D3 Examples - Country Centroids on a Map

Country Centroids on a Map

This example shows one way to plot the centroids of geometies using d3.path.centroid. You can also pan and zoom.

The idea here is to provide a starting point for sized donut charts, or Polar Area Small Diagrams, for all countries, on a map. The centroids however are not quite right. The US looks off-center, skewed I suppose by Alaska.

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<!DOCTYPE html>
<meta charset="utf-8">
<style>
​
.graticule {
  fill: none; 
  stroke: #777;
  stroke-width: .5px;
  stroke-opacity: .5;
}
​
.land {
  fill: #222;
}
​
.boundary {
  fill: none;
  stroke: #fff;
  stroke-width: .5px;
}
​
</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 = 480,
    radius = 6,
    fill = "rgba(255, 49, 255, 0.388)",
    stroke = "rgba(0, 0, 0, 0.5)",
    strokeWidth = 0.1;
​
var projection = d3.geo.equirectangular()
    .scale(153)
    .translate([width / 2, height / 2])
    .precision(.1);
​
var path = d3.geo.path()
    .projection(projection);
​
var graticule = d3.geo.graticule();
​
var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);
  
var zoom = d3.behavior.zoom()
  .scaleExtent([1, 20])
  .on("zoom", zoomed);
​
var g = svg.append("g");
​
// This invisible rect catches events for the zooming interaction.
var rect = svg.append("rect")
  .attr("width", width)
  .attr("height", height)
  .style("fill", "none")
  .style("pointer-events", "all")
  .call(zoom);
​
g.append("path")
  .datum(graticule)
  .attr("class", "graticule")
  .attr("d", path);
  
function zoomed(){
  g.attr("transform", "translate(" + d3.event.translate + ")scale(" + d3.event.scale + ")");
}
​
d3.json("https://gist.githubusercontent.com/mbostock/4090846/raw/d534aba169207548a8a3d670c9c2cc719ff05c47/world-50m.json", function(error, world) {
  if (error) throw error;
  g.insert("path", ".graticule")
      .datum(topojson.feature(world, world.objects.land))
      .attr("class", "land")
      .attr("d", path);
​
  g.insert("path", ".graticule")
      .datum(topojson.mesh(world, world.objects.countries, function(a, b) { return a !== b; }))
      .attr("class", "boundary")
      .attr("d", path);
​
  var features = topojson.feature(world, world.objects.countries).features;
  var centroids = features.map(function (feature){
    return path.centroid(feature);
  });
​
  g.selectAll(".centroid").data(centroids)
    .enter().append("circle")
      .attr("class", "centroid")
      .attr("fill", fill)
      .attr("stroke", stroke)
      .attr("stroke-width", strokeWidth)
      .attr("r", radius)
      .attr("cx", function (d){ return d[0]; })
      .attr("cy", function (d){ return d[1]; });
​
});
function type(d){
  
  // This should actually be [d.longitude, d.latitude]
  // It looks like these are reversed in the data.
  d.projected = projection([d.latitude, d.longitude]);
  return d;
}
  
d3.select(self.frameElement).style("height", height + "px");
​
</script>
​
    