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What is a fog of war map app?

The idea comes from strategy games: the world starts hidden and is revealed by going there. Applied to real streets, the interesting part turns out to be what happens after you leave.

In a strategy game, the map begins black. You can see the ground your units are standing on and nothing else, and the only way to learn what is over the ridge is to walk over the ridge. That is fog of war.

A fog of war map app does the same thing to the actual planet. Your personal map of Earth starts blacked out. You carry a phone, you go somewhere, and the fog pulls back over exactly the ground you covered — a corridor the width of a street, following the route you actually took. Everything you have never been to stays dark.

The idea is old enough to have been done well several times. What separates the apps that use it is not the visual. It is what happens to the fog after you leave.

The permanence is the whole feature

There are two ways to build this, and they produce completely different products.

The first is a decaying map: recent trips are bright, older ones dim, and after a few months a route you ran every day fades back into the dark. This is what a heat map is, and it is genuinely useful — it shows you where you go now, which is a real question with a real answer.

The second is a permanent map: a square you have been through is open, and it never closes again. There is no fading, no half-brightness, no decay. What you get is not a picture of your habits but a record of your life's coverage — a slowly growing shape you can look at in ten years.

The second is much less useful for training and much more interesting to look at. It answers a question a training log cannot: where have I actually been?

Why the resolution matters more than it sounds

Every fog of war map divides the world into some grid and marks squares as open or closed. How big those squares are decides what the map means.

Make them a kilometre across and passing through on a motorway opens a whole district you have never seen. Make them ten metres across and GPS error alone will open squares you never entered, and the map becomes a picture of your phone's accuracy rather than your movement.

Jerv uses squares of roughly fifty metres a side at European latitudes, a little larger nearer the equator. That is about the width of a street plus its buildings, which means walking a road opens the road and not the block behind it. Cross a bridge and you get the bridge. Cut through a park and you get the path, not the park.

What the empty squares are for

Once the map is permanent, something else becomes possible: the squares nobody has ever been through.

If the app knows which squares every user has opened, then the first person through a given square is the first person through it, full stop — and that can be recorded. Jerv calls these first footers, and they are the part people find unexpectedly compulsive. A city you have lived in for a decade turns out to be full of alleys, cut-throughs and dead ends that nobody has walked with the app running.

That is a different motivation from a training app's. It is not "go further" or "go faster". It is "go somewhere else", which is a much better reason to take an unfamiliar left turn.

What it is not

A fog of war map is not a fitness tracker, and an app built around one will be worse at pace, splits, intervals and heart rate than an app built around those. It is also not a navigation tool — it tells you where you have been, not how to get anywhere.

What it replaces, if it replaces anything, is the vague sense of a city you build up over years without ever being able to see it. The map makes that legible, and then it makes the gaps annoying, and then you go and fill them in.

Where to start

Pick somewhere you already walk. The first week of a fog map is the most satisfying week it will ever have, because everything is new and every trip visibly changes the picture. After that the frontier moves outward and the game changes from cover ground to find the gaps — which is when the empty squares in the middle of your own neighbourhood start to bother you.

Jerv is free, and the iPhone app is in open beta: get it through TestFlight. The Android beta opens soon, and the web app works in any browser today.