Geofence: a virtual boundary that raises an alert

Every geofence has two parts. The first is the shape: a polygon around a building, a circle around a hazard, a corridor along a route. The second is the rule that says what counts as an event. The common triggers are:

• Enter: a team member walks into a closed danger zone.
• Exit: a patrol vehicle leaves its assigned sector.
• Dwell: someone has been inside an area for longer than a set time.
• Threshold: a sensor inside the area passes a value, such as a river gauge above 200 cm.

The security lead of a three-day festival draws a geofence around the backstage area and another around the emergency exit lane by the north gate. The exit lane must stay clear, so any staff vehicle that remains inside it for more than five minutes raises an alert to the operator. A second rule warns when the last steward leaves the backstage zone during the headline set. Nobody has to keep an eye on the map for either rule, because the operator gets an alert. The event security plan guide shows where rules like these fit into the wider plan.