What Is a Map Projection and Why Do Maps Distort?

A map projection is a mathematical method for showing the curved surface of the Earth on a flat sheet or screen. Because a sphere cannot be flattened without distortion, every projection sacrifices something. Choosing one means deciding which properties matter most for the map you are making.

The orange peel problem

Try to flatten an orange peel and it will tear or stretch. The Earth's surface behaves the same way. A projection decides how to stretch it, and where. Some distortions are small near the centre of the map and grow towards the edges.

What gets distorted

Four properties can be affected:

• Shape: the outline of countries and the angles between lines
• Area: the relative size of regions
• Distance: whether measured lengths match real ones
• Direction: whether compass bearings are correct

No projection keeps all four. Designers choose to preserve one or two, or to balance the errors.

Common families

• Cylindrical: imagine wrapping a cylinder around the globe and projecting onto it. Mercator is the best-known example. It keeps angles and shapes correct locally, but inflates areas near the poles. See what Web Mercator is.
• Conic: projecting onto a cone, which works well for mid-latitude regions that are wider east to west, such as a country or a state.
• Azimuthal: projecting onto a plane that touches the globe at one point, often used for polar maps and for showing distance from a centre.
• Equal-area: projections designed so that regions keep their relative sizes, which suits thematic maps such as population or land use.

Why Greenland looks so large

On a Mercator-style map, Greenland appears comparable in size to Africa, although Africa is about 14 times larger in area. This is a side effect of how the projection stretches high latitudes, not an error in the data.

Projections and coordinates

Latitude and longitude describe positions on the Earth itself, so they do not depend on a projection. A projection is applied when those positions are drawn or when distances are computed on a plane. Choosing the right coordinate reference system ensures that everyone agrees on both the shape of the Earth and the projection used.

What this means for everyday maps

Most web maps use one projection for the whole world so that tiles fit together and zooming is smooth. That choice makes the map convenient to use and means that sizes near the poles should not be trusted. For navigation and finding an address, the distortion at city scale is small. For comparing the size of countries or measuring long distances, use a method designed for that purpose, as discussed in how long a degree is.