Globes and flat maps

Earth is round, like a ball.

A globe is a ball with a map of the Earth on it. You can spin it around. You can find the land, the oceans and where you live.

A globe is a model of Earth, made much smaller. Because Earth is round, a globe is round too, shaped like a ball. It shows the land, the oceans and the countries in the right places.

A map is different: it is flat, like a page in a book or a picture on a screen. People use both. In this topic we find out what a globe does best, what a flat map does best, and why a flat map of the whole world can never be perfect.

National Geographic defines a globe as a three-dimensional scale model of Earth or another round body. Because it is spherical, it can represent surface features, directions and distances more accurately than a flat map. A flat map, by contrast, is a two-dimensional image on paper or a screen.

The U.S. Geological Survey (USGS), the primary civilian mapping agency of the United States, puts the difference plainly: no flat map can rival a globe in truly representing the surface of the entire Earth, so every flat map misrepresents the surface in some way. The rest of this topic weighs the strengths and weaknesses of each, and explains why mapmakers keep using both.

On a globe, every country has its true shape.

Big places look big. Small places look small. That is because the globe is round, just like the Earth.

On a globe, four things are all true at the same time: directions (which way is north), distances (how far apart places are), shapes (what a country's outline looks like) and areas (how big a place is).

A globe also makes it easy to find the shortest way between two places across Earth's curved surface. That shortest path is called a great circle.

The USGS Map Projections poster (1993) lists the globe's properties in four words: directions true, distances true, shapes true, areas true. On a globe the lines of latitude (parallels) are equally spaced; the lines of longitude (meridians) meet at the poles, and their distance apart shrinks from the Equator toward the poles. At 60° latitude, the meridians are half as far apart as the parallels, and the scale at each point is the same in every direction.

The globe also makes great circles easy to find. A great circle is the shortest route between two points on Earth's surface, and on a globe it can be found quickly along the surface. On flat maps the two come apart: USGS notes that a straight line on a Mercator map is a line of constant direction but usually not the shortest distance, while on a Gnomonic map every straight line is a great circle.

A globe is too big to put in your pocket.

It is hard to carry and hard to store. And it cannot show your street. So people also use flat maps.

Globes have problems too. They are bulky, so they are hard to carry and hard to store, and they cost a lot to make and to keep up to date. And because the whole Earth has to fit on a ball, even a big globe is far too small to show your town's streets.

Flat maps fix these problems. A flat map can be folded into a pocket, printed in a book or shown on a phone. It can show just your neighborhood, in lots of detail, or the whole world at once.

USGS lists the globe's disadvantages: even the largest globe has a very small scale and shows relatively little detail; globes are costly to reproduce and update, difficult to carry around and bulky to store. National Geographic adds that globes may be less practical for travelers, since they are much bulkier than flat maps and often carry less detailed information.

Flat maps answer those needs. They can be large-scale, showing a small area in great detail, as a neighborhood street map does, or small-scale, showing a continent or the world. They can be printed, folded and updated cheaply, and digital maps let the user zoom from the world to one street. The price of that convenience is distortion, explained next.

Try to make the peel of an orange lie flat.

It tears or stretches! A round Earth is the same. When we make it flat, some places stretch and look the wrong size.

Imagine cutting a globe in half and pressing each half flat. The map would come out warped and torn, and it would not be much use. Even after tidying it up, the sizes, shapes and positions of the continents would be changed.

So mapmakers choose a method, called a projection, for turning the round Earth into a flat map. Every projection gets some things right and some things wrong. A flat map can show true directions, true distances, true areas or true shapes, but never all four at once.

National Geographic describes the thought experiment: cut a globe in half and flatten each side, and the result is warped, torn in places and unusable; even cleaned up, the size, shape and relative location of the continents would differ from the globe. Some distortion is therefore unavoidable, and cartographers decide which parts of the map should appear most true to life. The resulting perspective is called a projection.

USGS states the rule precisely: a flat map, or parts of one, can show one or more, but never all, of true directions, true distances, true areas and true shapes. A conformal map keeps the shapes of small areas but distorts sizes; an equal-area map keeps sizes in proportion but distorts shapes. The orange-peel comparison above is a common classroom analogy, not a measurement.

Different maps are good at different jobs.

Some help sailors find their way. Some show how big countries really are. Mapmakers pick the best map for each job.

Because no flat map is perfect, mapmakers choose the projection that suits the job. Some world maps are made for navigation: on a map Gerardus Mercator introduced in 1569, a straight line shows a steady compass direction, which helped sailors. But that map makes places far from the Equator look much too big. On it, Greenland looks bigger than South America, even though Greenland is smaller than the Arabian Peninsula.

Other maps are made to show the true sizes of countries. When you want the whole truth about the world's shapes and sizes, a globe is still the best tool.

USGS stresses that there is no "best" projection: the mapmaker must select the one best suited to the needs, reducing distortion of the most important features. Some projections are used for navigation, others show better representations of the true relative sizes of continents. Britannica describes the classic case: Gerardus Mercator's projection of 1569 is a useful navigation tool, because a straight line on it indicates a straight compass course, but it is less practical for world maps, because areas farther from the Equator appear disproportionately large; Greenland appears larger than South America, although it is smaller than the Arabian Peninsula.

Scale changes the picture too. USGS notes that the extreme distortion at high latitudes on a small-scale Mercator world map almost disappears on a properly oriented Transverse Mercator map of a small area at the same latitudes, and that its 1:24,000 topographic maps based on the Transverse Mercator are nearly correct in every respect.