This is in answer to this Stack Overflow question about how to place points on a map and how projections work.
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September 9, 2018 12:23
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Plotting points on a map in D3
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| <!DOCTYPE html> | |
| <html> | |
| <head> | |
| <meta charset="utf-8"> | |
| <title>Point on a map D3</title> | |
| <script src="http://d3js.org/d3.v3.min.js" charset="utf-8"></script> | |
| <script src="http://d3js.org/topojson.v1.min.js"></script> | |
| <style type="text/css"> | |
| .feature { | |
| fill: none; | |
| stroke: grey; | |
| stroke-width: 1px; | |
| stroke-linejoin: round; | |
| } | |
| .mesh { | |
| fill: none; | |
| stroke: lightgrey; | |
| stroke-width: 2px; | |
| stroke-linejoin: round; | |
| } | |
| h1 { | |
| font-family: sans-serif; | |
| } | |
| </style> | |
| </head> | |
| <body> | |
| <h1>Point in the north west part of SF</h1> | |
| <script type="text/javascript"> | |
| var width = 950, | |
| height = 550; | |
| // set projection | |
| var projection = d3.geo.mercator(); | |
| // create path variable | |
| var path = d3.geo.path() | |
| .projection(projection); | |
| d3.json("us.json", function(error, topo) { console.log(topo); | |
| states = topojson.feature(topo, topo.objects.states).features | |
| // set projection parameters | |
| projection | |
| .scale(1000) | |
| .center([-106, 37.5]) | |
| // create svg variable | |
| var svg = d3.select("body").append("svg") | |
| .attr("width", width) | |
| .attr("height", height); | |
| // points | |
| aa = [-122.490402, 37.786453]; | |
| bb = [-122.389809, 37.72728]; | |
| console.log(projection(aa),projection(bb)); | |
| // add states from topojson | |
| svg.selectAll("path") | |
| .data(states).enter() | |
| .append("path") | |
| .attr("class", "feature") | |
| .style("fill", "steelblue") | |
| .attr("d", path); | |
| // put boarder around states | |
| svg.append("path") | |
| .datum(topojson.mesh(topo, topo.objects.states, function(a, b) { return a !== b; })) | |
| .attr("class", "mesh") | |
| .attr("d", path); | |
| // add circles to svg | |
| svg.selectAll("circle") | |
| .data([aa,bb]).enter() | |
| .append("circle") | |
| .attr("cx", function (d) { console.log(projection(d)); return projection(d)[0]; }) | |
| .attr("cy", function (d) { return projection(d)[1]; }) | |
| .attr("r", "8px") | |
| .attr("fill", "red") | |
| }); | |
| </script> | |
| </body> | |
| </html> |
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Hi there. I'm just getting started with D3, and I came across this example on bl.ocks.org. It's really helped me quickly understanding plotting. Thanks!