Geofencing

There are two common forms. On a server, a geofence is a polygon or a circle, and a point-in-polygon check answers questions such as 'is this address inside the delivery area?' at checkout; PostGIS, Turf.js and most map SDKs can do the maths. On a phone, the operating system watches for entry and exit (Android's Geofencing API, iOS region monitoring) and wakes the app when the device crosses a boundary, even in the background.

Device geofences have limits: Android allows 100 active geofences per app and iOS 20 monitored regions, alerts can arrive a few minutes late to save battery, and background location needs an extra permission that app stores review closely. Delivery zones drawn by postcode or by drive time (an isochrone) often match real delivery times better than a plain radius.