Mercator projection

The Mercator projection transforms positions on the curved Earth onto a cylinder and then unwraps that surface into a rectangular map. Meridians and parallels appear as straight lines crossing at right angles, while spacing between parallels increases toward the poles. This geometry preserves local angles and shapes of sufficiently small features, making the projection conformal. In its standard aspect, a line of constant compass bearing—a rhumb line or loxodrome—appears straight, allowing a navigator to plot a steady course as a straight line on the chart.

The ability to plot constant-bearing courses simply made Mercator charts valuable in marine navigation, and the projection remains common on nautical charts. The U.S. National Oceanic and Atmospheric Administration notes that many nautical charts use it for this reason. Web mapping services also often use a variant, in part because the projection's regular grid is convenient for tiled maps and preserves local angles. Suitability depends on purpose: a projection that is useful for course plotting or local shape is not automatically suitable for comparing the sizes of distant regions.

The projection's scale grows rapidly with latitude. As a result, areas near the poles appear much larger relative to equatorial regions than their true areas warrant; the poles themselves cannot be represented at finite map coordinates. Greenland, for example, looks far larger on a world Mercator map than its area is relative to equatorial land regions. The projection does not preserve area, and straight segments on the map do not generally represent shortest routes over the globe. Distortion is especially pronounced on small-scale maps covering the entire world, while a suitably centered transverse Mercator projection can be effective over a limited region.

Flemish cartographer Gerardus Mercator published his world map using the projection in 1569. It became influential in navigation because a course of constant compass direction could be represented as a straight line. The projection's navigational utility and its distortion of high-latitude areas are separate consequences of its geometry; choosing a map projection therefore depends on which properties matter for the task.