It can be hard to discern the colors for many small-population counties, especially if you’re viewing this on a small screen. This approach has some disadvantages compared to the choropleth map. This map takes proportional symbols a step further, varying both their sizes and colors to produce bivariate proportional symbols. Where are the children? A bivariate proportional symbol map Click map for larger version But they have some big disadvantages, and there are some good alternatives, such as proportional symbol maps, which place a point symbol in each area and vary the symbol sizes. Where are the children? A choropleth map Click map for larger versionĬhoropleth maps are intuitive, familiar, and about as easy to produce as any other map type: all good reasons for their popularity. There are several standard ways to map summary data for geographic areas, including proportional symbol maps, but in the digital era, the prevalent approach has been choropleth mapping: varying the fill symbols of areas to correspond with statistical characteristics. I share lots of examples down below to demonstrate, but first, let’s consider the general advantages of proportional symbol maps compared to a more common alternative: choropleth maps… Three cheers for proportional symbols OK, how should we use them? In the case of statistical maps-my focus here-centers of population are wonderfully effective for placing proportional symbols. I’m hoping this blog will help get the word out, and if it does, you might now be reading this in some future age, marveling how we ever went so long without using them! What are they used for? At the moment, not much! But there are dozens of settings where they’d be helpful. To facilitate mapping and analysis, IPUMS NHGIS has transformed the coordinates into point shapefiles, available for download through the NHGIS Data Finder. Where can you get them? Through the Census Bureau website, you can download files containing the latitude and longitude coordinates for centers of population.
states, counties, census tracts, and block groups. For the last three decennial censuses (2000, 2010, 2020), the Census Bureau has published centers of population separately for U.S. What are they? A center of population represents the mean location of residence for an area’s population, roughly the average latitude and longitude, adjusting for the curvature of the earth.
Before I explain why, let’s cover some basics… population mapping, the Census Bureau’s centers of population may be the nation’s most underused data resource.
Jonathan Schroeder, IPUMS Research Scientist, NHGIS Project Manager The best mapping resource no one’s using?