Drawing Region Polygons on Google Maps API v.3

August 2013 ยท 2 minute read

You can “draw” all sorts of stuff on Google Maps using Layers but mor simply you can also directly create polygons and with a bit of extra work make them looks pretty cool. In this case I’m tracing a country using data from the Natural Earth Google Fusion Tables Dataset. Here is the final product showing where New York is.

Drawing a Polygon

The Natural Earth Dataset provides outline coordinates using the EPSG 4326 projection. We can build our polygon(s) with those points like:

var regionCoords = [];
for(var i=0 ; i < data.length ; i++)
	regionCoords.push(new google.maps.LatLng(data[i][1], data[i][0]);

var overlayOptions = {
  paths\: regionCoords,
  strokeColor\: "#FF0000"
var regionOverlay = new google.maps.Polygon(overlayOptions);

In the above code we are building an array of LatLng points(notice the Natural Earth Dataset uses Long/Lat format so we need to flip the order of the values). Then we are overlaying them on our map.

Positioning and Centering Map

To display the map at a position and scale so that our polygon(s) are visiable we need to create a bounds object that contains our polygon(s).

var regionBounds = new google.maps.LatLngBounds();
for(var i=0 ; i < data.length ; i++) {
  var point = new google.maps.LatLng(data[i][1], data[i][0]);

var allPolygonBounds = new google.maps.LatLngBounds();

//combine all of your polygons together into a single bounds object that contains them all using union

In the above code we are creating a combined bounds object that we can center our map on at a scale that all polygons are visiable.

We center and scale the map like:


Creating Pretty Polygons

You can create some nice looking polygons by drawing multiple polygons on top of each other in conjustion with the google.maps.StrokePosition. In the case of our [where is British Columbia] example, I’m using two polygons with different stroke opacity and stroke position to create a faded outline and fill.