nav10003 / redistricting

otl-metromerger

Metropolitan merger simulation with Leaflet choropleth map framework for On The Line project (http://OnTheLine.trincoll.edu)

Demo

http://jackdougherty.github.io/otl-metromerger

To Do

• add reset button to clear selected polygons (remove and readd geoJson layer? or just refresh browser?)
• can click event listener be modified to toggle? Turn off selected polygons that have already been turned on?
• find way to present infowindow data as a table or grid
• add link to data source
• start a second version: a School District merger tool, possibly with CT only data layer, or EdBuild national data layer?

Steps to construct this

1. Begin with Leaflet choropleth tutorial at http://leafletjs.com/examples/choropleth.html

2. Create simplified geoJson layer of Connecticut town boundaries

• Download town boundaries shapefile from UConn MAGIC http://magic.libraries.uconn.edu
• Simplify the polygon shapes to reduce size in geoJson format with this general approach http://chimera.labs.oreilly.com/books/1230000000345/ch12.html#_simplify_the_shapes
• Use MapShaper web tool (http://www.mapshaper.org/) to reduce size without changing too much detail. In this case, I reduced Connecticut boundaries down to 1.5%, around 100k.
• Export "shapefile - polygons", then copy and rename new .shp and .shx files into new folder with original .dbf (to retain data) and original .prj (projection) files

3. Join spreadsheet town data to the new simplified polygon shapefile

• in GIS application (such as ArcGIS or QGIS), load the new simplified polygon shapefile and spreadsheet data
• join data to shapefile, then export > save as new shapefile in new layer, and open attributes to delete unneeded columns
• create a new folder with four files (.shp, .shx, .dbf, .prj) and zip folder for next step

4. Convert new joined shapefile (simplified boundaries + data) into js format to use in Leaflet map

• Use ogre to ogre web client (http://ogre.adc4gis.com/) to convert zipped shapefile to geojson
• Copy and paste the geojson output into a new file, and modify into json this way:
• At top of new file, declare contents as a variable by adding this text:

var data =

• At the bottom of the new file, add a semicolon (;)
• Rename file suffix as .js, place inside local directory, and upload in the Leaflet HTML index template

5. See code comments in the Leaflet index.html template

Credits

• Thanks to the creator of the Leaflet Interactive Choropleth Map tutorial (http://leafletjs.com/examples/choropleth.html)
• Thanks to @alvinschang at CT Mirror, since this map borrows functions to display info window data, based on examples such as http://projects.ctmirror.org/content/2014/05/raceSchools/
• Thanks to @erose for helping me with the JavaScript logic