What Makes a Map Good

From its earliest days, humankind has put thought into the world: cave art depicting animals, tallies on sticks and bones, and especially a wide variety of maps, suited to different purposes, from a small portable stone map found in Spain that was used for navigation by foot to the floating bamboo-and-shell “stick” maps that the Marshall Islanders used for navigating the open ocean. Different as they are, the best maps share core features. They aren’t always to scale. They mix perspectives. They depict as well as map. They omit masses of information. This isn’t just from ignorance or lack of technical sophistication—it’s by design. What’s included is exactly the information that the map users need, uncluttered by information they don’t need.

Take, for instance, the London Tube map. Harry Beck’s map of the London Underground entered the world in 1931 and has since been imitated by transportation systems all around the globe. It shows — but distorts — a simplified skeleton of the train lines, depicted as lines running vertically, horizontally, or diagonally, by no means an accurate reflection of their pathways.

The insight that inspired the map came from electronic circuit diagrams, a fascinating instance of anachronistic analogical reasoning. Geography doesn’t matter for electricity. What matters are paths and connections, gateways to other paths. The same for commuters, Beck reasoned. What commuters needed were the paths from station to station and the connections to other Tube lines, not geographic accuracy.

His design met resistance from the powers that be but was an instant hit with commuters. It’s so legible. The Tube lines are color-coded. The horizontal, vertical, or diagonal lines are easy for the eye to follow. The stops are indicated by name and perpendicular blips, and the connections to other Tube lines are clearly marked by circles.

The Tube map includes only a small fraction of the possible information, and what it includes is exactly what users typically need: paths and points where actions can be taken—specifically, switching Tube lines, entering, or exiting. It distorts distances and directions.

People preferred schematic route maps that showed the relevant paths and places to turn, even if directions and distances weren’t accurately depicted.

The London Tube map also adds words and symbols. Most good maps and diagrams are multimodal, like natural conversation, which uses far more than words, such as intonation, gesture, and stuff in the world.

Maps can be designed for multiple purposes or different maps for different purposes. Maps can allow wayfinding and exploring an environment and planning excursions and rerouting traffic and locating bike lanes and so much more. Maps can form a foundation for explaining history, as the Aztecs did in their codices, colorfully showing the migrations of their ancestors over space and time.

Maps can explain wars, as the newspapers did during World War II, showing the size and movements and alliances of troops in Europe day by day. They can track the spread of disease, the first step in finding causality, as the dogged physician John Snow famously did in the cholera epidemic in London in 1854. No one knew then what caused cholera. Snow asked that each case be recorded on a map of central London. He observed that many cholera cases clustered around the Broad Street pump and ordered the pump handle removed.

That virtually ended the epidemic and at the same time initiated the science of epidemiology, still strongly based on maps. Maps can foster sleuthing and inferencing and discovery and prediction, whether it’s tracking the spread of disease or terrorists or the paths of hurricanes. They can allow making sense of voting patterns, famine and flood, and demographic data like population shifts and economic disparities. They can allow explanations of social, religious, political, linguistic, genetic, and technological change and their consequences.

Route maps are a special case of maps, but a common one. Route maps take you from A to B, from one location to another. Long before smartphones, a pair of grad students in computer graphics just down the street from my office came up with a prescient idea: Develop an algorithm to generate route maps that would allow people to easily get from A to B.

At the time, the custom routes that could be downloaded from websites were superimposed on highway maps and were almost useless. They were on a single scale, so the tricky parts, getting to and from the freeways, were too small to be seen, and the route itself was buried in irrelevant clutter.

The students had found my work on effective sketch maps. Together with the work of others, we had shown that people produced, preferred, and performed better with schematic route maps that showed the relevant paths and places to turn, even if directions and distances weren’t accurately depicted. Exact distance and direction are less important. The students applied those principles to create a terrific algorithm that quickly produced an enormous number of As to Bs and that users in beta testing loved. It was the beginning of rapid-paced developments in map technology.

Here are four rules of thumb for map design that we’ve gleaned from maps, ancient and modern. Most have been tested in one way or another. Together, they conform to the two cognitive design principles: They help to ensure that the representation corresponds to the targeted concepts and that the representation is straightforward to use for the targeted tasks.

Map elements and relations in real space to elements and relations in representational space. That’s the page, virtual or actual.

Include only the information that’s useful for the task, uncluttered by irrelevant information that can distract or confuse.

Exaggerate, even distort, the useful information to make it easy to find and follow.

Add words and symbols where useful to clarify the critical information.

These rules of thumb apply to maps and many other diagrams as well. These are best practices gathered from observation, analysis, and experience.

Try thinking of activities central to our lives that do not involve movement in space. Not easy. Maps are a natural for showing space, and because the eye quickly sees locations and clusters and direction, maps foster inferences about phenomena in space and movement in space. Space and movement in space are the ground on which individual, social, political, biological, chemical, and physical processes take place. By simplifying and sometimes even distorting the way we represent space and action in space, we can better understand and reason about those processes.