adebas/census-tracts

Illinois census tracts changes

About

This one page mapping tool shows differences between 2000 and 2010 census tracts in Illinois.

Data Source

Tract relationships data is from Census relationship files.

Geo data is from Tiger shapefiles.

The detailed explanation of census tract changes provides helpful background reading.

Processing

process.py takes the CSV in the data folder and creates a 2000.json and 2010.json. The script takes each row of the CSV related to one tract, and create an object containing all related tracts from the alternate year. For example, it takes a 2010 tract and fills the object with all related 2000 tracts, as seen in this snippet from the 2010.json.

These files are then referenced in site.js when a user clicks on the map, to pull the appropriate 2000 tracts that appear under the 2010 tracts. See inline comments in script.js.

TopoJSON

Tiger shapefiles of Illinois tracts were converted to TopoJSON since GeoJSONs would take too long to load in the browser.

First install topojson (requires node)

npm install topojson

Then specify the input file, output file name, and any additional options. The topoJSONs used on this site are simplified to 20% quality to reduce file size. The ID of the features is also promoted to the top of the properties, so that it can later be referenced with the findwhere function in script.js.

topojson -s [input file name] -0 --simplify-proportion=0.2 --id-property=GEOID -o il_tracts_2000.json

The input shapefiles are not included in this repository since only TopoJSONs are used on the site.

See the command line reference for more options when creating TopoJSON files.

Limitations of TopoJSON

Since the TopoJSON is very simplified, it limits the tool's ability to detect fine changes. For example, tract 17111871500 is composed of parts of four 2000 tracts, two of which don't appear to fall within the 2010 border due to simplification.

The purple 2000 tract contributes 0 percent population and area because Census calculates all percentages to 2 decimal places.

Javascript

The site uses underscore.js, and jquery (included in ext folder).

Mapbox.js and Topojson libraries are referenced externally.

Inline comments in script.js explain functionality.

Contribute

Contributions to this site are welcome, please open a ticket for features, bugs or to discuss pull requests. Or fork the site to replicate the process for your own state.