Polymaps - Statehood

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Statehood

This simple visualization uses color to encode the date each of the fifty United States joined the union; lighter states joined earlier, and darker states joined later. Choropleth maps are easy with vector tiles and CSS styling. And, unlike static images, you can zoom in for higher resolution. You can also experiment with different color scales, such as reds, greens and blues.

The map background is a monochrome image layer from CloudMade. Register a developer account with CloudMade for your own API key. We’re hosting GeoJSON tiles for states and counties on Google App Engine, and as with image tiles, you can always roll your own!

This example also uses a data visualization library called Protovis for computing quantiles and formatting dates. Protovis is optional and not required to use Polymaps.

Note that we’re not using Protovis to render anything—just data processing.

Source Code

var po = org.polymaps;

// Compute noniles.
var quantile = pv.Scale.quantile()
    .quantiles(9)
    .domain(pv.values(states))
    .range(0, 8);

// Date format.
var format = pv.Format.date("%B %e, %Y");

var map = po.map()
    .container(document.getElementById("map").appendChild(po.svg("svg")))
    .center({lat: 40, lon: -95})
    .zoomRange([3, 7])
    .zoom(4)
    .add(po.interact());

map.add(po.image()
    .url(po.url("http://{S}tile.cloudmade.com"
    + "/1a1b06b230af4efdbb989ea99e9841af" // http://cloudmade.com/register
    + "/20760/256/{Z}/{X}/{Y}.png")
    .hosts(["a.", "b.", "c.", ""])));

map.add(po.geoJson()
    .url("http://polymaps.appspot.com/state/{Z}/{X}/{Y}.json")
    .on("load", load))

map.add(po.compass()
    .pan("none"));

map.container().setAttribute("class", "Blues");

function load(e) {
  for (var i = 0; i < e.features.length; i++) {
    var feature = e.features[i], d = states[feature.data.id.substring(6)];
    if (d == undefined) {
      feature.element.setAttribute("display", "none");
    } else {
      feature.element.setAttribute("class", "q" + quantile(d) + "-" + 9);
      feature.element.appendChild(po.svg("title").appendChild(
          document.createTextNode(feature.data.properties.name + ": "
          + format(d).replace(/ [ ]+/, " ")))
          .parentNode);
    }
  }
}

Polymaps is a project from SimpleGeo and Stamen.