Zoomable Geography

An example of d3.behavior.zoom and d3.geo.path. By modifying the transform, the browser can rapidly redraw geographic features while panning and zooming, without the overhead of reprojection. This technique can be extended by combining with d3.geo.tile.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.overlay {
  fill: none;
  pointer-events: all;
}

.state {
  fill: #aaa;
}

.county-border,
.state-border {
  fill: none;
  stroke: #fff;
  stroke-linejoin: round;
  stroke-linecap: round;
}

</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 path = d3.geo.path()
    .projection(null);

var zoom = d3.behavior.zoom()
    .translate([0, 0])
    .scale(1)
    .scaleExtent([1, 8])
    .on("zoom", zoomed);

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

var features = svg.append("g");

svg.append("rect")
    .attr("class", "overlay")
    .attr("width", width)
    .attr("height", height)
    .call(zoom);

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

features.append("path")
      .datum(topojson.feature(us, us.objects.states))
      .attr("class", "state")
      .attr("d", path);

features.append("path")
      .datum(topojson.mesh(us, us.objects.states, function(a, b) { return a !== b; }))
      .attr("class", "state-border")
      .attr("d", path)
      .style("stroke-width", "1.5px");

features.append("path")
      .datum(topojson.mesh(us, us.objects.counties, function(a, b) { return a !== b && !(a.id / 1000 ^ b.id / 1000); }))
      .attr("class", "county-border")
      .attr("d", path)
      .style("stroke-width", ".5px");
});

function zoomed() {
  features.attr("transform", "translate(" + d3.event.translate + ")scale(" + d3.event.scale + ")");
  features.select(".state-border").style("stroke-width", 1.5 / d3.event.scale + "px");
  features.select(".county-border").style("stroke-width", .5 / d3.event.scale + "px");
}

d3.select(self.frameElement).style("height", height + "px");

</script>

https://d3js.org/d3.v3.min.js

https://d3js.org/topojson.v1.min.js