Maps and Navigation

Why We Make Maps

Imagine trying to describe your whole town to a friend who has never seen it — every street, river, hill and building — using only words. It would take hours, and your friend would still get lost. This is the problem a map solves. A map is a picture of a place seen from above, drawn to help us understand and find our way.

People have made maps for thousands of years. Ancient peoples scratched maps onto clay tablets and animal skins. Sailors drew charts of coastlines. Today maps live on the screens in our pockets. Whatever the age, a map does the same clever thing: it takes a huge, complicated piece of the world and shrinks it down so we can hold it in our hands and understand it at a glance.

In this book we will learn how maps shrink the world without losing the truth, how every spot on Earth has its own address, and how, for centuries, explorers found their way across empty oceans long before there were screens to guide them.

Shrinking the World: Scale

A map cannot be life-sized — a map of your country the size of your country would be useless. So every map is shrunk by a fixed amount, and that amount is called the scale.

Scale is often written as a ratio, such as 1:50,000. This means that one unit of distance on the map stands for 50,000 of the same units in the real world. So 1 centimetre on the map equals 50,000 centimetres — that is 500 metres — on the ground.

• A large-scale map (like 1:10,000) zooms in close. It shows a small area in lots of detail, perfect for walking around a town.
• A small-scale map (like 1:10,000,000) zooms far out. It shows a whole country or continent, but with much less detail.

Many maps also show a scale bar, a small ruler printed on the map. You can measure a distance on the map against the bar to find the real distance. Scale is what makes a map trustworthy: it lets you measure the real world without leaving your chair.

Symbols, Keys and Colours

A map cannot draw every tree, house and lake exactly as it looks. Instead it uses symbols — small pictures or shapes that stand for real things. A little blue line might mean a river, a green patch a forest, a red triangle a campsite.

Because different maps use different symbols, each map includes a key (sometimes called a legend). The key is a small box that explains what every symbol and colour means. Reading the key first is the secret to understanding any map quickly.

Colours carry meaning too. Blue almost always means water. Green often means low, flat or forested land, while browns and oranges show higher ground and mountains. On many maps, thin brown lines called contour lines join places that are the same height above sea level. When contour lines are packed close together, the slope is steep; when they are spread far apart, the land is gentle. Learning to read contours lets you "see" hills and valleys on a flat sheet of paper.

Finding Direction: The Compass Rose

A map is only useful if you know which way is which. That is the job of the compass rose, the star-shaped symbol that shows direction. Its four main points are the cardinal directions: North, East, South and West. Between them lie the in-between directions — northeast, southeast, southwest and northwest.

By tradition, north is placed at the top of almost every modern map. So if you turn the map so its north matches the real north, everything lines up: roads on the map run the same way as roads in front of you.

But how do you find real north? For nearly a thousand years the answer was the magnetic compass. The Earth behaves like a giant magnet, and a small magnetised needle, free to spin, always swings to point roughly north. The compass, probably first used in ancient China, was one of the most important inventions in the history of travel. For the first time, a sailor far out at sea, with no land in sight, could still know which way the ship was heading.

Every Place Has an Address: Latitude and Longitude

How do you describe the exact position of a lonely island in the middle of an ocean? Mapmakers solved this by drawing an imaginary grid over the entire planet.

• Lines of latitude run east to west, parallel to the Equator — the line around the middle of the Earth. Latitude tells you how far north or south you are, measured in degrees, from 0° at the Equator to 90° at each pole.
• Lines of longitude run north to south, from the North Pole to the South Pole. They tell you how far east or west you are. The starting line, 0° longitude, is the Prime Meridian, which passes through Greenwich in London.

Together, latitude and longitude give every single point on Earth its own unique address, a pair of numbers shared by no other place. The peak of Mount Everest, your school, a shipwreck on the seabed — each has its own coordinates. This grid is the invisible skeleton behind every map and every navigation system in the world.

The Tricky Job of Flattening a Globe

Here is a puzzle that troubled mapmakers for centuries. The Earth is round, like a ball, but a map is flat, like a page. You cannot peel a globe and lay it flat without tearing or stretching it. Try flattening an orange peel and you will see the problem at once.

So every flat world map is a kind of clever compromise called a projection. A projection is a method for spreading the round Earth onto a flat surface, and every method distorts something. The famous Mercator projection, useful for sailing because it keeps directions true, badly stretches the lands near the poles, making Greenland look as huge as Africa when it is really far smaller. Other projections keep sizes honest but bend the shapes.

There is no perfect flat map. Mapmakers simply choose which truth matters most for their purpose — true direction, true size, or true shape — and accept the distortion elsewhere. Understanding this stops you from being fooled by a map.

Navigating Before Satellites

For most of history, travellers had no map of the open ocean and certainly no screen. Crossing the sea meant solving a hard puzzle: Where am I, and which way should I go? Sailors became brilliant problem-solvers.

The oldest trick was to follow the stars. The North Star, Polaris, sits almost exactly above the North Pole, so its height in the sky tells a sailor their latitude. By day, the rising and setting Sun showed east and west. Pacific Islander navigators crossed thousands of kilometres of open ocean reading the stars, the swell of the waves and the flight paths of birds — all without instruments.

Later, clever tools appeared. The astrolabe and then the sextant let sailors measure the angle of the Sun or a star above the horizon, turning that angle into a latitude. Finding longitude was far harder and was only solved in the 1700s by John Harrison's marine chronometer, a clock accurate enough to keep the time of the home port during a long voyage. By comparing that time with the local time worked out from the Sun, a navigator could finally calculate longitude — and know exactly where in the world the ship was.

Maps in the Age of Satellites

Today most of us navigate using GPS, the Global Positioning System. High above the Earth circles a fleet of satellites, each constantly broadcasting a signal that says, in effect, "I am here, and the time is exactly now."

A GPS receiver — in a phone, a car or a ship — listens to several satellites at once. Because radio signals travel at a known speed, the receiver can measure the tiny differences in how long each signal took to arrive. From those differences it calculates its distance from each satellite, and where those distances cross is your position. With four or more satellites, GPS can pinpoint you to within a few metres, almost anywhere on the planet.

Satellites also photograph the whole Earth from space, which is why digital maps can show real images of streets, mountains and coastlines. Yet the old ideas have not gone away. Behind every glowing blue dot on a phone lies the same grid of latitude and longitude, the same idea of scale, and the same dream that has driven travellers for thousands of years: to know exactly where we are.

What We Have Learned

A map is one of humankind's greatest inventions — a way to shrink the whole world onto a page without losing the truth. We have seen how scale controls the shrinking, how symbols and keys pack in meaning, and how the compass rose and contour lines show direction and height.

We met the invisible grid of latitude and longitude that gives every place its own address, and learned why no flat map can show the round Earth perfectly. Finally we followed navigators from the star-readers of the ancient oceans, through the sextant and the chronometer, all the way to the satellites of GPS.

Next time you glance at a map or follow a moving dot on a screen, remember: you are using one of the oldest and cleverest ideas in human history.

Keen to keep exploring the planet? Travel across the land in The Seven Continents, or follow the water downhill in The World's Great Rivers.