What a geofence is
A geofence is a defined region, often a circle or polygon, that triggers an enter or exit event when a tracked object crosses its boundary. Examples include alerting when a driver arrives at a pickup zone or when a phone leaves a warehouse.
The matching problem
For each position update you must answer which fences contain this point. Checking every fence per update does not scale, so fences are stored in a spatial index like an R tree or cell index. A position lookup retrieves only the candidate fences whose bounding region overlaps, then a precise point in polygon test confirms membership.
Detecting transitions
A fence cares about changes, not just current state. The system remembers the previous inside or outside flag per object and fence pair, and emits an event only when that flag flips. This avoids repeated alerts while a device sits inside.
Key idea
Geofencing narrows candidate regions with a spatial index, confirms with a precise test, and emits events only when inside or outside state flips.