Free tools for making nonprofit maps: choropleths with uDig!
In last week's article about cartograms, we discussed one type of thematic map (a map that displays a theme, such as poverty statistics or disease infection rates). This week, we'll look at some free programs to make choropleths - thematic maps that preserve distance and shape, but use colors or patterns to differentiate between different regions with different values.
Choropleths are useful when comparing data such as rates of regions that are measured across each region. For instance, one very common choropleth familiar to most Americans is the "red states vs. blue states" electoral results map: no matter whether the vote in each state is close or a landslide, the electoral votes for each state are delivered as a bloc (for the most part, anyway). As we discussed in the cartogram article, these maps can sometimes be misleading due to differences between area and various data that we wish to share. However, cartograms are also useful because the recognizable shape of the map allows most people to easily find particular areas. We'll ignore discussions about geographic illiteracy for now :)
The data for these maps is usually comprised of one value per region, such as "population density per state" where the urbanized states are colored differently than the rural states along a spectrum. Some choropleths might depict a "binary" theme, such as the red vs. blue example above. If you're working on researching your state's budget for community programs, you might want to analyze data on a county-by-county basis. The example that I used in this article uses data from neighborhood-by-neighborhood census results for Chicago.
As an open source software advocate for nonprofits, I prefer to use free/open source/libre software whenever possible, so for this project, I used OpenOffice.org Base for major data entry and manipulation of the DBF part of my source shapefiles, as well as uDig for generating the maps themselves. For source data, I used shapefiles of the 77 neighborhood areas from the City of Chicago GIS page and integrated data from the US Census 2000 summary tables. The result, after a little tweaking, is an easy-to-view (even for the color blind) map of Chicago neighborhoods, shaded according to the percentage of individuals living at or below the poverty income level. I'm thrilled that free software tools are out there allowing anyone to make these maps using their own data in just minutes!
Map of Chicago neighborhood areas shaded according to percentage of citizens living in poverty.
P.S. My apologies to the map sticklers out there for leaving off the legend - I had it suppressed in uDig while working and forgot to re-enable it when exporting my images :) I'll make sure to enable that properly for my MAPresso demo later this month!