src/geo/albers-usa.js

import "albers";
import "conic-equal-area";
import "geo";

// A composite projection for the United States, configured by default for
// 960×500. Also works quite well at 960×600 with scale 1285. The set of
// standard parallels for each region comes from USGS, which is published here:
// http://egsc.usgs.gov/isb/pubs/MapProjections/projections.html#albers
d3.geo.albersUsa = function() {
  var lower48 = d3.geo.albers();

  // EPSG:3338
  var alaska = d3.geo.conicEqualArea()
      .rotate([154, 0])
      .center([-2, 58.5])
      .parallels([55, 65]);

  // ESRI:102007
  var hawaii = d3.geo.conicEqualArea()
      .rotate([157, 0])
      .center([-3, 19.9])
      .parallels([8, 18]);

  var point,
      pointStream = {point: function(x, y) { point = [x, y]; }},
      lower48Point,
      alaskaPoint,
      hawaiiPoint;

  function albersUsa(coordinates) {
    var x = coordinates[0], y = coordinates[1];
    point = null;
    (lower48Point(x, y), point)
        || (alaskaPoint(x, y), point)
        || hawaiiPoint(x, y);
    return point;
  }

  albersUsa.invert = function(coordinates) {
    var k = lower48.scale(),
        t = lower48.translate(),
        x = (coordinates[0] - t[0]) / k,
        y = (coordinates[1] - t[1]) / k;
    return (y >= .120 && y < .234 && x >= -.425 && x < -.214 ? alaska
        : y >= .166 && y < .234 && x >= -.214 && x < -.115 ? hawaii
        : lower48).invert(coordinates);
  };

  // A naïve multi-projection stream.
  // The projections must have mutually exclusive clip regions on the sphere,
  // as this will avoid emitting interleaving lines and polygons.
  albersUsa.stream = function(stream) {
    var lower48Stream = lower48.stream(stream),
        alaskaStream = alaska.stream(stream),
        hawaiiStream = hawaii.stream(stream);
    return {
      point: function(x, y) {
        lower48Stream.point(x, y);
        alaskaStream.point(x, y);
        hawaiiStream.point(x, y);
      },
      sphere: function() {
        lower48Stream.sphere();
        alaskaStream.sphere();
        hawaiiStream.sphere();
      },
      lineStart: function() {
        lower48Stream.lineStart();
        alaskaStream.lineStart();
        hawaiiStream.lineStart();
      },
      lineEnd: function() {
        lower48Stream.lineEnd();
        alaskaStream.lineEnd();
        hawaiiStream.lineEnd();
      },
      polygonStart: function() {
        lower48Stream.polygonStart();
        alaskaStream.polygonStart();
        hawaiiStream.polygonStart();
      },
      polygonEnd: function() {
        lower48Stream.polygonEnd();
        alaskaStream.polygonEnd();
        hawaiiStream.polygonEnd();
      }
    };
  };

  albersUsa.precision = function(_) {
    if (!arguments.length) return lower48.precision();
    lower48.precision(_);
    alaska.precision(_);
    hawaii.precision(_);
    return albersUsa;
  };

  albersUsa.scale = function(_) {
    if (!arguments.length) return lower48.scale();
    lower48.scale(_);
    alaska.scale(_ * .35);
    hawaii.scale(_);
    return albersUsa.translate(lower48.translate());
  };

  albersUsa.translate = function(_) {
    if (!arguments.length) return lower48.translate();
    var k = lower48.scale(), x = +_[0], y = +_[1];

    lower48Point = lower48
        .translate(_)
        .clipExtent([[x - .455 * k, y - .238 * k], [x + .455 * k, y + .238 * k]])
        .stream(pointStream).point;

    alaskaPoint = alaska
        .translate([x - .307 * k, y + .201 * k])
        .clipExtent([[x - .425 * k + ε, y + .120 * k + ε], [x - .214 * k - ε, y + .234 * k - ε]])
        .stream(pointStream).point;

    hawaiiPoint = hawaii
        .translate([x - .205 * k, y + .212 * k])
        .clipExtent([[x - .214 * k + ε, y + .166 * k + ε], [x - .115 * k - ε, y + .234 * k - ε]])
        .stream(pointStream).point;

    return albersUsa;
  };

  return albersUsa.scale(1070);
};