The Mercator projection is a common method for transforming the Earth’s three-dimensional surface onto a flat, two-dimensional map. This technique was introduced by the Flemish geographer and cartographer Gerardus Mercator in 1569. It was specifically designed to aid sailors navigating the world’s oceans during the great age of exploration. Mercator’s innovation provided a systematic way to represent the globe on a flat sheet.
Mapping the Curve: How the Projection Works
The Mercator projection is classified as a conformal cylindrical map projection. The concept involves imagining a cylinder wrapped around the globe, touching it precisely along the equator. Points on the Earth are then projected outward onto the inner surface of this cylinder before it is unrolled into a flat map. This geometric process results in lines of longitude appearing as equally spaced vertical lines and lines of latitude appearing as parallel horizontal lines.
The map’s defining feature is its mathematical property of preserving angles, known as conformality. This characteristic is achieved by making the lines of latitude farther apart as they move away from the equator. The necessary stretching in the north-south direction compensates for the natural east-west stretching that occurs when the globe’s converging lines of longitude are forced into parallel straight lines.
This preservation of angles made the Mercator map an indispensable tool for marine navigation. A trajectory of constant compass bearing, known as a rhumb line or loxodrome, appears as a straight line on the map. Navigators could simply draw a straight line between their starting point and destination and follow that single, constant compass direction. For centuries, this practical advantage outweighed the projection’s other shortcomings.
The Price of Accuracy: Distortion and Area
The benefit of accurate direction comes at the cost of distorting the accurate representation of land area. To maintain the correct shape and direction everywhere on the map, the distance scale must increase exponentially as one moves further away from the equator. This means that the Mercator projection is accurate only at the equator, where the cylinder touches the globe. As the latitude increases, the distortion of size and distance becomes progressively greater.
This effect is most noticeable at the poles, where the scale becomes infinite and the areas are impossibly exaggerated. On a Mercator map, for example, Greenland appears to be roughly the same size as the entire continent of Africa. In reality, Greenland’s area is approximately 2.17 million square kilometers, while Africa covers about 30.37 million square kilometers, making Africa over 14 times larger.
Similarly, the size of Canada, located at high northern latitudes, is greatly overstated compared to its actual area. The map can make Canada appear larger than China, when in fact they are nearly the same size. This distortion also causes Antarctica to look immense, appearing much larger than its true proportion to the rest of the world.
Why We Still Use Mercator Today
Despite its well-known area inaccuracies, the Mercator projection remains widely used in modern applications. Its primary purpose is no longer oceanic navigation, but rather the foundation of many popular digital mapping services. Many major online street mapping providers, including Google Maps, use a variant called Web Mercator.
The reason for this continued use is the projection’s ability to preserve local angles and shapes, which is referred to as conformality. This property is particularly useful for web mapping because the map can be seamlessly tiled and zoomed. When a user zooms in to a local level, the shapes of roads, buildings, and coastlines remain virtually free from distortion, and local navigation directions are preserved.
The Mercator projection is excellent for displaying localized, large-scale maps where the distortion is minimal. While other projections like the Gall-Peters or Robinson prioritize accurate area, they sacrifice the angular precision beneficial for digital navigation and displaying map tiles.