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Kentucky Population Density
license: gpl-3.0

Created via the U.S. Atlas project as make topo/ky-counties.json, after modifying the Makefile to include population and area statistics. These per-county values are then used to compute the population density of each county.

topojson \
	--id-property=FIPS,STATE_FIPS \
	-p name=COUNTY,name=STATE,+population,area=+SQUARE_MIL \
	-e KY-counties-pop.csv \
	-- shp/ky/counties.shp shp/ky/states.shp

Colors by ColorBrewer.

<!DOCTYPE html>
<meta charset="utf-8">
<style>
svg {
font: 10px sans-serif;
}
.caption {
font-weight: bold;
}
.key path {
display: none;
}
.key line {
stroke: #000;
shape-rendering: crispEdges;
}
.county {
fill: #eee;
}
.county-border {
fill: none;
stroke: #fff;
}
.state-border {
fill: none;
stroke: #333;
}
</style>
<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.threshold()
.domain([30, 60, 120, 360])
.range(["#ffffcc","#c2e699","#78c679","#31a354","#006837"]);
// A position encoding for the key only.
var x = d3.scale.linear()
.domain([0, 390])
.range([0, 240]);
var xAxis = d3.svg.axis()
.scale(x)
.orient("bottom")
.tickSize(13)
.tickValues(color.domain());
// Kentucky State Plane, Single Zone
// ftp://kygeonet.ky.gov/kygeodata/standards/Ky_StatePlane.pdf
var projection = d3.geo.conicConformal()
.rotate([85 + 45 / 60, -36 - 20 / 60])
.parallels([37 + 05 / 60, 38 + 40 / 60])
.scale(7090)
.center([-0.04940, 1.50406])
.translate([width / 2, height / 2]);
var path = d3.geo.path()
.projection(projection);
var svg = d3.select("body").append("svg")
.attr("width", width)
.attr("height", height);
var g = svg.append("g")
.attr("class", "key")
.attr("transform", "translate(40,40)");
g.selectAll("rect")
.data(color.range().map(function(d, i) {
return {
x0: i ? x(color.domain()[i - 1]) : x.range()[0],
x1: i < color.domain().length ? x(color.domain()[i]) : x.range()[1],
z: d
};
}))
.enter().append("rect")
.attr("height", 8)
.attr("x", function(d) { return d.x0; })
.attr("width", function(d) { return d.x1 - d.x0; })
.style("fill", function(d) { return d.z; });
g.call(xAxis).append("text")
.attr("class", "caption")
.attr("y", -6)
.text("Population per square mile");
d3.json("ky-counties.json", function(error, ky) {
if (error) throw error;
var counties = topojson.feature(ky, ky.objects.counties);
svg.append("g")
.attr("class", "county")
.selectAll("path")
.data(counties.features)
.enter().append("path")
.attr("d", path)
.style("fill", function(d) { return color(d.density = d.properties.population / d.properties.area); })
.append("title")
.text(function(d) { return d.properties.name + ": " + d.density.toFixed(0) + "/mi.²"; });
svg.append("path")
.datum(topojson.mesh(ky, ky.objects.counties, function(a, b) { return a !== b; }))
.attr("class", "county-border")
.attr("d", path);
svg.append("path")
.datum(topojson.mesh(ky, ky.objects.counties, function(a, b) { return a === b; }))
.attr("class", "state-border")
.attr("d", path);
});
</script>
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