64.2°N 51.7°WNuuk, Greenland
Why you cannot flatten a globe
Try to press an orange peel flat and it tears or stretches. A sphere cannot be laid flat without changing some distances, and mathematicians proved this in the 1800s. So every flat map has to give up accuracy somewhere. Mapmakers choose which properties to keep.
The four things a map can keep
- Shape (conformal maps): angles and small shapes look right, but sizes change across the map.
- Area (equal-area maps): sizes are in the right proportion, but shapes get squashed or stretched.
- Distance (equidistant maps): distances are correct from one point or along certain lines only.
- Direction (azimuthal maps): directions from the center point are true.
No map keeps both shape and area across the whole world. Some keep neither exactly and balance the errors instead.
Projection families
| Family | Imagine the paper as | Most accurate | Examples |
|---|---|---|---|
| Cylindrical | A tube wrapped around the equator | Near the equator | Mercator, Gall-Peters |
| Conic | A cone set over one hemisphere | Mid-latitudes, along one or two chosen parallels | Albers, Lambert conformal conic |
| Azimuthal | A flat sheet touching one point | Near the center point | Polar maps, azimuthal equidistant |
| Pseudo-cylindrical | A tube with curved meridians | Spread across the map | Robinson, Mollweide, Equal Earth |
Conic projections suit countries that stretch east to west in the mid-latitudes, such as the United States, which is why many national maps use them.
The Mercator projection
Gerardus Mercator published his world map in 1569 for sailors. On it, a line of constant compass bearing is a straight line, which made plotting a course simple. The cost is size. Areas are stretched more and more toward the poles, and the poles themselves cannot be shown at all.
The classic example: on a Mercator map, Greenland looks about as big as Africa. In reality Africa is roughly 14 times larger. Russia, Canada and Antarctica look far bigger than they are, too.
Most online street maps use a version called Web Mercator. Because it keeps local shapes and angles right, streets meet at the correct angles at any zoom level, which matters more on a city map than comparing continents.
Equal-area and compromise maps
Equal-area projections such as Mollweide, Gall-Peters and Equal Earth show every region at its true relative size. They are the right choice for maps of population, forests or anything measured per square kilometer, because a stretched area would exaggerate the data.
Compromise projections such as Robinson and Winkel tripel let shape, area and distance all be a little wrong so that no single error becomes large. They are popular for classroom wall maps and atlases because the world looks familiar.
How to spot distortion on any map
- Look at the grid. If parallels are evenly spaced straight lines and meridians never meet, high latitudes are stretched.
- Compare a high-latitude country with one near the equator whose size you know.
- Check the map's notes or legend. Good maps name their projection.
- When in doubt, check on a globe. It is the only map with no distortion at all.
Which projection for which job?
| Task | Good choice |
|---|---|
| Navigating by compass bearing | Mercator |
| Comparing country sizes or mapping data by area | An equal-area projection |
| A general world wall map | A compromise projection |
| Polar regions or distances from one city | Azimuthal |
| A mid-latitude country | A conic projection |
Map projection FAQ
What is the most accurate map projection?
None is accurate for every purpose. Each one keeps some properties and distorts others. The best projection depends on whether you care most about shape, area, distance or direction.
Why is Greenland so big on maps?
Most familiar world maps, including many web maps, use Mercator or something close to it. Mercator stretches areas more the farther they are from the equator, and Greenland lies far north.
Do topographic maps have this problem?
Yes, but over a small area the distortion is tiny. Local maps use projections chosen so that errors stay well below what you could measure on the sheet. See topographic maps.