Displaying Census Data

This post should give you an idea of how census data can be displayed geographically using choropleth (an area that marks property on a map) outlines to communicate insight derived from analytical investigation to a broad audience.

Every ten years, the US government makes a thorough counting of its citizens. With an estimated population of over 310 million people, the only questions posed to everyone are those necessary to identify the population uniquely.

But businesses and policy makers need more data, on more subjects, updated more frequently. As a result, the U.S. Census Bureau also rigorously interviews approximately 250,000 people each month. The answers are then aggregated and reported as the American Community Survey (ACS), in 1-year, 3-year and 5-year collections.

Depending on the questions posed, one might integrate the available private data with state, county, city, zip code or census tract detail to get answers. Presenting such results geographically is often the best way to make a point.

TIGER/Line Shapefiles from the Census Bureau describe these detail areas. They are designed for use in geographic information systems, and provided in the shapefile (.shp) file format. We prefer to present visualizations in web-standard browsers using the D3 (Data-Driven Documents) library, which requires conversion to geojson and then topojson formats.

This D.C. Choropleth example shows 2012 District of Columbia census data displayed by census tract relative to other tracts. Home owner occupancy rate presents well, as it is relevant to everyone in the census tract and normally distributed. An overlay shows points of interest in and near the different tracts.

Hopefully, you take away from this example the basic use of the choropleth in the presentation of information geographically. It’s a work horse for stories that rely on demographic and/or economic data.

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