This graph shows the topology of U.S. counties. Each node in the graph represents a county, positioned at the county centroid, while each link represents two neighboring counties sharing a border. (This graph is only approximate, as it is based on simplified geometry.)
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March 7, 2019 16:27
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County Topology
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| license: gpl-3.0 | |
| height: 600 | |
| border: no | |
| redirect: https://observablehq.com/@mbostock/county-topology |
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| <!DOCTYPE html> | |
| <svg width="960" height="600"></svg> | |
| <script src="https://d3js.org/d3.v4.min.js"></script> | |
| <script src="https://d3js.org/topojson.v2.min.js"></script> | |
| <script> | |
| var svg = d3.select("svg"), | |
| width = +svg.attr("width"), | |
| height = +svg.attr("height"); | |
| var path = d3.geoPath(); | |
| d3.json("https://d3js.org/us-10m.v1.json", function(error, us) { | |
| if (error) throw error; | |
| var features = topojson.feature(us, us.objects.counties).features, | |
| neighbors = topojson.neighbors(us.objects.counties.geometries); | |
| features.forEach(function(feature, i) { | |
| feature.centroid = path.centroid(feature); | |
| if (feature.centroid.some(isNaN)) feature.centroid = null; // off the map | |
| feature.neighbors = feature.centroid ? neighbors[i] | |
| .filter(function(j) { return j < i && features[j].centroid; }) | |
| .map(function(j) { return features[j]; }) : []; | |
| }); | |
| svg.append("path") | |
| .attr("fill", "#ddd") | |
| .attr("d", path(topojson.feature(us, us.objects.nation))); | |
| svg.append("path") | |
| .attr("fill", "none") | |
| .attr("stroke", "#fff") | |
| .attr("stroke-linejoin", "round") | |
| .attr("d", path(topojson.mesh(us, us.objects.counties, function(a, b) { return a !== b; }))); | |
| svg.append("path") | |
| .attr("fill", "none") | |
| .attr("stroke", "#000") | |
| .attr("stroke-linejoin", "round") | |
| .attr("d", d3.merge(features.map(edges)) | |
| .map(function(edge) { return "M" + edge[0] + "L" + edge[1]; }) | |
| .join("")); | |
| function edges(feature) { | |
| return feature.neighbors.map(function(neighbor) { | |
| return [feature.centroid, neighbor.centroid]; | |
| }); | |
| } | |
| }); | |
| </script> |
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