How to create a missing sidewalk map in QGIS

How to create a missing sidewalk map in QGIS
Missing sidewalks are highlighted in black here. 

If you've got a GIS map of your city's roads and a map of your city's sidewalk, you can use the QGIS software to create a map of your missing sidewalks. A missing sidewalk map can be useful to plan where new sidewalks should go or to audit transportation equity.

The steps I used are explained in this video and also below.

  1. Download and install QGIS if you haven't already.
  2. Obtain your City's road and existing sidewalk layers in a vector format if you haven't already. I used GeoJSON.
  3. Determine the UTM Zone of your location. The answer will be something like UTM Zone 16, also noting "N" for Nothern hemisphere and "S" for south.
  4. In QGIS, use "Layer: Add Layer: New Vector Layer" to load each map layer.
  5. In the properties for each layer, convert the CRS layer to a projected CRS based on your local UTM Zone, like "WSG 84 / UTM zone 16N". This is necessary to use meters or feed in the next step instead of degrees. When I made the video, I hadn't learned about Coordinate Reference Systems in QGIS and converted feet to degrees.
  6. In QQIS, use "Vector: Geoprocessing Tools: Buffer" and select the sidewalk layer. Buffer it out 50 feet converted to degrees.
  7. In QGIS, use "Vector: Geoprocessing Tools: Difference", select the road layer as the Input layer and the buffered sidewalk layer as the overlay.

The resulting layer is a map of roads of with no sidewalks. The resulting map is not particularly clean. There's a probably a better way to create a missing sidewalk map. Let me know in the comments!

I wanted to highlight the missing sidewalk layer on another map, so I followed the process above to "buffer" it out 10 more feet. If your map display software supports handling this as a display/styling issue-- that's a better way to do it.

This was originaly done for the Sidewalk Equity Audit I conducted for Bloomington, Indiana.