An intrusion zone — in standards terms a field detector — is a behavioural rule that fires when an object enters (or stays within, or exits) a defined area drawn on the scene, rather than crossing a single line. The operator paints a polygon over the area that matters — a restricted yard, a platform edge, an equipment compound — and the system alerts when a person or vehicle is present inside it. Where a line answers "did they cross?", a zone answers "are they where they shouldn't be?".
Zones are standardised in the ONVIF Analytics specification as a normative Field Detector, so the rule and its events are portable across conformant cameras and VMS. They are more expressive than a tripwire: they can require a minimum dwell time before alerting (present for N seconds, not just passing through), can be limited to an object class, and can combine with schedules so the same yard is "allowed" by day and "intrusion" at night. This makes them the backbone of perimeter and area protection.
The pitfalls are shape, dwell, and overlap. A zone drawn too large catches incidental passers-by and floods operators with false alarms; drawn too tight, it misses an intruder skirting the edge. Without a dwell requirement, someone briefly clipping the corner trips it; with too long a dwell, a fast intruder slips through. Size and shape the zone to the real risk, set dwell and class filters deliberately, and accept that tuning trades false alarms against missed ones — it is never a perfect fence.
