OpenLayers 3 Beginner’s Guide

Making rectangle export to GeoJSON with ol.interaction.DragBox

<!doctype html>
  <head>
    <title>Simple example</title>
    <link rel="stylesheet" href="../assets/ol3/css/ol.css" type="text/css" />
    <link rel="stylesheet" href="../assets/css/samples.css" type="text/css" />
  </head>
  <body>
    <div id="map" class="map"></div>
    <script src="../assets/ol3/js/ol.js"></script>
    <script>
      var osm_default = new ol.layer.Tile({
        source: new ol.source.OSM()
      });

      var dragBoxInteraction = new ol.interaction.DragBox({
        condition: ol.events.condition.shiftKeyOnly,
        style: new ol.style.Style({
          stroke: new ol.style.Stroke({
            color: 'red',
            width: 2
          })
        })
      });

      dragBoxInteraction.on('boxend', function(e) {
        var format = new ol.format.GeoJSON();
        var geom = e.target.getGeometry();
        geom.transform('EPSG:3857', 'EPSG:4326');
        var feature = new ol.Feature({
          geometry: geom
        });
        var obj = format.writeFeatures([feature]);
        console.log(JSON.stringify(obj));
      });

      var map = new ol.Map({
        layers: [osm_default],
        interactions: ol.interaction.defaults({
          shiftDragZoom: false
        }).extend([dragBoxInteraction]),
        target: 'map',
        view: new ol.View({
          center: ol.proj.transform([-1.81185, 52.443141], 'EPSG:4326', 'EPSG:3857'),
          zoom: 6
        })
      });
    </script>
  </body>
</html>