Symmetric Difference Creator Processor

Summary

The Symmetric Difference Creator Processor can be used to clip or remove the intersecting portion of an event record’s geometry and a geofence from the union of the two geometries. The symmetric difference of two geometries is essentially an exclusive OR statement.

Illustration

Examples

• Use the Symmetric Difference Creator Processor to union and clip the intersection of a live tornado and a zone of interest. As the tornado travels throughout the zone, the areas directly impacted can be clipped and removed from the zone. The remaining parts of the zone can be used to show area not directly impacted by the tornado.

Usage notes

• The Symmetric Difference Creator Processor requires two discrete geometries be specified. For example, a regular expression cannot be used to specify several different geofences and obtain a symmetric difference for the collection of geometries.
• The processor also requires geometries have the same type. For example, the symmetric difference between a point and a polyline as a union of the two cannot be computed.
• The processor requires the field containing the event record’s geometry be specified. A geometry field name can be specified or a GEOMETRY tag applied to a field within a GeoEvent Definition can be used to identify the geometry field.
• The Geometry Field parameter will display the name of every field from every GeoEvent Definition with a geometry data type. When selecting the geometry field by name, choose the name of the correct geometry field that corresponds to the event record’s GeoEvent Definition.
• The processor can be configured to replace the event record's geometry with the computed symmetric difference or place the symmetric difference geometry into a new geometry field using the Replace Geometry parameter. Adding a new field to an event record alters the event record's schema, which requires GeoEvent Server to create a new GeoEvent Definition. The new GeoEvent Definition will be managed by GeoEvent Server and deleted if changes are made to the processor or the GeoEvent Service in which the processor is used.
• Choosing Yes for the Tag the New Geometry Field as Geometry parameter explicitly tells GeoEvent Server which geometry field should be treated as the event record’s realized geometry for subsequent processing or output.

Parameters

Parameter	Description
Name	A descriptive name for the processor used for reference in GeoEvent Manager.
Processor	The name of the selected processor.
Geometry Field	The name of the geometry field or the GEOMETRY tagged field from the inbound event record the processor uses to calculate the symmetric difference with an intersecting geofence.
Replace Geometry	Specifies whether to replace the current event record geometry with the symmetric difference of the event record and geofence geometries. The default is Yes. Yes - The event record geometry will be replaced with the geometry of the symmetric difference. No - The event record geometry will remain the same. The geometry of the symmetric difference will be stored in a new geometry field as part of the event record. Altering an event record’s schema by adding a new field name requires a new GeoEvent Definition.
New Geometry Field Name (Conditional)	The name of the new geometry field where the geometry of the calculated symmetric difference will be stored. Property is shown when Replace Geometry is set to No and is hidden when set to Yes.
Tag the New Geometry Field as Geometry (Conditional)	Specifies whether the new geometry field created by the processor will be tagged as GEOMETRY. The GEOMETRY tag is used by GeoEvent Server to identify the event record’s realized geometry when multiple geometry fields are present. The default is No. Property is shown when Replace Geometry is set to No and is hidden when set to Yes.
New GeoEvent Definition Name (Conditional)	The name assigned to the new GeoEvent Definition. The new GeoEvent Definition will combine the schema of the inbound event record with the additional new geometry field. Property is shown when Replace Geometry is set to No and is hidden when set to Yes.
Geofence	The category and name of the geofence. The processor uses the geofence to calculate the symmetric difference with the geometry of the intersecting event record. The resulting geometry is the union of the event record and geofence geometries with the intersecting portion clipped (or removed). The required format for specifying a geofence is category/name for example: Hurricanes/Doe AVL/Vehicle1

Considerations and limitations

• Consider replacing the existing event record geometry with the geometry representing the symmetric difference prior to sending the event data to a feature service. Feature services only support one geometry type field and therefore whichever field has not been tagged as GEOMETRY will be converted and kept as a string.