Geofence: a zone, 3 triggers, and the hysteresis that stops it lying

A geofence is a zone with a trigger rule and a hysteresis policy attached. The zone says where. The trigger says which crossing you care about — entry, exit, or having stayed. The hysteresis policy says how much crossing counts, and it is the part that gets skipped. A zone and a trigger with no hysteresis fires 9 alerts for 1 arrival.

The reason sits in the physics of the fix, not the code. A parked vehicle does not report a constant position: consecutive fixes scatter by tens of metres, more in a yard surrounded by steel. Park on a boundary and the position crosses it repeatedly while the engine is off.

Radius or polygon, and when each one is wrong

• A radius is a point and a distance: cheap to draw, defensible for small round sites — a kerbside drop, a weighbridge, a fuel stop.
• A radius fails on long sites. A 400-metre distribution centre needs 200 metres from its centre, capturing the access road and the neighbouring unit.
• A polygon is the site's real footprint: right for depots, ports and shared estates, and it needs redrawing when the yard is re-laid.
• A stale polygon beats no polygon only until the site changes; then it is confidently wrong.

The 3 triggers, and the question each answers

• Entry. The vehicle has arrived and the detention clock starts. It is wired straight into what a carrier status message actually says, so a noisy fence becomes a customer-visible lie.
• Exit. The vehicle has left, the dwell clock closes and the next leg starts. It fires late more often than entry: a vehicle can sit 20 metres outside a fence for an hour.
• Dwell. Still inside after a stated period — 40 unplanned minutes at a service area, a trailer parked overnight. The only trigger needing a duration rather than a crossing.

A dwell rule meaning 'stopped working' rather than 'still inside' needs vehicle state as well as position, and that state has provenance problems of its own — see what an ignition event actually proves.

Hysteresis: the third part, and the one that gets skipped

The accuracy gate surprises people. A fix accurate to 300 metres says nothing about a 150-metre zone either way, so admitting it is worse than no fix at all. It needs the feed to carry the field, which is why it belongs in the 8 fields a position report must carry.

Sizing a zone by what kind of site it is

That last row is the general rule in disguise: zone size follows fix quality, not the site plan. A 75-metre zone needs positions good to 15 metres — reasonable from a hardwired unit with a roof-line antenna, optimistic from a handset in a pocket, the trade-off in installed unit or the driver's phone.

Where a correctly configured fence still fails

1. The zone is smaller than the gap between pings. At 90 km/h a vehicle covers 750 metres in 30 seconds, so a 100-metre zone on that cadence is never observed.
2. The fence covers a road it should not, so any vehicle driving past produces an arrival.
3. Fences overlap. On a shared estate a vehicle sits inside 3 zones at once, and the trigger cannot say which site it arrived at.
4. The dwell rule crosses midnight in the wrong time zone, so a 4-hour parking alert fires on the wrong side of the shift change.

A zone with no hysteresis does not detect arrivals. It detects the moment a fix fell on the far side of a line — a different event that looks identical in a database.

When events go wrong, check the breadcrumb trail the vehicle actually drove first: it shows whether the vehicle crossed or the position wandered. The zone editor, state machine and event log that keep this maintainable are the small, specific product work we describe under MVP and product builds. Related definitions sit in telematics and the live vehicle data plane, inside our logistics and mobility work.

• geofence
• telematics
• arrival detection
• fleet operations