendered map for true squares measured on the surface on Earth will slightly change with latitude.

True North on OpenStreetMap is not shown, but for a normal Spherical Mercator projection, grid north and true north will coincide and it will follow any vertical line (or meridian) to the top of the map.