A map is a representation of all or part of the Earth's surface, and its basic purpose is to show where things are. Maps can present both visible features, such as rivers and lakes, forests, buildings, and roads, and things that cannot be seen, such as boundaries and temperatures. Most maps are drawn on a flat surface. A map represented on the surface of a sphere is called a globe. Maps and globes also depict the surfaces of bodies other than the Earth, such as the moon.

Types of maps and globes

In addition to such general maps, there are many others that deal with a particular subject or serve a particular purpose. Many of these are concerned with travel and finding the way. Examples are road maps and the specialized maps called charts, which are used for navigation at sea (nautical charts) or in the air (aeronautical charts). Other common types are geological maps, which show different kinds of rock; land-use maps, which show how land is used; and population maps, which show the distribution of people. Historical maps show how the land was divided in the past.

Globes provide the same kinds of information that flat maps do. Because the Earth is nearly a sphere, a globe represents it best. A globe shows the Earth as it actually looks when seen from outer space. Only on a globe can distances, directions, and the shapes and sizes of areas on the Earth be shown accurately.

Scale

A map is always smaller than the area it represents. The map's scale indicates how much smaller it is. It shows how distances on the map are related to distances on the surface it depicts.

Map scale can be indicated in a number of ways. The simplest of the devices is the graphic scale, a ruled line or bar that is usually marked off in miles or kilometers. Any distance on the map can be marked off and measured on the graphic scale. Some maps have a verbal scale, which uses words such as “one inch equals 60 miles.” A third kind of map scale uses a mathematical expression called a ratio. For example, the scale ratio 1:50,000 states that one unit of measurement on the map is equal to 50,000 such units on the ground. Therefore, one centimeter on the map is equal to 50,000 centimeters (500 meters) on the ground. This type of scale works best with the metric system of measurement.

Maps are sometimes classified by the amount that distances are reduced in their scales. The reduction is greater on a small-scale map than it is on a large-scale map. For example, one inch may represent 20 miles on a small-scale map but only 2 miles on a large-scale map. Therefore, a large-scale map shows a small area of the Earth's surface in considerable detail. A small-scale map shows a much larger area but in much less detail.

Finding a place

A common way of describing the position of a place on a map is through the latitude and longitude system. This system divides the Earth into sections using two sets of numbered lines. Lines of latitude run in an east-west direction. Lines of longitude run in a north-south direction. The index of a typical atlas, or bound collection of maps, lists the location of places by their latitude and longitude. (See also latitude and longitude.)

Some maps are divided into a rectangular grid, sort of like squares on a chessboard. Rows across may be lettered “A,” “B,” “C,” and so on, and rows that go up and down may be numbered “1,” “2,” “3,” and so on. If the atlas index lists the location of a place as “B 2,” for example, this means that the place can be found on the map somewhere within the area in both the “B” and the “2” rows. Road maps commonly use this system.

Projections

If the Earth were flat, representing its surface on a flat sheet of paper would not be a problem. Because the Earth is spherical, however, its surface cannot be “flattened” out into a map without being distorted, or twisted out of its true shape. Distances, for example, may appear longer or shorter than they actually are, or the sizes or shapes of landmasses may be incorrect. For a large-scale map of a very small area, this is not noticeable to the map user. But in any small-scale map representing a large area such as a country or a continent, the spherical shape of the Earth causes significant problems.

On a globe (which is a small-scale model of the Earth), the lines of longitude get closer together as they approach the North and South poles, where they meet. They are all the same length, and they always intersect with the lines of latitude at right angles. On a map of a large area, it is not possible to show the lines of latitude and longitude so that they correspond exactly to those on the globe. On a map, therefore, the surface of the Earth is distorted in some way. It follows that the scale of such a map cannot be correct all over.

Although this problem cannot be avoided, it can be dealt with by means of projection. Projections are methods of representing the Earth's lines of latitude and longitude on a flat surface in a way that represents at least one property correctly. Some projections distort shape badly but show the relative sizes of areas correctly. These are known as equal-area, or equivalent, projections. Others distort size but show shapes correctly. These are known as conformal projections. A third type is designed to show distances correctly from one central point. These are called equidistant projections. No map projection can have all these properties at the same time.

When viewing a globe, only half of it can be seen from any angle. This is the largest area that can be shown continuously on a map projection. Therefore, maps of the entire Earth's surface are made by dividing the surface, usually along a line of longitude, so that all of it can be seen at once. Whatever arrangement of lines of latitude and longitude is chosen, the distortion around the edges of the map is always very great.

As with other maps, it is possible to make a world map so that either the shapes of landmasses or their areas are relatively correct, but it is impossible to do both in one map. Many world maps are a compromise between the two extremes. There are many different projections for maps of the world, but all world maps are highly distorted. For example, the best-known projection, the Mercator, preserves the shape of landmasses but not their areas. Because Mercator maps have been used widely in classrooms and textbooks, many people have mistaken ideas about the relative size of various landmasses. For example, on the Mercator maps Greenland is larger than South America, when in reality it is only one eighth as large.

Representation of relief

Map symbols

In the past mapmakers commonly used drawings of features—for example, a castle or a mountain chain—to indicate where they were located. These drawings were greatly exaggerated in size so they could be clearly seen. Such artwork was nice to look at and easy to understand, but it produced maps that were cluttered and imprecise.

Gradually, maps were simplified as pictures were replaced by symbols. Each symbol stands for, or represents, one particular thing. For example, a map may have different symbols for cities, rivers, lakes, different types of roads, railroad tracks, boundary lines, buildings, and so on. The information that is regarded as most important will likely be shown by symbols that are relatively large and strongly colored. For example, a main road may be shown by double black lines with another color between them. The symbols are described in the map's legend.

History of mapmaking

The oldest known maps were drawn on clay tablets by the people of ancient Mesopotamia. They date back to about 2300 BC. Nearly as old as these are certain Egyptian drawings and paintings discovered in early tombs. The ancient Greek mathematician and astronomer Ptolemy had a great influence on geography and cartography, or mapmaking. His eight-volume Guide to Geography, written in the 2nd century AD, was a major reference work for almost 1,000 years.

After Ptolemy, little progress was made in mapmaking until the age of exploration, which began in the late 15th century. Exploration encouraged the development of navigation, ship design and construction, instruments for observing and measuring the land, and use of the compass. In turn, these developments improved the accuracy of existing information used to make maps and encouraged further exploration and discovery.

The leading mapmaker of the age of discovery was Gerardus Mercator of Flanders (now in Belgium). He developed the famous projection that was named for him. In 1569 he published a map of the world based on this projection. The Mercator projection is still widely used.

In the 18th and 19th centuries mapmaking was transformed by the development of precision scientific instruments. Improvements to the telescope helped raise the quality of astronomical observations. A new type of clock called a chronometer made it easier to figure longitude. Extensive national surveys were begun in several countries, notably in France, Great Britain, and the United States. Surveyors measure positions, points, and lines on or near the surface of the Earth for mapmaking and other purposes. All of these advances led to maps with more accurate details.

Even so, much of the world remained largely unmapped until aerial photography came into use during World War II. Much of the mapping at this time was done for military purposes. Today most countries have national mapping organizations. They produce maps at many different scales for official, commercial, and scientific purposes. Nautical charts are also the responsibility of government departments. The United Nations provides help in mapping to countries wishing it. In addition to official maps, many private companies also produce and publish maps, especially road and tourist maps. Electronic equipment is now used in the production of maps. Computers can be programmed to trace boundaries and coastlines, to provide printouts of map projections, and to produce a variety of statistical maps. (See also geography.)