Finding Latitude and Longitude on Projected TopoJSON / Christopher Manning

Published

Edited

Apr 13, 2020

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2 stars

{

const height = 600;

const canvas = DOM.canvas(width, height);

var context = canvas.getContext("2d");

// don't set the projection here since us/10m.json is already projected

// alternatively, the projection `d3.geoIdentity()` can be used

var path = d3.geoPath(null, context);

context.beginPath();

path(us_states);

context.stroke();

// create a separate projection that uses the same parameters as the topojson:

// https://github.com/topojson/us-atlas#us-atlas-topojson

var projection = d3

.geoAlbersUsa()

.scale(1300)

.translate([487.5, 305]);

// get an x,y coordinate for a latitude and longitude

us_state_capitals.forEach(o => {

var capital = projection([o.longitude, o.latitude]);

context.fillStyle = "red";

context.fillRect(capital[0], capital[1], 5, 5);

});

// get the latitude and longitude of an x,y coordinate

us_states.features.forEach(o => {

var centroid = path.centroid(o);

// these two lines are unnecessary, but demonstrate how to convert from x,y to latlon and back

var latlon = projection.invert(centroid);

centroid = projection(latlon);

context.fillStyle = "blue";

context.fillRect(centroid[0], centroid[1], 5, 5);

});

return canvas;

}

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