Clustering can be applied to point layers that allow configuration of symbology. This includes ArcGIS Server feature layers, Spatial Data Service layers, geoRSS layers, and graphics layers. For more information on the different layer types and their capabilities, see Layer types.

About clustering

When a layer contains a large number of point features, showing each feature individually on the map is often not useful. In this scenario, point features often overlap, making it difficult to distinguish between features. Even when they do not overlap, it is usually difficult or impossible to visually extract meaningful information when hundreds or thousands of points are shown all at once.

One approach to resolving this issue is to aggregate groups of point features into one symbol. This is called clustering. Within the ArcGIS Viewer for Silverlight, clustering will aggregate the points in a layer that are within a certain distance of one another on the screen. Since the clustering is dependent on screen distance, more points are aggregated into fewer groups as you zoom out. Conversely, points are divided into more and more groups as you zoom in. When you zoom in to a level where the clustering area around one point feature no longer contains any other features, that feature will not be clustered but rather will be shown in its location with the symbology specified by the layer's renderer.

Configuring clustering

To get started configuring clustering, complete the following steps:

  1. Choose the desired point layer on the Configure Layers panel.
  2. Right-click the layer and click Configure. Alternatively, click the Configure button at the bottom of the panel:
    Configure layer
  3. On the Configuring panel that appears, expand the Clustering group and check Use Point Clustering to apply clustering to the selected layer. The configuration controls will also be enabled:
    Use point clustering
  4. Use the Maximum Points Up and Down arrows to choose a number for the maximum number of points in a cluster, or click the text box and enter the number manually. This number determines the maximum number of points that can be included in an interactive cluster. The Maximum points setting has an upper limit of 20 that cannot be overridden. Interactive clusters flare out when you move the mouse pointer over them, displaying one flare for each feature within the cluster. You can then pause the pointer on each flare to view the associated feature's pop-up display:
    Feature pop-up display

    Clusters that contain more features than the number specified here, such as the one with 65 features in the upper left corner of the image above, are not interactive.

  5. Use the Radius Up and Down arrows to choose the distance (in screen pixels) for the cluster, or click the text box and enter the number manually. Features that are within this distance of one another will be aggregated into a cluster.
  6. Click the Cluster Background Color button and choose the background (fill) color of the cluster symbol:
    Cluster background color
  7. Click the Cluster Foreground Color button and specify the text and border color of the cluster symbol:
    Cluster foreground color

For more detailed information on setting colors, see the "Changing symbol color" section in Single symbol rendering.