Why does Greenland look so large on a map?

Why does Greenland look so large on a map?

Look at a world map, and Greenland often stands out. On a Mercator world map, it can even appear larger than South America. Yet the United States Geological Survey's manual on map projections gives a very different comparison: Greenland's actual area is approximately one eighth of South America's.

How can the difference be so dramatic? It begins with something easy to overlook. The Earth's surface is curved, while a map is usually flat.

Imagine peeling an orange and trying to lay a piece of its skin flat on a table. It curls up. To flatten it, you would have to make cuts, stretch it, or compress it. Turning the Earth's curved surface into a flat map involves a similar difficulty. The orange is a useful analogy, though mapmakers use mathematics to do the actual work.

A map projection is a way to transform positions on the Earth's surface into positions on a flat surface. It is not simply a photograph of the planet. Nor does it necessarily involve putting a light at the Earth's center and casting a shadow. Different mathematical methods preserve different features while changing others.

One feature the Mercator projection preserves is angles within very small areas. To understand the tradeoff, picture the lines of longitude on a globe. They are farther apart near the equator and converge toward the poles. On a Mercator map, however, they become parallel vertical lines. Places where those lines were closer together are spread out horizontally.

If the map stretched only east to west, small shapes would become flattened. Mercator's mathematics also adjusts the north to south scale so that, around any one location, the magnification is the same in every direction. Local angles are preserved, but area increases. This enlargement becomes more pronounced toward the poles.

Here is a number that makes the effect easier to grasp. Consider the spherical version of the Mercator projection, with the equator as the scale reference. Around latitude sixty degrees, a very small patch has roughly twice the length scale it would have at the equator. Twice as wide and twice as tall means about four times the area.

That is a local scale factor near a particular latitude. It is not the enlargement factor for the whole of Greenland. A large island spans different latitudes, where the amount of enlargement varies. Greenland's high latitude makes it look especially large on this kind of world map.

You may also hear that Mercator preserves shapes. That needs a qualification: very small shapes. A tiny circle can remain circular at different latitudes while becoming larger or smaller. A whole continent or island covers a much bigger region, with different scales in different places. Its overall outline can still change. Preserving local angles does not mean preserving the size and shape of an entire landmass.

Why use Mercator at all, then? Maps are tools for particular tasks. Mercator has a useful navigation property. A route that maintains a constant heading relative to true north appears as a straight line. This helps navigators plan what is called a rhumb line, or constant-heading route.

Such a route is usually not the shortest path between two points on the Earth's surface. So a straight line on a map does not automatically mean the shortest real-world distance. A projection's usefulness for one task does not make it suitable for every task.

To compare the areas of countries, continents, or regions of forest cover, an equal-area projection is a better choice. It preserves the proportions between areas on the map and their corresponding areas on the Earth. The tradeoff is that angles and shapes change in some places. Landmasses may look flatter or longer, and some maps have interruptions.

Other world maps take a compromise approach, balancing different kinds of distortion. They may look more pleasing, but that does not make their areas, shapes, and distances completely accurate. The USGS explanation is straightforward: there is no single best projection for every purpose. The choice depends on the task.

Next time you compare two landmasses on a map, look for the projection name before judging their areas by eye. Ask what you want to understand: area, heading, distance, or local detail.

Greenland has not suddenly grown. Its appearance has changed through a particular method of mapping. Understanding that step gives us a useful habit: match the map to the question we want it to answer.