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© 2010 CloudMade, OpenStreetMap contributors, CCBYSA.

Affine Transform

Image tiles don’t have to be in spherical Mercator coordinates. Polymaps can translate, rotate, scale and shear a tileset using an affine transformation, so that any zoomable image can be overlaid onto a map. In this example, we transform an old map of Brooklyn courtesy of the New York Public Library. Use the '1' and '2' keys to toggle the opacity of the map overlay.

The background map is an image layer from CloudMade. Register a developer account with CloudMade for your own API key.

The affine transform here is hard-coded based on the tile coordinates of three locations in image space and Earth space (with spherical mercator projection), respectively.

There’s an additional helper library used by this example to load the image metadata from the NYPL Map Warper. The image coordinates are defined such that ⟨0,0⟩ is the top-left corner, and ⟨1,1⟩ is the bottom-right corner of the smallest square that contains the image.

Source Code

var po = org.polymaps;

var transform = derive(
  // broadway & kent
  {column: 2.7943515, row: 7.5530137, zoom: 4},
  {column: 0.2945320, row: 0.3759881, zoom: 0},
  // havemeyer & north 6th
  {column: 12.818008, row: 5.0867895, zoom: 4},
  {column: 0.2945710, row: 0.3759732, zoom: 0},
  // river & north 1st
  {column: 5.3814725, row: 1.9752101, zoom: 4},
  {column: 0.2945396, row: 0.3759645, zoom: 0}

var map = po.map()
    .center({lon: -73.96115978350677, lat: 40.712867431331716})
    .zoomRange([15, 18])

    + "/1a1b06b230af4efdbb989ea99e9841af" // http://cloudmade.com/register
    + "/998/256/{Z}/{X}/{Y}.png")
    .hosts(["a.", "b.", "c.", ""])));



function key(e) {
  var nypl = document.getElementById("nypl");
  switch (e.keyCode) {
    case 49: nypl.style.opacity = e.type == "keydown" ? 1 : .5; break;
    case 50: nypl.style.opacity = e.type == "keydown" ? 0 : .5; break;

window.addEventListener("keydown", key, false);
window.addEventListener("keyup", key, false);
Polymaps is a project from SimpleGeo and Stamen.