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Point Cluster:

The Point Stacker rendering transformation is a Vector-to-Vector transformation which displays a data set of points with nearby points aggregated together.

Often, this produces a more readable map in situations when there are many close points displayed at once.

The stacking is performed dynamically, so it can be used to visualize changing data.

It provides good performance even when applied to very large datasets.

The stacked view is created by configuring a layer with an SLD style which invokes the PointStacker rendering transformation.

Let's take a dense point layer such as certified vernal pools:

 http://giswebservices.massgis.state.ma.us/geoserver/wms?service=WMS&version=1.1.0&request=GetMap&layers=massgis:GISDATA.CVP_PT&styles=&BBOX=-25257,736552,402626,995132&WIDTH=786&HEIGHT=475&srs=EPSG:26986&format=image/png 

Response:

And let's group up the vernal pool into 1, 2-50 and > 50 clumps.  Use 3 sizes of dots - bigger for each bigger group of points.  Also label each dot with how many points it contains.  Style name: GISDATA.CVP_PT::Clustered

Request:

http://giswebservices.massgis.state.ma.us/geoserver/wms?service=WMS&version=1.1.0&request=GetMap&layers=massgis:GISDATA.CVP_PT&styles=GISDATA.CVP_PT::Clustered&BBOX=-25257,736552,402626,995132&WIDTH=786&HEIGHT=475&srs=EPSG:26986&format=image/png

Response:

Rendering transformations can also be chained