OpenLayers 3 Beginner’s Guide

Geometries in action

Open sample in a new windows

<!doctype html>
    <title>Geometry Examples</title>
    <link rel="stylesheet" href="../assets/ol3/css/ol.css" type="text/css" />
    <link rel="stylesheet" href="../assets/css/samples.css" type="text/css" />
    <div id="map" class="map"></div>
    <div id="closestFeature"></div>
    <script src="../assets/ol3/js/ol.js"></script>
      var countries = new ol.layer.Vector({
        source: new ol.source.GeoJSON({
          projection: 'EPSG:3857',
          url: '../assets/data/countries.geojson'
      var view = new ol.View({
        projection: 'EPSG:3857'
      var map = new ol.Map({
        target: 'map',
        layers: [countries],
        view: view

      // zoom to the extent of the vector source once the map has been rendered
      // for the first time
      map.once('postrender', function() {
        view.fitExtent(countries.getSource().getExtent(), map.getSize());

      // capture mouse move events so we can 'look' at the feature
      // underneath the mouse
      map.on('pointermove', onMouseMove);

      function onMouseMove(browserEvent) {
        // the mousemove event sends a browser event object that contains
        // the geographic coordinate the event happened at
        var coordinate = browserEvent.coordinate;
        // we can get the closest feature from the source
        var feature = countries.getSource().getClosestFeatureToCoordinate(coordinate);
        // to compute the area of a feature, we need to get it's geometry and do
        // something a little different depeneding on the geometry type.
        var geometry = feature.getGeometry();
        var area;
        switch (geometry.getType()) {
        case 'MultiPolygon':
          // for multi-polygons, we need to add the area of each polygon
          area = geometry.getPolygons().reduce(function(left, right) {
            return left + right.getArea();
          }, 0);
        case 'Polygon':
          // for polygons, we just get the area
          area = geometry.getArea();
          // no other geometry types have area as far as we are concerned
          area = 0;
        area = area / 1000000;
        // display the country name and area now
        var text = feature.getProperties().name + ' ' + area.toFixed(0) + ' km<sup>2</sup>';
        document.getElementById('closestFeature').innerHTML = text;