Field notes 5 MIN READ
What Strava's privacy zones do not cover
They hide the map. They do not hide the file, the distance, or the pattern — and one of those is enough to find an address.
Strava's privacy zones do something genuinely useful and something people believe they do which they do not.
What they do
Draw a circle around an address. Activities that start, end or pass through it have that portion hidden from other people. The rest of the activity stays visible.
For casual browsing by strangers, this works.
What they do not do
They do not change your export. If you download the GPX of an activity, you get your data, complete, including the section inside the zone. That is correct behaviour — it is your recording — but it means the file you forward to anyone is unprotected.
They do not hide the shape of the gap. The activity still shows a distance and a duration. If the visible track resumes 600 m from where it started, the hidden portion is 600 m of road, and only so many roads within 600 m of that point are candidates.
That is the substance of the KU Leuven work in 2023: combine the reported distance inside the zone with the street grid, and a set of candidate addresses falls out. They reported success rates that make the feature a speed bump rather than a wall.
They do not cover the pattern. One activity is a route. Two hundred is a routine — the same start time, the same first turn, the same three roads. A zone hides the last hundred metres of each. The convergence is still visible.
What to do anyway
Use them, and make them big. A generous zone loses you nothing but a little of the map.
Do not centre a zone precisely on the address. The centre is the most probable point in it, and an attacker who knows you drew it knows that too.
And treat any file you export as unprotected — because it is. If it is going to somebody else, it should be trimmed before it leaves.
What we do with it
We trim every route by default and cannot be told not to. Routes read from a connection are trimmed on our side before they are sent to the browser, so the untrimmed coordinates never travel — not even to their owner, because a response sits in a browser cache and a network log.
Questions people actually ask
- Do privacy zones affect an exported GPX file?
- Your own export contains your own untrimmed data — the zone is about what others see, not about what you download. Anything you then email carries the real start point.
- How big should a zone be?
- Larger than feels necessary, and not centred exactly on the address. A zone whose centre is your house makes the house the most likely point in it.
- Does hiding the whole activity map fix it?
- For that activity, yes. It also removes the map from every activity that passes through, which is most of them if you live in the zone.
- Should I still use them?
- Yes. They are a real improvement over nothing. They are just not a guarantee, and they do nothing about files you hand to anyone else.
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