Seventy-five years ago, the United States dropped an atomic weapon on Hiroshima at the end of World War II. Since then, survivors have been studied for long-term effects of radiation from the bomb.
To visualize the results of this research for a feature in the 24 July issue of Science, we envisioned an infographic of Hiroshima displaying both the radiation levels experienced by survivors and the resulting health effects, appearing largely as cancers.
This seemed like a straightforward project. Hiroshima has been closely studied for decades by the Radiation Effects Research Foundation (RERF). Jointly operated by the United States and Japan, the organization monitors affected populations in both Hiroshima and Nagasaki, which was bombed a few days later.
As we began conceptual development, RERF provided us with statistical data. We obtained a map showing the location of all the Hiroshima survivors in the study, each location color-coded according to the amount of radiation they received during the blast.
With this information in hand, we felt it would be easy to find a map of Hiroshima, allowing us to plot the epicenter of the explosion and show damaged areas of the city. After all, this was one of the most widely known and studied historical events in the modern era. Surely, we’d have plenty of cartographic data available to us.
So, we had a clear and straightforward concept. We’d create a graphic with:
- A map of Hiroshima as it was in 1945, showing bomb damage along with locations and radiation dosages of the survivors.
- A series of charts using data collected by RERF highlighting cancer risk among the survivors depending on radiation received during the explosion, and noting distribution of age, sex, and other information.
But as we began to build the graphic, things quickly became less straightforward.
First, we found that RERF carefully protects survivors’ personal data. They declined to share the geotagged information that we needed to reproduce their visualizations of survivors in our own graphic.
Second, we could not find files showing the complete city layout as it was in 1945. Although many military aerial photographs showed the city before and after the explosion, none of them covered a large enough area of Hiroshima for us to plot our planned diagram of radiation effects.
Finally, we could not find elevation data with enough resolution to generate a relief layer in the map. That would have been informative, allowing us to show how the mountains surrounding the city affected the destruction.
We found a simple solution for our first problem, showing radiation exposure with a series of concentric circles, rather than using points to show individuals. Each circle marked the outer limit of an exposure level. We used simple lines for each circle, after finding that zones of solid colors were too intrusive and obscured underlying information.
The second problem was not so easy to solve. Without a historic map of the city that our software could use, we had only two options that would work within our deadline. We could use a modern satellite image of the city, or we could manually trace a historic map into our system.
The first option seemed inappropriate. Modern Hiroshima’s urban area is significantly larger than in 1945. And it felt out of context to build a visualization about effects of the A-bomb over the modern city.
So we chose the second option, searching multiple historical maps. After reviewing all the candidates, we selected one created by the U.S. Army Map Service in 1946. This had everything we needed: a detailed layout of the city streets and a detailed account of the areas totally and partially destroyed by the bomb.
The only problem was that we needed to trace the map’s detailed information into our software by hand. This was a tedious yet strangely satisfying task, recalling the 1990s, when print maps were routinely scanned and traced into computers in newsrooms across the country. It also called to mind the exacting work of earlier generations of cartographers who meticulously created maps like this on paper in the era before modern geographic information systems were developed.
For the final version of the map, we chose a minimalist design, using a limited and desaturated color palette that would play well with the other layer of data we needed to place over it: the radiation chart.
We applied the same style to the rest of the graphic, limiting the palette to three basic colors of red, gray, and black. We used a more diverse color gradient for the radiation chart, where we needed to display six different values.
This understated approach, seen here in the final version, seemed appropriate for presenting effects of Hiroshima’s bombing, three-quarters of a century after the event.
Alberto Cuadra is the Graphics Managing Editor at Science.