Food security is an extremely important issue for the livability of cities. To educate citizens about it, accuracy in mapping is vital as well.

Upon first glance two weeks ago, I and a number of like-minded folks in the region sent along links and tweets to our contacts about “When Healthy Food is Out of Reach,” a joint report from D.C. Hunger Solutions and Social Compact.

Sadly, it comes as no surprise that food deserts or or “grocery gaps,” defined as an “area in the United States with limited access to affordable and nutritious food, particularly such an area composed of predominantly lower income neighborhoods and communities,” exist in DC. Similarly, it comes as no surprise that, based on that definition, Wards 5, 6, 7, and 8 are the hardest hit.

However the maps used to convey the District’s food deserts are misleading, particularly Maps 3 and 4 on pages 15 and 18 of the report. Below is an annotated version of Map 4, “Food Deserts in the District of Columbia.”

Click to enlarge.

This illustrates what Mark Monmonier calls a “Blunder that Misleads” in his book How to Lie With Maps. According to the report, map 4 combines “census tracts where 51 percent or more of the population lives at incomes below 200 percent of the federal poverty level” and census tracts with “below-average access to full-service grocery stores.”

The blunder — not a lie but rather a “cartographic fallibility” — comes from using census block group data. Entire block groups get designated as food deserts that either have no living residents (such as cemeteries and parks) and so do not have much income and require no access to food, or house institutions (like hospitals) and so do not have much income but likely provide food on site. Nearly 1/4 of the areas designated as food deserts may technically, but not realistically, fit the working definition.

Particularly during this time of increasing unemployment, homelessness, and hunger, advocacy groups across the District and the country are working overtime. Like many others, I want to help in the fight for social justice, but we need to be sure our data are airtight in order to effectively convey our messages.

Cross-posted at The District Curmudgeon.