Multiple Choice, Multiple Answers

Every flat map of the world is, in some sense, wrong. The surface of a sphere cannot be spread flat without stretching or tearing it, so mapmakers must decide which properties to keep and which to sacrifice. A map can show the correct shapes of small areas, or the correct relative sizes of countries, but it cannot do both at once. The projection published by the Flemish geographer Gerardus Mercator in 1569 chose shape. Its great advantage was practical: a line of constant compass bearing appears on it as a straight line, so a navigator could draw a course between two ports and follow it with a compass. For sailors, this mattered far more than accurate areas. The price of this choice is that areas are increasingly exaggerated towards the poles. On a Mercator map, Greenland looks roughly as large as Africa, although Africa is about fourteen times bigger. Critics have argued that, because the projection became so familiar in schools, it has led many people to underestimate the size of regions close to the equator, which include much of Africa, South America and South-East Asia. Alternatives exist. Equal-area projections show the true relative sizes of land masses, though they squash or stretch their shapes, and compromise projections try to keep both kinds of distortion moderate. Many atlases now prefer such compromises for world maps. Yet Mercator's design has not disappeared: a version of it underlies most online street maps, where users zoom into small areas and correct local shapes and directions matter more than comparisons between continents.