How to cluster Google Maps kml overlay

Question

I have several thousand markers stored in a kmz file that I load into a Google Maps (v3 API).

I've looked around to find a way to cluster the overlay data, but to no avail.
MarkerClusterer looks nice, but it only seems to work with a marker array?

function initialize() {
    geocoder = new google.maps.Geocoder();
    var latlng = new google.maps.LatLng(25,-20);
    var myOptions = {
        zoom: 2,
        center: latlng,
        mapTypeId: google.maps.MapTypeId.ROADMAP
    };
    map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);
    var ctaLayer = new google.maps.KmlLayer('thedata.kmz');
    ctaLayer.setMap(map);

    //cluster
    var mcOptions = {gridSize: 50, maxZoom: 15};
    var mc = new MarkerClusterer(map, ctaLayer, mcOptions);
}

How can I cluster the markers in the overlay, just like markerclusterer does it?

Answer 1

The KML file is actually read by a Google server, then served to the map as image tiles. Thus, you don't have access to the actual point objects. If you want to do clustering, you would have to load the KML another way and parse the points into an array, and then you can use whatever client-side clustering you want.

From the Google Maps V3 API docs:

Because the components within a KmlLayer are rendered on demand, the layer allows you to easily manage the rendering of thousands of markers, polylines, and polygons. Note that you can't access these constituent objects directly, though they each provide click events which return data on those individual objects.

Answer 2

This is possible. You can use a Javascript KML parser library that gets you access to all the data in each marker.

In Maps V2 the best one was here:

econym.org.uk/gmap/egeoxml.htm

In V3 you can use one of these 2:

code.google.com/p/geoxml3/

code.google.com/p/geoxml/

Once used, you can do things like loop through each KML marker, and use javascript to group points in the same location, and use JS to show different info windows for each marker in the same place.

I do that on my Your Mapper site here. Click a black numbered marker to see it in action:

http://www.yourmapper.com/map/111/crime-reports/crime-in-metro-louisville-ky-since-jan-2003.htm