The most famous world map is the one Gerardus Mercator, a Flemish mapmaker, published in 1569. It was made for sailors, and it still shows up on classroom walls and web maps. You learned in Why Flat Maps Stretch that every flat map stretches something. Here is what Mercator stretches.
Key idea
A Mercator map keeps angles and directions true, so small areas have the right shape and a straight line is a steady compass course. To do that, it stretches places more and more toward the poles: areas far from the equator look much too big.
Why it works
On a globe, meridians come together at the poles. Mercator draws them as straight, parallel lines instead, so lands near the poles are stretched east-west. To keep shapes from looking squashed, Mercator also spreads the parallels farther apart toward the poles, stretching lands north-south by the same amount. Shapes stay right, but sizes grow: at 60° N, lengths are doubled and areas look four times too big. The poles themselves would be infinitely far away, so Mercator maps stop before them.
| Latitude | Areas look bigger by about |
|---|---|
| 0° | 1 time (true) |
| 30° | 1.3 times |
| 45° | 2 times |
| 60° | 4 times |
| 70° | 8.5 times |
Worked example: Greenland and Africa
On a Mercator map, Greenland looks about as big as Africa. Greenland covers about 2.17 million km² and Africa about 30.3 million km². How many times bigger is Africa?
- Divide: 30.3 ÷ 2.17.
- That is about 14.
- Greenland looks huge because it is far north, where Mercator enlarges areas many times.
Answer: Africa is about 14 times bigger.
Worked example: Why sailors liked it
A captain draws a straight line on a Mercator chart from Lisbon to a port in Brazil. What does the line tell her?
- On a Mercator map, angles are true.
- So a straight line crosses every meridian at the same angle.
- She can hold one compass heading the whole way.
Answer: The one compass direction to steer for the whole trip.
Common mistake
Believing Greenland is as big as Africa, or that Europe and North America are bigger than they are, because a Mercator map shows them that way. Check sizes on a globe or an equal-area map. Remember the rule: on Mercator, the farther from the equator, the bigger things look.