Why Map Literacy Matters for Evidence-Based Policy

By Camille Galdes & Maya Brennan

By orienting us in space, maps provide a strong sense of place and reality, despite the fact that they are—by their very nature—abstractions of the real world. In an increasingly visual age, maps are being used more and more by policy makers and laymen alike. Yet, many creators and users of maps do not view them critically as the product of people and their editorial decisions.

As a snapshot of the world, a map can convey complex data in a simple and understandable way—or it can obscure the nature of the data with a compelling yet confused narrative. Understanding what maps say (and do not say) may open doors to improved application of housing research—and other work that connects people and places.

The interactive maps presented by the New York Times as part of its reporting on Raj Chetty’s mobility research, while fascinating, are oversold, according to Andrew Wiseman, a geography professor at George Washington University in Washington, D.C. Wiseman, in an article for CityLab, uses these maps to illustrate the importance of accurately describing the map rather than sensationalizing it. While Chetty found a connection between the county where children grow up and their subsequent earnings, that is not sufficient basis for labeling one county as a better or worse place to grow up than another. The overpromise of the article’s title, “The Best and Worst Places to Grow Up: How Your Area Compares,”is good for social media hits, but a more careful reading of the map shows what the data actually say.

Sensational titles are just the start of Wiseman’s tour of common map interpretation problems. He urges readers not to be taken in by a map’s beauty or promises. Instead, they should look under the hood: What is the source of the data? Why are the data divided as they are? Are the visual elements, such as size or color, creating a false conclusion? What does the map really mean? Does it even make sense to display the data spatially?

A lot of variation in maps is driven by population. There is more of everything—deaths, jobs, crime, schools, hospitalization, and households that cannot afford the rent—where there are more people. Data that have not been adjusted for population do not show anything more than the distribution of people.

The geography selected for presentation matters, too. Display of housing affordability data by state, metropolitan area, county, city, and census tract can lead to different interpretations. Depicting affordability at the state level does not say much about the location or magnitude of affordability challenges. A household in the Buffalo, New York, metropolitan area lives in a different market than a household in Manhattan. Too granular a view may not be helpful either. When analyzing children’s educational opportunities, anything smaller than a school’s catchment area is meaningless (in an area where such boundaries still exist, that is).

How data are divided also point to specific interpretations. When states show up the same color on a map, it sends an immediate message that something about those states is the same. A mapmaker should be transparent in the legend about how the data are divided so a viewer can decide if these dividing lines make sense.

Fortunately, many reputable research institutions put housing data into compelling and interactive maps. Major think tanks such as the Urban Institute, Pew Research Center, and the Brookings Institution provide maps along with traditional research reports, or even in place of them. For instance, and interactive map on population change by the Urban Institute provides the user with all the data parameters and allows them to be manipulated. The Joint Center on Housing Studies’ interactive map on renters’ cost burdens also explains how the categories of data are defined and allows users to toggle across each data category.

The level of transparency in these maps better ensures that users know what they are looking at. By functioning more as data analysis tools, the maps show their own inner workings to the user rather than present all the information as a static image that one must quickly interpret.

As data profilerate, maps have also become a staple of the online news landscape, particularly through blogs associated with traditional news outlets, like the Atlantic’s CityLab and the Washington Post’s Worldview. The New York Times regularly publishes maps on such pressing topics as segregation, marriage, and migration. Because of their pithy, visual format, the maps generated by these online news outlets are quickly sent out across social media platforms. Whether these maps are edited with the same critical eye that a strictly word-based piece would demand is still to be determined.

By opening access to mapmaking by large and small institutions, resources like PolicyMap, Social Explorer, and OpenStreetMap enable users to explore spatial data without expensive geographic information system (GIS) software. These tools have facilitated more visibility of the place-based nature of people’s lives.

For example, the Urban Institute created an animated map and graph of mortgages using OpenStreetMap. The New York Times produced a map depicting segregation that is rich with information. These maps stimulate debate and may spark further research into mortgage activity and segregation. Yet, it is often just as easy to use mapping platforms to inadvertently create a map that leads the debate astray with badly interpreted data.

Knowing how to create maps and how to discern their meaning is an increasingly important skill in the Digital Age. Maps entertain yet also convey information for making decisions, analyzing policies, and understanding the world around us. As both producers and users of maps, it is more important than ever that we understand what they are and what they are not. We’ve all been told “don’t believe everything you read.” Likewise, we should apply a critical eye to the maps that have become a regular part of the flood of online information. While a good map is just one part of sound place-based decision making, a bad map can create an epic myth that may be impossible to dispel.