You use maps all the time. And on the highway map in your car, the topographic map in your backpack and the world map on your wall, north is almost always at the top.

Why? It is, after all, a completely arbitrary choice. There is no "up" or "down" in space. And the only factor that determines the "top" of a map is the way the type is placed. So how did we get stuck with the north-up orientation? The answer can be found in a brief tour through the history of maps.

Cartography, the science of map making, was established through the efforts of early scholars such as Eratosthenes, head of the library in Alexandria, Egypt, in the 3rd century B.C. He used simple trigonometry to calculate the world's circumference by measuring the angular difference in noonday shadows at two places a known distance apart.

Eratosthenes realized that, for cartographic purposes, the globe could be depicted as if a grid of "parallels" (lines of latitude) and "meridians" (lines of longitude) were laid over it.

Although the magnetic compass would not come into use for another thousand years, certain parallels with an east-west trend already were well established through celestial observations. These included the "equinoctial line" (the equator), the "Summer Tropic" (Tropic of Cancer) and the "Winter Tropic" (Tropic of Capricorn). To these, Eratosthenes added the concept of meridians with a north-south trend.

But Eratosthenes drew his parallels and meridians to intersect known places of the world, mostly around the Mediterranean; consequently, the lines were spaced irregularly. In the following century, the Greek astronomer Hipparchus made the point that this grid should be geometrically regular, a true system of latitude and longitude. That concept led to one of the most influential of all cartographers, Ptolemy.

Like Eratosthenes, Ptolemy was a Greek who lived in Alexandria. None of his maps has survived. But Geographia, his great contribution to cartography, lists the latitude and longitude of all important places in the world as they were known then. That allowed later map makers to reconstruct his world view.

Ptolemy invented the concept of the atlas, in which the world is subdivided into a series of maps. The modern word, however, comes from the later practice of decorating maps with an image of the mythological titan Atlas holding up the world.

Ptolemy recognized a problem in portraying Earth's spherical surface on a flat sheet. So he devised the first real map "projection," with north at the top [Figure 1].

Why north up? In Ptolemy's projection, the habitable world -- particularly the Mediterranean region -- extended in an east-west direction in the northern hemisphere. Most of the equatorial region and southern hemisphere was considered uninhabitable and occupied by monsters. In addition, convergence of the meridians toward the top of the map may have been aesthetically more pleasing.

Perhaps also, as Lee De Cola of the U.S. Geological Survey points out, a globe with most of the areas of interest on its lower half would be harder to use, and Ptolemy's projection was created with a globe in mind.

But that didn't set the standard. In Europe during the Middle Ages, Christian doctrines guided much of the thought and scholarly work. Maps of the time, known as mappaemundi, portray the world as a circle with Jerusalem in the center and east at the top, in the presumed direction of the garden of Eden [Figure 2].

The traditional importance of east as a direction is reflected in the verb "orient," whose original meaning is "to place facing the east" (from the Latin word for "rising," as in sunrise) -- a common element, for example, of medieval cathedral design.

By the 15th century, Europeans were gaining a truer sense of the world through exploration. The state-of-the-art 1459 world map by the Italian monk Fra Mauro was still circular but, for the first time, correctly showed the Indian Ocean connecting with the Atlantic around the Horn of Africa. On this map, south is to the top, perhaps reflecting the influence of Islamic maps made centuries earlier.

In contrast to these world maps, whose creators commonly had religious or cosmological considerations foremost in their minds, another class of map was developed by the 14th century. They, too, had a variety of orientations.

Known as "portolan" or "portolano" charts from the Latin word for harbor or entrance, they focused on the correct depiction of coastlines, a very real concern to maritime traders of the Mediterranean and North Atlantic.

The portolan chart, sometimes drawn on animal skin for durability, was designed for use with a magnetic compass. Its offshore areas were covered with radiating direction or "rhumb" (compass point) lines that permitted accurate marine navigation anywhere on the map. The coastlines on some Mediterranean charts are quite modern in their accuracy, whereas those of more distant places are less so.

And, as seen in this 1547 portolan chart of the East Coast of North America [Figure 3], north-to-the-top still was not the predominant orientation. But it soon would be.

In the 15th century, scholars "rediscovered" Ptolemy's work. Maps depicting his north-up perspective were reconstructed from his text and widely distributed with the help of the newly invented printing press.

Then came Flemish geographer Gerardus Mercator (1512-94) and his famous projection. Mercator's 1569 map was an enormous boon to mariners because compass headings could be plotted as straight lines. Incidentally, it put north to the top.

To be sure, many maps that are produced are not north-up. Australians, for example, frequently use south-up versions. But over time, the rise in popularity of the scientific method, with its emphasis on reproducible results and standardization, helped to make other orientations increasingly less frequent. Eventually, a single cartographic convention came to predominate -- that of a 2nd-century Egyptian way ahead of his time.

William C. Burton is a geologist with the U.S. Geological Survey.

For questions about maps, inquire at the Earth Science Information Center (1-888-ASK-USGS or esicmail@usgs.gov) or the USGS Library (703-648-4302 or library@usgs.gov).