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Have you wonder how do some map app provide functions to cluster icons/markers?

Here is one solution:

http://google-maps-utility-library-v3.googlecode.com/svn/tags/markerclustererplus/2.1.2/docs/examples.html

Inside the code:

MarkerCluster obj

hide all markers first, calculate the markers distance, cluster them together in pixel distance and then show the clustericon

add markers
resetviewport
redraw
createClusters_
addToClosestCluster_
distanceBetweenPoints_
if two markers are closer than 40000, cluster them together
new Cluster

Cluster obj
ClusterIcon obj, extended from overlayview