Practical OpenLayers 3 & Leaflet

Info for GeoJSON layer

Displaying information associated with a marker requires a popup overlay and an event handler that finds the closest marker and displays the popup showing the associated features information.

Again add the following to the bottom of ol3.js:

// -- Display information on singleclick --

// Create a popup overlay which will be used to display feature info
var popup = new ol.Overlay.Popup();

// Add an event handler for the map "singleclick" event
map.on('singleclick', function(evt) {

    // Hide existing popup and reset it's offset
    popup.setOffset([0, 0]);

    // Attempt to find a feature in one of the visible vector layers
    var feature = map.forEachFeatureAtPixel(evt.pixel, function(feature, layer) {
        return feature;

    if (feature) {

        var coord = feature.getGeometry().getCoordinates();
        var props = feature.getProperties();
        var info = "<h2><a href='" + props.caseurl + "'>" + props.casereference + "</a></h2>";
            info += "<p>" + props.locationtext + "</p>";
            info += "<p>Status: " + props.status + " " + props.statusdesc + "</p>";
        // Offset the popup so it points at the middle of the marker not the tip
        popup.setOffset([0, -22]);, info);




OpenLayers 3 doesn't currently provide a popup so we are using one based on an example provided by Tim Schaub one of the OL3 core team. The popup is simply a positioned HTML element which is added to the map as an overlay which is fixed to a geographic coordinate.

Cross-domain requests

The GeoJSON features are requested from a server on a different domain to the one that our OL3 map is served from. Normally these requests would fail due to your browser respecting the same-origin policy which by default prevents a page served from one domain loading textual content such as GeoJSON from another domain.

Both the GeoJSON and WMS services used for this workshop allow cross-domain requests as they support CORS which is the modern way of permitting such requests. CORS is supported by all modern browsers. The alternative would be to use JSONP which has wide browser support but requires requests be JSONP requests which specify a callback parameter.