Writing for Smithsonian.com, Greg Millar looks at the maps of pioneering naturalist Alexander von Humboldt.

What’s often omitted, however, in discussions of Humboldt’s scientific legacy is the role that his pioneering maps and scientific illustrations played in shaping his thinking. By creating visualizations of data that had previously been bound up in tables, Humboldt revealed connections that had eluded others, says historian Susan Schulten of the University of Denver. “He’s really a visual thinker,” she says.

According to Schulten, Humboldt was one of the first scientists to use maps to generate and test scientific hypotheses. One example was his use of what he called “isotherm” lines to indicate regions of the globe with the same average temperature. These lines are ubiquitous on weather maps today, and they seem so obvious we take them for granted. But when Humboldt published a map using them in 1817, it caused scientists to rethink the widely held assumption that the average temperature of a region depends primarily on its latitude. The isotherm lines on Humboldt’s map had ups and downs that deviated from lines of latitude. This prompted him and others to look for explanations, and eventually led to an understanding of how ocean currents, mountain ranges, and other features of geography contribute to local climates.

For more on Humboldt generally, Andrea Wulf’s biography, The Invention of Nature, is a marvellous read.