Geofencing is the process by which points of interest are categorized by the area which contains them. For example, when sending political solicitations, politicians will often use ZIP code geofencing to ensure that mail is sent only to residents of certain ZIP codes (that is, ZIP codes in the politician's constituency).

This example uses Turf.js and Mapbox GL JS to geofence live data of shipping vessels tracked by Fintraffic's Vessel Traffic Services and by the Finnish Maritime Administration. Geofencing is used to enumerate vessels by port and to count vessels in Finland's exclusive economic zone. Port geofences have been provided by Kestrel Insights.