Most conventional maps have a few key pieces: title, scale bar, legend, and north arrow or compass rose.  It’s easy to see why these are necessary: what map am I looking at, and what’s its purpose?  How big is this map?  What do these colors and symbols mean?  And finally, which way do I need to go?  These pieces make the map usable to the viewer.

However, some geographers and cartographers discuss the need for a compass rose or north arrow.  The arguments are varied:
1. North at the top, south at the bottom is the convention.  People assume this, so don’t take up valuable map space with a redundant piece of information.  People are also familiar with particular land masses, so a display of the United States for an American audience doesn’t really need the explanation.
2. Depending on the datum, projection, and display, north may not be accurately described.  When dealing with the Robinson projection, or other display-based projections, “north” isn’t a uniform direction.  If you were standing in Italy, north would be a different direction than if you were in Papua New Guinea.  Placing a single arrow isn’t accurate.
3. In some maps, it’s just not useful or necessary information.  If you’re displaying a map of general music preference, is “north” really relevant?  Will the viewer be using this map for navigation or any directional activity?  Does it add to the dataset, or is it just there because of the “required map elements” checklist?

But in that first question comes the assumption that the general viewer will “know” that north is at the top of the map and that south is at the bottom.  Why is that?

The astronomer Claudius Ptolemaeus (Ptolemy) seems to be the most popular origin story for the north-up map.  In his Geographia, the maps are among the first known to use latitudinal and longitudinal lines.  His coordinate system, while not entirely accurate, was useful to other geographers and cartographers to compare, measure, and verify their own world maps.  A standardized system is often appealing, because it allows different sets of knowledge to be displayed and analyzed together, by the same criteria.

Ptolemy’s maps placed north at the top, south at the bottom, and this was a convention that remained for a while, as cartography is an expensive, time-consuming occupation that was quite difficult without modern-day assistance such as aerial and satellite photos.  Many cartographers “borrowed” the work of others, and so mistakes and conventions were replicated for many years, as not every cartographer had the luxury of exploring the world they were representing.

Outside of the Western world, the cardinal directions were not always the most important.  Instead, the most important pieces were at the center, not the top.  Palaces, capital cities, and even celestial bodies were at the center of the map, and less important, but still related, items radiated outwards from them.  This, like Ptolemy placing his own society at the “top” of his map, can be an example of egocentrism.  This isn’t necessarily a negative thing – maps represent not only the globe, but local areas.  Generally, a local person cares most about his or her local area – how do I get from Here to There?  Where am I in the world?  When making maps for local people, most societies chose to put themselves at the top of the hierarchy, making maps from other areas of the world strikingly different.

Many medieval maps didn’t use Ptolemy’s convention, and the T-O map became much more popular.  East was often oriented towards the top of the map, and the word “orientation” itself comes from the Latin word for “east”, oriens.  Some suggest this is because the sun rises in the east, so orienting yourself becomes easier and does not require the use of any tools.

So what changed it back?  Why do we now use the north-up convention?

One suggestion was the expansion of the Western world.  The use of Portolan charts, and other navigational tools, changed the needs of the general audience.  Where were these trading posts?  Where did this item come from?  Where were the wars happening?

Another was egocentricity.  As the Western world expanded, leaders may have wanted to assert their cultural dominance.  The most important pieces of the map are at the top – the northern world.  This convention continued, as the global North prioritized their own thinkers and creators over other areas that were wrongly perceived as less civilized.

Further suggestions include the discovery and importance of magnetic North: a compass points towards magnetic North, and that would make it easier for a person to use a compass and compare it to a map.  This standard would expand the use of maps, as well as increase their accuracy.

Likely, it was a combination of many elements.  Few events in history can be simplified to one sole cause.  While there has been criticism of the north-up convention, even in popular media, it seems to persist.  Whether or not it will be the preferred convention in the future is unknown, as certainly political power and perceptions alter the way in which we view maps and representations of our world

For futher reading, consider these:
1. Cartastrophe‘s more humorous take on the “required map elements
2. Medievalist.net‘s collection of 10 medieval maps with some interesting orientations (take note of the Tabula Rogeriana, by the famed Al-Idrisi, which is oriented with south at the top and north at the bottom, usually displayed upside-down for modern audiences)
3. Wikipedia’s entry on the compass rose itself.