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Neighboring Congressional Districts
license: gpl-3.0

This example uses topojson.neighbors to compute neighboring congressional districts (from the 113th congress) from geographic boundaries by detecting shared edges (arcs). The red district is the hovered district, and the orange district is the neighboring districts.

<!DOCTYPE html>
<meta charset="utf-8">
path {
stroke-linejoin: round;
stroke-linecap: round;
.land {
fill: #bbb;
.land :hover {
fill: red;
.land .neighbor {
fill: orange;
.border {
pointer-events: none;
fill: none;
stroke: #fff;
.border--district {
stroke-width: .5px;
.border--state {
stroke-width: 1.5px;
<script src="//"></script>
<script src="//"></script>
<script src="//"></script>
var width = 960,
height = 600;
var projection = d3.geo.albersUsa()
.translate([width / 2, height / 2]);
var path = d3.geo.path()
var svg ="body").append("svg")
.attr("width", width)
.attr("height", height);
.defer(d3.json, "/mbostock/raw/4090846/us.json")
.defer(d3.json, "/mbostock/raw/4090846/us-congress-113.json")
function ready(error, us, congress) {
if (error) throw error;
var districts = congress.objects.districts,
neighbors = topojson.neighbors(districts.geometries);
.attr("id", "land")
.attr("d", path);
.attr("id", "clip-land")
.attr("xlink:href", "#land");
var district = svg.append("g")
.attr("class", "land")
.attr("clip-path", "url(#clip-land)")
.data(topojson.feature(congress, districts).features)
.attr("d", path);
.text(function(d) { return; });
.each(function(d, i) { d.neighbors = d3.selectAll(neighbors[i].map(function(j) { return district[0][j]; })); })
.on("mouseover", function(d) { d.neighbors.classed("neighbor", true); })
.on("mouseout", function(d) { d.neighbors.classed("neighbor", false); });
.attr("class", "border border--district")
.datum(topojson.mesh(congress, congress.objects.districts, function(a, b) { return a !== b && ( / 1000 | 0) === ( / 1000 | 0); }))
.attr("d", path);
.attr("class", "border border--state")
.datum(topojson.mesh(us, us.objects.states, function(a, b) { return a !== b; }))
.attr("d", path);
}"height", height + "px");
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