Correct algorithm to determinate if the user is really inside/outside a region

What is geofencing algorithm?

A mobile app or software uses the Global Positioning System (GPS), radio frequency identification (RFID), Wi-Fi or cellular data to define a virtual geographical boundary and trigger a targeted marketing action when a device enters or exits that boundary. This boundary is known as a geofence.

How do you calculate geofencing?

To do this, just calculate the distance between the center of the circle and your location (longitude, latitude). If the distance is smaller than your circle radius, then you’re inside the geofence, otherwise you’re outside the geofence. This formula is called the Haversine formula.