How a GPS audio guide knows when to start talking

An audio guide that plays by itself looks like magic the first time. It is not: it is a distance calculation that runs a few times a second. Here is what happens between the GPS chip in your phone and the narrator starting a sentence, and the choices behind it.

Where the phone thinks you are

Your phone combines satellite positioning with Wi-Fi networks and cell towers it can see, and reports a position with an accuracy figure: "you are here, give or take 12 metres". In open squares that figure is small. In the lanes of a medieval quarter, with five-storey walls on both sides, satellites are blocked and reflected, and the error grows to 20 or 30 metres. Sometimes the dot jumps across a block and back.

An audio guide has to live with that. If it demanded that you stand within 10 metres of a doorway, it would miss you half the time in exactly the places that are most worth stopping at.

The 50-metre rule

Xploreo starts a stop's story when your position comes within about 50 metres of it. That number is a compromise between two failures: a radius so small that a reflection off a wall keeps you "outside" the stop while you are standing in front of it, and a radius so large that a story starts a street early, before you can see the building it is about.

Fifty metres is roughly the length of a city block side. In practice it means the narrator begins as the place comes into view, which is when you would want a guide to start talking anyway.

What the app watches, and when

While you are walking a route, the app keeps your position updating and checks the distance to the next unvisited stop. The check runs against the route's stops in order, so passing close to stop five on the way to stop two does not set stop five off early; the app knows where you are in the sequence and only listens for the stop that is next.

Outside a route the app does nothing unless you ask. Tap a place on the map and its story plays; turn on autoplay for nearby places and the same 50-metre rule applies to whatever is around you. That is a choice: a map full of places would otherwise talk at you every time you stopped at a crossing.

Keeping the screen in your pocket

Audio continues when the screen locks, with play and pause on the lock screen like any other audio app. That is standard iOS behaviour and it matters more here than for a podcast: the whole point is to look at the building, not at the phone.

When the signal is poor

Under a railway bridge or inside a covered market the phone may stop reporting a position for a while. The app does not guess; it waits. When positions come back it recalculates the distance to the next stop. If you have walked past a stop entirely while the signal was gone, nothing plays for it, and you can tap it from the route list.

Positions with a very poor accuracy figure are ignored for the purpose of triggering a story. A reading that says "somewhere within 200 metres" is not good enough to start talking about a specific doorway.

What is not tracked

Only your phone sees your position while you walk. On your own walk nothing about where you went leaves the device: the line you walked is saved on the phone for your own history. If you join a friend's walk through a guest pass, you see where the leader is on the map, but guests' positions are not shared with anyone.

Why not just play in order?

Some audio tours play track after track and leave you to match them to buildings. The problem is pace: people stop for a coffee, take a detour for a photograph, or walk the loop backwards. Distance-based triggering means the walk sets the pace and the stories keep up. The route is a loop rather than a line for the same reason: wherever you start, you end there too, without retracing your steps.