How does Wi-Fi triangulation determine location

Definition

How does Wi-Fi triangulation determine location?

Wi-Fi triangulation estimates position by matching the identifiers of nearby wireless networks against a database of known network locations, which is how most desktop location works, and why it fails when the surrounding networks change.

How it works

Every Wi-Fi access point broadcasts an identifier called a BSSID, usually its hardware address, whether or not you connect to it. A device scans, collects the BSSIDs it can see and their signal strengths, and sends the list to a location service. The service has a database of where those access points have been seen before, built from phones that reported them alongside a GPS fix. It returns an estimated position.

Strictly this is closer to fingerprinting than triangulation, because it matches a pattern against known locations rather than measuring distances. The name stuck.

Why desktop location depends on it

Laptops and desktops rarely have GPS or a cellular radio. Wi-Fi is usually the only device signal they have. That is why desktop location checks generally require Wi-Fi to be switched on and the machine to have a wireless adapter, even when it is plugged into Ethernet. The check uses the adapter to scan for nearby networks, not to connect.

When it fails, and what to try

A new or replaced router. The database has not seen the new BSSID yet. Neighbouring networks usually make up for it. In a detached house with no neighbours in range, there may be nothing else to match. A phone hotspot will not help, because it moves.
Mesh systems. Several nodes can share or rotate identifiers. Results are usually fine but can be less stable.
Few networks in range. Rural areas give the service little to match. Moving closer to a window or to where neighbours’ networks reach can help.
Offices, campuses and hotels. Large networks with hundreds of access points sometimes appear in the database at the wrong building, especially after equipment moves.
Near a border. An estimate accurate to tens of metres can still fall on the wrong side of a state line. Most checks report their uncertainty and refuse rather than guess.
Wi-Fi switched off, or location services denied. On a Mac, the app doing the check needs Location Services permission in System Settings. Without it there is nothing to scan.