Now click Add and select Spatial Expression to display the Spatial expression type dialog:

You can also access this command from the Overlays dialog > Schema tab > Column drop-down:

Option	Description
Count	Counts the number of Items on another Overlay whose origin is within a Polygon on this Overlay.
Sum	Adds up the values of an expression on Items on another Overlay whose origin is within a Polygon on this Overlay.
Average	Averages the values of an expression on Items on another Overlay whose origin is within a Polygon on this Overlay.
Overlap	Calculates the area of overlap between a Polygon on this Overlay and Polygons on another Overlay.
Contained	Retrieves a property value from an Item on another Overlay that is within a Polygon on this Overlay.
Container	Retrieves a property value from a Polygon on another Overlay that contains the origin of an Item on this Overlay

Examples of spatial expressions

The Map Window below shows an SWD with Polygon Items on each of three overlays:

For examples of each spatial expression options, click here.

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