Population Choropleth by mbostock

Note: the counties aren’t clipped to water boundaries, so they look a bit weird and the corresponding density calculation is inaccurate. So don’t copy this example without fixing those problems first, ok? See the us-atlas project for a more comprehensive Makefile of examples, and the projected choropleth example for a better style. Also, you’ll want to add a legend for the color scale.

To build this example, git clone this repo, npm install, and then make.

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960,
    height = 500;

var color = d3.scale.log()
    .range(["hsl(62,100%,90%)", "hsl(228,30%,20%)"])
    .interpolate(d3.interpolateHcl);

var path = d3.geo.path()
    .projection(null);

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

d3.json("us-albers.json", function(error, us) {
  if (error) throw error;

  var counties = topojson.feature(us, us.objects.counties).features;

  var densities = counties
      .map(function(d) { return d.properties.density = d.properties.pop / path.area(d); })
      .sort(function(a, b) { return a - b; });

  color.domain([d3.quantile(densities, .01), d3.quantile(densities, .99)]);

  svg.append("g")
      .attr("class", "counties")
    .selectAll("path")
      .data(counties)
    .enter().append("path")
      .style("fill", function(d) { return color(d.properties.density); })
      .attr("d", path);
});

</script>

Makefile

all: \
	us-albers.json

clean:
	rm -rf -- County_2010Census_DP1.*

County_2010Census_DP1.zip:
	curl -o $@ --raw 'http://www2.census.gov/geo/tiger/TIGER2010DP1/County_2010Census_DP1.zip'

County_2010Census_DP1.shp: County_2010Census_DP1.zip
	rm -rf $(basename $@)
	mkdir -p $(basename $@)
	unzip -d $(basename $@) $<
	for file in $(basename $@)/*; do chmod 644 $$file; mv $$file $(basename $@).$${file##*.}; done
	rmdir $(basename $@)
	touch $@

# projected (giving node extra memory since input shapefile is big)
us-albers.json: County_2010Census_DP1.shp
	node --max_old_space_size=8192 node_modules/.bin/topojson \
		-q 1e5 \
		-s 1 \
		--projection 'd3.geo.albersUsa()' \
		--id-property=GEOID10 \
		-p name=NAMELSAD10,pop=+DP0010001 \
		-o $@ \
		-- counties=County_2010Census_DP1.shp

# non-projected (not used by this example, but included for reference)
us.json: County_2010Census_DP1.shp
	node_modules/.bin/topojson \
		-q 1e5 \
		-s 7e-7 \
		--id-property=GEOID10 \
		-p name=NAMELSAD10,pop=+DP0010001 \
		-o $@ \
		-- counties=County_2010Census_DP1.shp

package.json

{
  "name": "anonymous",
  "version": "0.0.1",
  "private": true,
  "devDependencies": {
    "topojson": "1"
  }
}