Geofence Builder

What a geofence actually is

A geofence is a virtual boundary around a real place. Software compares a device's reported position against that boundary and reacts when the device crosses it — entering, leaving, or lingering inside. That is the whole idea. Everything else is detail about how the boundary is stored and how crossings are detected.

Two shapes cover almost every real use. A circle is a centre point plus a radius: trivial to store, trivial to test, and the right choice when you just need "near this address". A polygon is a list of corners: more work to define, but it can follow the actual outline of a school campus, a depot yard, or a park that a circle would either miss or overshoot.

Drawing a zone that won't cause false alerts

Most geofencing complaints are not bugs — they are boundaries drawn too tightly for the accuracy of the hardware reporting against them.

• Add a buffer of at least 50 metres. Consumer GPS is accurate to roughly 5–10 metres with a clear sky view, and 30–50 metres or worse between tall buildings. A boundary drawn exactly on the fence line will report exits while the device sits parked inside.
• Include the car park and the driveway. If someone arriving counts as "there", the zone has to cover where they actually stop, not just the building footprint.
• Don't trace every architectural detail. Twelve well-placed points beat two hundred. Extra vertices add processing cost and no accuracy that GPS noise doesn't erase anyway.
• Keep zones apart. Overlapping fences produce simultaneous enter and exit events and make alert histories very hard to read.

About the GeoJSON this produces

Output follows RFC 7946, so it drops straight into Leaflet, Mapbox, PostGIS, Turf.js, or any API that accepts standard GeoJSON. Two properties of the format catch people out constantly:

• Positions are [longitude, latitude] — the reverse of how humans write coordinates. Our coordinate converter shows both orders side by side.
• A polygon ring must be closed: the last position repeats the first. The builder does this for you; hand-written GeoJSON often forgets, and strict parsers reject it. The GeoJSON viewer checks for it.

How the area is calculated

Area comes from the spherical excess of the polygon, not from projecting it onto a flat plane. Flat-plane approximations are fine near the equator and increasingly wrong toward the poles — a shape in northern Norway can be overstated by a wide margin. Treating the surface as a sphere keeps the error under about 0.5% anywhere on Earth. The area calculator covers this in more depth, including how holes are subtracted.

From a drawn shape to a working alert

This tool defines the boundary. Turning it into something that notifies you needs three more pieces: a device reporting its position, a service testing each position against the zone, and a notification channel to reach you.

That is what Geoblip is. You draw the same kind of zone in the app, assign a phone or a hardware GPS tracker, optionally restrict it to certain hours — school hours, a work shift, overnight — and get an email or push notification the moment the boundary is crossed. See how the setup works, or check the pricing.