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albers.js
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albers.js
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// Derived from Tom Carden's Albers implementation for Protovis.
// http://gist.github.com/476238
// http://mathworld.wolfram.com/AlbersEqual-AreaConicProjection.html
d3.geo.albers = function() {
var origin = [-98, 38],
parallels = [29.5, 45.5],
scale = 1000,
translate = [480, 250],
lng0, // d3_radians * origin[0]
n,
C,
p0;
function albers(coordinates) {
var t = n * (d3_radians * coordinates[0] - lng0),
p = Math.sqrt(C - 2 * n * Math.sin(d3_radians * coordinates[1])) / n;
return [
scale * p * Math.sin(t) + translate[0],
scale * (p * Math.cos(t) - p0) + translate[1]
];
}
albers.invert = function(coordinates) {
var x = (coordinates[0] - translate[0]) / scale,
y = (coordinates[1] - translate[1]) / scale,
p0y = p0 + y,
t = Math.atan2(x, p0y),
p = Math.sqrt(x * x + p0y * p0y);
return [
(lng0 + t / n) / d3_radians,
Math.asin((C - p * p * n * n) / (2 * n)) / d3_radians
];
};
function reload() {
var phi1 = d3_radians * parallels[0],
phi2 = d3_radians * parallels[1],
lat0 = d3_radians * origin[1],
s = Math.sin(phi1),
c = Math.cos(phi1);
lng0 = d3_radians * origin[0];
n = .5 * (s + Math.sin(phi2));
C = c * c + 2 * n * s;
p0 = Math.sqrt(C - 2 * n * Math.sin(lat0)) / n;
return albers;
}
albers.origin = function(x) {
if (!arguments.length) return origin;
origin = [+x[0], +x[1]];
return reload();
};
albers.parallels = function(x) {
if (!arguments.length) return parallels;
parallels = [+x[0], +x[1]];
return reload();
};
albers.scale = function(x) {
if (!arguments.length) return scale;
scale = +x;
return albers;
};
albers.translate = function(x) {
if (!arguments.length) return translate;
translate = [+x[0], +x[1]];
return albers;
};
return reload();
};
// A composite projection for the United States, 960x500. 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
// TODO allow the composite projection to be rescaled?
d3.geo.albersUsa = function() {
var lower48 = d3.geo.albers();
var alaska = d3.geo.albers()
.origin([-160, 60])
.parallels([55, 65]);
var hawaii = d3.geo.albers()
.origin([-160, 20])
.parallels([8, 18]);
var puertoRico = d3.geo.albers()
.origin([-60, 10])
.parallels([8, 18]);
function albersUsa(coordinates) {
var lon = coordinates[0],
lat = coordinates[1];
return (lat > 50 ? alaska
: lon < -140 ? hawaii
: lat < 21 ? puertoRico
: lower48)(coordinates);
}
albersUsa.scale = function(x) {
if (!arguments.length) return lower48.scale();
lower48.scale(x);
alaska.scale(x * .6);
hawaii.scale(x);
puertoRico.scale(x * 1.5);
return albersUsa.translate(lower48.translate());
};
albersUsa.translate = function(x) {
if (!arguments.length) return lower48.translate();
var dz = lower48.scale() / 1000,
dx = x[0],
dy = x[1];
lower48.translate(x);
alaska.translate([dx - 400 * dz, dy + 170 * dz]);
hawaii.translate([dx - 190 * dz, dy + 200 * dz]);
puertoRico.translate([dx + 580 * dz, dy + 430 * dz]);
return albersUsa;
};
return albersUsa.scale(lower48.scale());
};
var d3_radians = Math.PI / 180;