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@mbostock
Last active Feb 9, 2016
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EPSG:2163 Coordinates
license: gpl-3.0

Use the brush to select extent coordinates interactively.

The gdalwarp utility can crop the warped output image using the -te argument, which specifies a geographic extent as xmin ymin xmax ymax coordinates in the target projection. For example, the extent -2030000 -1250000 -1300000 -40400 shown above encompasses California when using EPSG:2163, the U.S. National Atlas projection.

The trick to computing these coordinates is to know the scale of the underlying Lambert azimuthal equal-area projection: 6,370,997 meters, as shown in the human-readable projection definition. From there, it is possible to translate between pixel space and EPSG:2163 coordinates.

function te(projection) {
  var radius = 6370997,
      scale = projection.scale(),
      rotate = projection.rotate(),
      translate = projection.translate(),
      offset = projection.rotate([0, 0]).translate([0, 0])([0, 0]);

  projection.rotate(rotate).translate(translate);

  return [
    Math.round((-offset[0] - translate[0]) / scale * radius),
    Math.round((offset[1] + translate[1] - height) / scale * radius),
    Math.round((-offset[0] - translate[0] + width) / scale * radius),
    Math.round((offset[1] + translate[1]) / scale * radius)
  ];
}
<!DOCTYPE html>
<meta charset="utf-8">
<style>
body {
position: relative;
height: 500px;
}
#info {
position: absolute;
left: 10px;
bottom: 10px;
}
.graticule {
fill: none;
stroke: #777;
stroke-width: .5px;
stroke-opacity: .5;
}
.land {
fill: #333;
}
.state-boundary {
fill: none;
stroke: #fff;
}
.brush .extent {
fill: red;
fill-opacity: .1;
stroke: red;
}
</style>
<div id="info"></div>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>
var width = 960,
height = 500,
formatNumber = d3.format(".3r");
var projection = d3.geo.azimuthalEqualArea()
.rotate([100, -45])
.scale(1050)
.translate([width / 2, height / 2 - 110]);
var extent = te(projection);
var brush = d3.svg.brush()
.x(d3.scale.linear().domain([extent[0], extent[2]]).range([0, width]))
.y(d3.scale.linear().domain([extent[1], extent[3]]).range([height, 0]))
.extent([[-2030000, -1250000], [-1300000, -40400]])
.on("brush", brushed);
var path = d3.geo.path()
.projection(projection);
var graticule = d3.geo.graticule()
.extent([[-140, 20], [-60, 60]])
.step([2, 2]);
var info = d3.select("#info");
var svg = d3.select("body").append("svg")
.attr("width", width)
.attr("height", height);
svg.append("path")
.datum(graticule)
.attr("class", "graticule")
.attr("d", path);
d3.json("/mbostock/raw/4090846/us.json", function(error, us) {
if (error) throw error;
svg.insert("path", ".graticule")
.datum(topojson.feature(us, us.objects.land))
.attr("class", "land")
.attr("d", path);
svg.insert("path", ".graticule")
.datum(topojson.mesh(us, us.objects.states, function(a, b) { return a !== b; }))
.attr("class", "state-boundary")
.attr("d", path);
svg.append("g")
.attr("class", "brush")
.call(brush)
.call(brushed);
});
function brushed() {
var e = brush.extent();
info.text(formatNumber(e[0][0])
+ " " + formatNumber(e[0][1])
+ " " + formatNumber(e[1][0])
+ " " + formatNumber(e[1][1]));
}
function te(projection) {
var radius = 6370997,
scale = projection.scale(),
rotate = projection.rotate(),
translate = projection.translate(),
offset = projection.rotate([0, 0]).translate([0, 0])([0, 0]);
projection.rotate(rotate).translate(translate);
return [
Math.round((-offset[0] - translate[0]) / scale * radius),
Math.round((offset[1] + translate[1] - height) / scale * radius),
Math.round((-offset[0] - translate[0] + width) / scale * radius),
Math.round((offset[1] + translate[1]) / scale * radius)
];
}
</script>
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