The Mercator projection is a map projection that translates the spherical Earth onto a flat, two-dimensional surface. Its core purpose was to aid nautical navigation, which it does exceptionally well by preserving angular relationships and direction. This means a line of constant compass bearing, known as a rhumb line, is represented as a straight line on the map. However, this angular accuracy comes at a trade-off, as the Mercator projection significantly distorts the relative size of landmasses as they move away from the equator.
How the Projection Creates Area Inflation
The Mercator projection is considered a cylindrical projection, conceptually created by wrapping a cylinder around the globe and projecting the Earth’s features onto it. On a globe, the meridians (lines of longitude) naturally converge at the North and South Poles. To represent these on a flat, rectangular map, the Mercator projection forces the meridians to be parallel and equally spaced vertical lines.
This forced parallelism creates the distortion because it requires stretching the map horizontally as latitude increases. The circumference of the Earth decreases significantly toward the poles, but on the map, every line of latitude is stretched to the same length as the equator. To maintain conformality, or the accurate representation of local shapes and angles, a corresponding vertical stretch must also be applied.
The spacing between the parallels (lines of latitude) increases exponentially as they move toward the poles to match the increasing horizontal stretch. The result is that the scale factor, which determines the amount of stretching, becomes progressively larger the farther one travels north or south from the equator.
Where Distortion Becomes Extreme
The greatest distortion in a Mercator projection occurs at the highest latitudes, increasing rapidly as the map approaches the North and South Poles. Distortion is negligible near the equator, where the cylinder conceptually touches the globe. However, the scale factor continuously increases until it becomes infinitely large at the poles themselves, which means the poles cannot be displayed on a standard Mercator map.
This extreme inflation of area is most evident when comparing landmasses at different latitudes. For example, on the Mercator map, Greenland appears to be roughly the same size as the continent of Africa. In reality, Africa is approximately 14 times larger than Greenland.
Similarly, Alaska appears disproportionately large compared to its true size relative to landmasses near the equator. The distortion is so severe that regions above 70 degrees latitude are often considered practically unusable for accurate area representation.
Visual Consequences and Alternative Maps
The visual consequence of the Mercator distortion is a misleading perception of relative global land area. The exaggeration of northern hemisphere landmasses, such as Canada, Russia, and Europe, can subconsciously lead to a misjudgment of their true size and global significance compared to equatorial nations. This effect has made the Mercator projection a subject of criticism when used for general world maps outside of navigation.
When the accurate comparison of land sizes is the priority, cartographers turn to alternative projections that sacrifice the Mercator’s angular accuracy to preserve area. These are known as equal-area projections, or authalic projections, which ensure that any area on the map is proportional to its actual area on the globe. Projections like the Gall-Peters or the Mollweide projection are often used to display global distributions or thematic data where area comparison is paramount.
The choice of map projection always involves a compromise, as it is mathematically impossible to flatten a sphere without introducing some form of distortion. While the Mercator excels at direction, its area distortion means it is less suitable for general world reference compared to projections that minimize the overall error across different properties, such as the Robinson projection.