Why do we need a new map?
Congressional districts in the United States have always been challenging to map for two reasons: their highly variable size (in terms of square mileage) and their highly irregular shapes. This results in maps that are essentially useless unless they're interactive and zoomable, features that are not always easy to implement or even desirable. For example, below we've mapped the results of the 2012 presidential election (as calculated by Daily Kos Elections), using a traditional map of congressional districts:
Urban areas become unidentifiable black smudges, with one district jammed in against another. Large, rural, Republican districts, however, paint the map red, leaving the impression that Mitt Romney must have defeated Barack Obama in a landslide. We can try to fix this problem with a type of map known as a cartogram, but it's far from an ideal solution. In the cartogram below, we resize each congressional district so that they all have approximately the same area:
We now can see all the urban areas that were missing in the standard map, and no longer does the election look like a Romney wipeout. But the distortion makes some state shapes ridiculous, like New York, and creatively shaped districts (such as those in North Carolina) remain a tangled spaghetti mess. We need a different kind of cartogram.
These problems prompted us to develop a map that conformed to the general shape of the United States, with recognizable state shapes and identifiable districts. We also wanted districts to appear proportional to their populations, as they do in the classic cartogram above, so we chose to make them all the same size, even though they range in population from about 528,000 (for Rhode Island's two districts) to 1.02 million (for Montana's lone at-large seat). This choice does, however, reflect the fact that every representative's vote counts equally on the floor of the House. The end result is a map based on hexagons—the one at the top of this post—with each district made up of five such polygons. Each state contains all of its constituent districts (so California, for instance, has 53, while Delaware has just one), and each district is placed about where it ought to be relative to other districts in the same state. It's still a cartogram, but one with far less distortion.
Using the new map
Here are the 2012 election results by congressional district using the new map:
The vast majority of districts are easy to find and identify, even with the limited dimensions of the image file allowed here
(though be sure to click on this map and all the others in this post for larger versions). They're not all exactly where they're supposed to be, of course—see, for example, New York City or Michigan's Upper Peninsula—but as long as the country's population remains spread so unevenly, compromises are necessary. That wide range in population density is also responsible for perhaps the most striking aspect of the map: the large blank space in the center. But it's really much more of a feature than a bug, since it accurately conveys just how sparsely populated the Plains and much of the Mountain West are, in stark contrast to their vast size.
There's another cool benefit of this map: It's easy to find the states that have been heavily gerrymandered. See any states with a few dark blue districts surrounded by pink districts? There are quite a few of them, and that's where Republicans have very shrewdly drawn the lines to benefit themselves, by packing urban Democratic voters into a handful of seats while carefully spreading Republican voters around the surrounding districts. The opposite should be true, too, in theory: Maps that heavily favor Democrats would have a few dark red districts surrounded by light blue ones—but you won't spot many of those. Just for fun, let's look at just the districts
that Romney won where Obama took less than 50 percent (a handful of which he won with a plurality):
There's an obvious pattern here: Republican support is centered in the South. There are plenty of districts that Romney won outside of the South, of course, but the vast majority of dark red seats are below the Mason-Dixon Line. What about Obama districts?
Obama seats are spread throughout most of the country, including some deep blue districts in the South where a majority of the population are minorities. We see a pattern of urban districts and minority districts, and rural areas in the Northeast and parts of the Midwest. Finally, here's a map of just those districts where Obama's share of the vote was between 45 and 55 percent. Since they were swingy on the presidential level, these districts are more likely to host competitive House races:
One of the more interesting features on this map is that large blank space stretching from the Carolinas to Texas, where neither Romney nor Obama won a single seat by fewer than 10 points.
More maps of congressional districts are in the works, and we plan to release a shapefile so that anyone can use our map with GIS software. In the meantime, we have blank template outlines that you can fill in manually (one with district numbers and one without). And feedback is, of course, very much welcome.