The mission of my Short-Term Fellowship was to research the development of thematic map technique in the West: the way that proportional circle, flow line, isopleth, choropleth, dasymetric, dot density, and cartogram techniques were invented and developed in Europe and the United States during the nineteenth century.

When I told friends and family about my project, there would be a pause, followed by either a confused search for a response ("Oh. Umm?") or a swift segue back to normal topics ("Huh. So, where are you staying?"). Hovering in the air between us was an incredulous elephant: this is how she spent the gift of four weeks in the Map Room, where masterpieces of cartography from across the centuries, elsewhere protected beyond the credentialed gates of private research libraries, are here available for public consultation? Instead of communing with maps from Joan Blaeu's seventeenth century masterpiece, the Atlas mayor, she chose to leaf through yellowed, crumbling international statistical congress proceedings and the dusty annual reports of departments of public works, in search of black-and-white foldout maps and line diagrams?

Yes, and it was glorious.

What cartographers vaguely refer to as "thematic techniques" are the very same that we encounter daily in the maps and graphics of the New York Times and the Wall Street Journal: the use of circle sizes ("proportional circles") to depict the number of people in Iraqi refugee camps, for example, or grids of squares ("cartograms") to visualize the latest proportions of electoral votes for each candidate by state. As modern as these techniques may appear, they are built with a visual language that developed in the West during the 1800s in response to conditions including changes in printing technology, the rise of something called "statistics," the search for a "universal language" for sharing statistical analyses across international borders, the availability of state funding, and the aesthetic pronouncements of a few particularly influential individuals. 

As new techniques were introduced and shared within and across disciplines, they were quickly put to use by scientists, politicians, social reformers, and engineers to explore data sets and reveal patterns in ways that powerfully influenced the development of railway lines, for example, or the allocation of funding to alleviate poverty. They were also immediately put to use to manipulate statistical data and inflame public opinion on political and social topics, for the same principles which support effective information design can also of course support effective mis-information design. (For important lessons regarding these design histories, see for example the work of Susan Schulten and Jason Hansen.) 

For my fellowship, I wanted to hear the dirt, to understand why some techniques thrived in certain circles and not elsewhere, what was rejected along the way, whose career blossomed as a result and whose was wrecked, and which topics were left for twentieth century cartographers to tackle. I sought to explore how the maps moved from a focus on the techniques themselves to the development of full thematic layouts expressing a comprehensive statement about the data variables. To achieve that understanding required a research library with an interdisciplinary, international collection of nineteenth-century maps, atlases, journals, and textbooks, which I found in the holdings of The New York Public Library. Access to paper copies of these publications enabled me to look closely at each map for the incremental changes to legend design, palette choices, data classification, line weight, and layout that gradually over time coalesced into design protocols for thematic cartographic language.

One aspect of this history that was revealed to me through this research was the ways in which cartographers developed the multivariate capacities of cartographic language. Whereas I had previously assumed that early statistical maps employed one thematic technique to portray one data set at a time, gradually becoming multivariate later in the century, in fact the research taught me that thematic technique was multivariate from the beginning. Statisticians immediately experimented with ways to synthesize different techniques in the same map in order to explore possible connections between variables from different data sets in the days before correlation became a measureable condition. At first, these multivariate effects were achieved through compositional adjacency, as when Alexander von Humboldt (see above image) mapped the global routes of mineral exports and imports, then placed his first line (depicting changes in quantities over time) and bar graphs (depicting relative proportions of extractive products by location) directly below and within peripheral view, to achieve a comprehensive statement about the global mineral trade in time and space.

A decade later, the radically innovative Atlas of the Irish Railway Commission took the language to the next level by inventing and then combining dasymetric, proportional symbol, and flow line techniques in the same mapped space, achieving multivariate comparisons through overlap rather than adjacency of separate graphics.

In another ten years, legendary French engineer Charles Minard would follow suit when he combined proportional circles and flow lines in the same map; Minard, however, then immediately advanced to a new level by designing multivariate symbols, achieving multivariate comparisons through the re-design of individual symbols, rather than through symbol overlap, the results of which can be seen online at the Bibliothèque Numérique Patrimoine des Ponts .

The research also gave me a deeper understanding of how politics influenced the acceptance or rejection of specific graphic marks. Just like today, the cool crowd defined what was aesthetically lovely statistics and what was not, and these assessments, in combination with the political, economic, and social needs of the moment, privileged certain techniques over others. What happened when Armand Joseph Frère de Montizon developed a graphic technique to depict population, today known as the dot density technique? Crickets. But when Minard embraced and advanced the flow line technique and publicized his work to engineering students and foreign dignitaries alike, he inspired an entire generation to devote themselves to the form. This next generation would in turn go on to produce masterpieces of multivariate flow line cartography in the annual Album de Statistique Graphique and the 2-volume Atlas de Statistique Graphique de la Ville de Paris. 

In England, where statisticians were loathe to leave the beloved intricacy of statistical tables, proportional symbols and isopleths finally gained traction mid-century because August Petermann moved to London and exalted about their usefulness for visualizing census data. After collaborating there with Edward Stanford and Samuel Clark, Petermann then moved on to Gotha to start his own geographical journal at Justus Perthes, Petermanns  Geographische Mitteilungen . Meanwhile across the sea, thematic technique found its power and funding with the staff of the United States Census Bureau, first with a focus on isarithmic and dasymetric techniques while under Petermann's influence, then shifting to a passion for the choropleth, a tradition that continues to this day. 

Though I come to thematic cartography from a very different demographic and perspective than these mapmakers, I feel deep kinship with those among them who perceived that thematic techniques are different from conventional, descriptive cartography, though no less grammatical in structure, comprising utterly under-developed, cross-cultural visual language that travels beyond words. Like these mapmakers, I too believe that with continuous collaborative development, such cartographic language holds the power to reveal in spatial interrelationships the otherwise invisible connections between our separated worlds. That our very lives depend on our ability to locate and express such common ground to each other.

To understand, then, how these men (and for now, only a few known women) of the 1800s went about shaping and sharing their work with one another, which techniques fell by the wayside and why, saves us from repeating past mistakes while also illuminating unexplored paths. For me, it is a must, not a perhaps, a responsibility to continue to expand that language as much as possible.

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Margaret Pearce is a cartographer based in Rockland, Maine. She is writing the entry on "Thematic Mapping Techniques" for History of Cartography, volume 5: Cartography in the Nineteenth Century, edited by Roger Kain and forthcoming from University of Chicago Press.