A world map intended for navigation
In 1569 Mercator published a large world map assembled from eighteen sheets. Its title identified navigation as its purpose. Place names, coasts, islands and explanatory texts accompanied a redesigned coordinate grid. European ships were crossing the Atlantic and Indian oceans, and navigators needed to relate compass directions at sea to routes on paper. Reports, earlier maps and measurements supplied uneven information about different regions, which the map gathered into one extensive printed surface.
Making constant-bearing routes straight
Meridians appear as parallel straight lines, crossed at right angles by parallels whose spacing increases toward the poles. The arrangement preserves local angles and turns a rhumb line, a route of constant compass bearing, into a straight line on the map. Navigators could draw a line between positions and read its bearing. The projection translated a useful directional relationship on a sphere into a convenient geometrical relationship on paper, later expressible through coordinate formulas.
Scale changes with latitude
Scale grows in both directions at high latitudes, making polar regions appear much larger. A single scale cannot measure all areas and distances uniformly. The shortest ocean route follows a great circle and generally curves on a Mercator map. Navigators and cartographers therefore choose projections for particular tasks. Mercator remained useful for sea charts, while equal-area projections offer other ways to compare the size of regions.