What is a map?

Imagine you are a bird flying over your home.

You look down. You see roofs, roads, trees and a park. A map is a picture of what you would see. It shows where everything is.

A map is a drawing or picture of a place, shown flat as if you were looking straight down from above. The real place has hills, buildings and trees, but the map turns it into a flat picture you can carry, fold or look at on a screen.

Maps can show almost anything: the way from your house to school, the streets of a town, the mountains of a whole country or the entire world. They help us know what to expect on a trip and check that we are going the right way.

National Geographic defines a map as a symbolic representation of the arrangement of features in an area. Most maps are two-dimensional images on paper or screens, but globes and terrain models are three-dimensional ones, and maps used to guide ships or aircraft are often called charts. The study and practice of making maps is cartography, and mapmakers are cartographers.

Every map is a choice. A cartographer decides what to include, what to leave out and how to show it, so a map always simplifies the real place. Britain's national mapping agency, Ordnance Survey, describes a map simply as a 2D picture of a 3D landscape; the Royal Geographical Society calls maps and plans views from above, a "bird's eye view" of a place. Ordnance Survey suggests that children start by drawing a sketch map of their own route to school.

Maps use little pictures, shapes and colors.

They can stand for roads, houses and rivers. The key is a box that tells you what each picture means.

If a map wrote a label on every house, road and river, it would be far too crowded to read. So mapmakers use symbols instead: shapes, colors and lines that stand for real things. A small dot might stand for a whole city, and colored lines might show roads or borders.

Every map can use different symbols, so most maps have a key, also called a legend. The key lists each symbol and what it means. When you find a symbol you do not know, look it up in the key.

Maps communicate with visual elements: lines, colors, symbols and words. A dot can stand for a city; colored lines can mark highways or borders; shading and tints can show changes in height. Because cartographers choose their own symbols, the same color can mean different things on different maps, which is why a map's legend (or key) matters.

Height is a special challenge on a flat page. Contour lines (topographic lines) join places of equal elevation: loops drawn inside one another can show a hill (or, where the ground drops, a hollow), and the closer the lines, the steeper the slope. National Geographic suggests the memory aid DOGSTAILS for a map's parts: Date, Orientation, Grid, Scale, Title, Author, Index, Legend and Sources.

Maps help us know which way to go.

The four main ways are north, east, south and west. Many maps have north at the top. A little arrow often shows which way north is.

To follow a map, you need to know directions. The four main directions, called cardinal directions, are north, east, south and west. Halfway between them are northeast, southeast, southwest and northwest.

Many maps have a compass rose, a drawing that shows the directions. Some have just one arrow, and that arrow usually points north. Many maps are printed with north at the top, so east is to the right and west is to the left, but not all of them: always check the arrow.

Orientation tells the reader how the map lines up with the real world. A compass rose shows the cardinal directions, and more detailed ones add the ordinal (intercardinal) directions halfway between them; a single arrow normally points north. In many cases maps are printed with north at the top, but it is a convention, not a rule, so the arrow is worth checking.

To describe exact positions, maps use grids. A simple grid labels columns with letters and rows with numbers, so a park map might say a trail starts at D7. Ordnance Survey maps are covered with numbered blue grid lines: eastings increase toward the east and northings toward the north, and you always give the easting first. Latitude and longitude form a worldwide grid, so a coordinate means the same place on every map and globe.

A real town is very big.

A map of it can fit on one page. That is because the map makes everything smaller. A scale tells you how much smaller.

A map is a small model of a big place. The scale tells you how much smaller it is. Many maps have a scale bar that looks like a ruler: it might show that 1 centimeter on the map stands for 1 kilometer on the ground.

Scales can also be written as numbers, such as 1:25,000. That means 1 centimeter on the map stands for 25,000 centimeters in real life, which is 250 meters. A map of your street shows lots of detail; a map of the whole world shows less.

Scale is the ratio between distance on the map and distance on the ground. A bar scale shows it as a ruler; a representative fraction states it as a ratio in any units. At 1:1,000,000, one centimeter on the map is 1,000,000 centimeters, or 10 kilometers, on Earth. At 1:25,000, a scale Ordnance Survey calls very useful for walking, 4 centimeters on the map represent 1 kilometer.

Cartographers call a map of a small area in great detail, such as a neighborhood street map, a large-scale map, because objects appear relatively large; a map of a continent or the world is a small-scale map. Zooming in on a phone changes the scale, from small to large, even though the screen stays the same size.

Earth is round like a ball.

A globe is a round map of Earth. A paper map is flat. Making a round world flat stretches some places, so they look the wrong size.

Earth is a sphere, and a globe shows its shape best. But globes are hard to carry, and you cannot see the whole world at once. So mapmakers flatten Earth onto paper. That always stretches or squashes some parts, a little like trying to flatten an orange peel.

The way a mapmaker flattens the globe is called a projection. On one famous projection, made by Gerardus Mercator in 1569, Greenland looks bigger than South America, even though Greenland is really smaller than the Arabian Peninsula in Asia.

Transferring Earth's curved surface onto a plane always distorts something, such as the size, shape or relative position of the continents. The method chosen is the map's projection. National Geographic groups projections into three families: planar (most accurate at the center, often used for the poles), conical (good in the mid-latitudes, often used for the United States) and cylindrical (most accurate near the Equator, often used for world maps).

Gerardus Mercator introduced his projection in 1569. Any straight line on it is a line of constant compass bearing, which made it valuable for navigation charts, but it enlarges areas far from the Equator: Greenland appears larger than South America, although in reality Greenland is smaller than the Arabian Peninsula. Choosing a projection means choosing which distortions are acceptable for the map's purpose.