Most SCADA events come from a process value crossing a limit, but some of the most useful events in field operations come from a position crossing a line on a map. A geofence entry/exit event is generated when a GPS location moves across a virtual boundary drawn around a site or around a moving asset. This guide explains how the boundary is defined, where the position comes from, how dwell logic keeps the event from flickering at the edge, and how the event is routed into the alarm and monitoring system.
Geofence Event in one line: A geofence entry/exit event is a SCADA event triggered when a tracked GPS position crosses a predefined virtual boundary - entering or leaving a circle or polygon on a map. It turns location into an alarmable signal, letting a system log crew arrivals, detect an asset leaving a lease, or automatically start polling a site the moment someone reaches it.
A geofence is a virtual perimeter described in geographic coordinates. It can be a simple circle - a center point and a radius, which is quick to set around a wellsite or a yard - or a polygon traced along real boundaries such as a lease line, a road, or the fence of a facility. The geofence can be anchored to a fixed place, or it can be attached to a moving object so the perimeter travels with it; a fence drawn around a generator, for instance, moves as the generator is hauled and fires if the generator leaves the trailer it belongs to.
The position that is tested against the boundary comes from a GPS receiver on the asset being tracked - a truck, a frac spread, a portable generator, or a crew member's device. Each reported fix is a latitude and longitude with some accuracy figure attached. The system compares the fix to the geofence: inside the boundary or outside it. A transition from outside to inside is an entry, and inside to outside is an exit. The event carries the coordinates, the time, and which asset and which fence were involved.
GPS accuracy shapes how a geofence has to be drawn. A fix can wander by a number of meters even when the asset is standing still, so a boundary drawn too tightly around a small target will produce false crossings from position noise alone. Fences are usually sized with a margin, and low-accuracy fixes may be discarded, so that the crossing being reported reflects real movement rather than the jitter of the receiver.
The classic failure of a naive geofence is flapping: an asset parked right on the boundary reports positions that fall alternately inside and outside as the GPS fix drifts, and the system fires entry, exit, entry, exit in a rapid stream of meaningless events. Because these events can trigger alarms, notifications, or actions, an unfiltered geofence at the edge can generate a flood of noise from an asset that is not actually going anywhere.
The fix is dwell and debounce logic. A dwell requirement says the position must stay on the new side of the boundary for a set time or a set number of consecutive fixes before the crossing counts as real. An asset that briefly dips across the line and comes back never satisfies the dwell, so no event is generated. Hysteresis achieves a similar effect with distance: the entry line and the exit line are separated slightly, so a fix has to move a meaningful distance past the boundary before the opposite event can fire.
These controls are what make a geofence trustworthy enough to alarm on. With a sensible dwell, an entry event means the asset genuinely arrived and stayed, and an exit event means it genuinely left, not that a single noisy fix crossed the line. The trade-off is a small delay between the true crossing and the confirmed event, which is almost always acceptable for the arrival, departure, and boundary-breach use cases geofences serve.
Once a crossing is confirmed, a geofence event is routed like any other event in the platform. It can be logged to the event history for a record of arrivals and departures, raised as an alarm for a boundary that should not be crossed, or used to notify a person. In practice the three headline uses are crew-arrival logging, off-lease theft detection, and automated actions. A crew arriving on a location produces an entry event that timestamps the visit; an asset leaving where it belongs produces an exit event that flags possible theft or misrouting.
Geofences also let location drive the monitoring itself. A remote site that is not worth polling continuously can be configured so that a crew's arrival - an entry event - automatically raises the polling rate or wakes fuller data collection, and their departure returns it to a quiet baseline. This ties the monitoring effort to where the people and activity actually are, which is efficient on constrained field networks and battery-powered sites where constant polling is costly.
Because geofence events flow into the same alarm and history pipeline as process events, they inherit the platform's routing, acknowledgment, and reporting. An off-lease exit can escalate the same way a high-pressure alarm would, an arrival can appear on the same activity timeline as an operator action, and a report can summarize how long crews spent inside each fence. Treating a boundary crossing as just another event is what lets location fit naturally into a system built around process monitoring.
A site geofence is anchored to a fixed place, such as a lease, a yard, or a facility, and fires when a tracked asset enters or leaves that area. An asset geofence is attached to a moving object so the boundary travels with it, which is how you detect that a generator or a piece of equipment has left the trailer or location it is supposed to stay with. Both test a GPS position against a boundary; only the boundary's mobility differs.
GPS fixes drift by several meters even when an asset is stationary, so an asset parked on a boundary can report positions that flip inside and outside repeatedly. Without filtering, that flicker fires a stream of false entry and exit events. Dwell logic requires the position to stay on the new side for a set time or number of fixes before the crossing counts, and hysteresis separates the entry and exit lines, so only real movement generates an event.
You draw a geofence around the location where a mobile asset belongs - a tank, a generator, a spread - and configure an exit event as an alarm. If the asset's GPS position leaves that boundary and stays out long enough to satisfy the dwell, the system raises an off-lease alarm that can notify staff immediately. This catches equipment being moved or hauled away outside of normal operations.
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