Mercator projection

The Mercator projection is a conformal cylindrical map projection introduced in 1569 by the Flemish cartographer Gerardus Mercator. It became the dominant projection for navigation during the eighteenth century due to its unique ability to represent rhumb lines—paths of constant compass bearing—as straight lines. This property made it indispensable for maritime travel, allowing sailors to plot directional courses with unprecedented ease. Although it preserves angles and shapes locally, the Mercator projection drastically distorts size, especially towards the poles, making high-latitude regions appear far larger than they actually are. Despite its limitations, the projection remains widely used in certain navigational contexts and in modern web-mapping systems.

Historical Development

Speculation about proto-Mercator methods in earlier cultures has been raised, such as Joseph Needham’s suggestion that Song dynasty star charts in China employed a Mercator-like projection. Cartometric analysis later demonstrated that these charts used an equirectangular projection, not Mercator’s. Likewise, medieval portolan charts, beginning in the thirteenth century, displayed remarkably accurate coastlines and windrose networks of rhumb-like bearings. Although the charts were not based on a formal projection, their limited geographic scope meant that constant-bearing courses appeared approximately straight.
In the early sixteenth century, German polymath Erhard Etzlaub created small compass maps that some scholars initially believed used the Mercator projection. Later analysis indicated that they more likely followed a central cylindrical projection, inspired by sundial geometry.
Portuguese mathematician Pedro Nunes made a crucial theoretical contribution in 1537 by describing the loxodrome, or rhumb line, and suggesting that a nautical atlas composed of matched equirectangular charts could approximate a projection suitable for navigation. Mercator, acquainted with Nunes’s work and his tabulated navigation data, included rhumb-line networks on a 1541 terrestrial globe and drew on these principles when producing his landmark 1569 world map.
Mercator’s new projection appeared in a grand eighteen-sheet map titled A new and augmented description of Earth corrected for the use of sailors. Although he never revealed his construction method, contemporary annotations show that he fully understood the projection’s navigational advantages. Formal mathematical tables for accurately constructing the projection were published by Edward Wright in 1599 and 1610, while later mathematical treatment appeared through the work of Henry Bond and earlier unpublished work by Thomas Harriot.
The full adoption of the Mercator projection in oceanic navigation was delayed until the eighteenth century. Its effectiveness depended upon accurate determination of longitude at sea and reliable knowledge of global magnetic declination—both unavailable until the invention of the marine chronometer and improved magnetic mapping. By the nineteenth century, the Mercator had become dominant in commercial and educational world maps. Its prominence, however, drew criticism due to its extreme size distortion, inspiring an array of alternative projections during the late nineteenth and early twentieth centuries. Publishers gradually reduced its use in general-purpose maps, although the rise of web mapping in the early twenty-first century—particularly through the Web Mercator projection—renewed its popularity for digital platforms.

Mathematical and Geometric Properties

The Mercator projection can be conceptualised by imagining a cylinder wrapped around a globe with tangency along the equator. The surface of the sphere is projected outward onto the cylinder in such a way that angles are preserved, a property known as conformality. After unrolling the cylinder into a plane, the meridians and parallels form a grid of right-angled straight lines.
Among cylindrical projections, Mercator is unique in balancing the east–west stretching of parallels with proportional north–south stretching, ensuring that the local scale is uniform in every direction. This quality preserves shapes at very small scales—an essential feature for navigation charts where bearings and angles must be accurate.
However, the projection severely distorts area. Because north–south stretching increases exponentially with latitude, regions near the poles expand disproportionately. For this reason, latitudes above 70° become essentially unusable; the poles themselves cannot be shown in finite space, as the scale approaches infinity.

Navigational Utility

In navigation, the Mercator projection’s ability to represent rhumb lines as straight segments revolutionised chart-making. Sailors could draw a straight course on a Mercator chart and follow a single compass bearing to approximate that path. This feature, combined with conformal accuracy, made the projection exceptionally practical before the development of more sophisticated navigational instruments.
The grid of perpendicular meridians and parallels enabled simple transfer of bearings using a protractor, compass rose or parallel rulers. Despite later advances, the projection has remained a staple of marine charts, where its navigational benefits still outweigh its distortions.

Applications and Modern Usage

While the Mercator projection once dominated classroom wall maps and atlases, modern cartographers tend to avoid it for general world maps because of its misleading size exaggerations. Many projections introduced between the late nineteenth and early twentieth centuries—such as the Robinson, Gall–Peters, and Winkel Tripel projections—were designed specifically to address this issue.
The projection has, however, seen a revival in the digital era. Major web-mapping services adopted variants of it, most notably the Web Mercator projection, which simplifies mathematical calculations by treating the Earth as a perfect sphere. This approach supports dynamic zooming and tiling algorithms essential for online map interfaces. Although Google Maps moved away from the projection for global-scale views in 2017, many online systems continue to rely on it, especially for local-area maps where distortion is minimal.

Characteristics Compared with Other Projections

Because the Mercator projection preserves angles but not area, it is frequently contrasted with equal-area projections, such as the Gall–Peters, which preserve relative landmass size but distort shapes. It also differs from compromise projections, like the Winkel Tripel, which seek a balanced representation of shape, area, and distance.