A student, a fitness map, and several military bases
In November 2017 the fitness service Strava published a global activity map: every activity its users had recorded, merged into a single image drawn from roughly a billion recordings and some three trillion GPS points. For two months nobody noticed anything.
In late January 2018, Nathan Ruser, a twenty-year-old Australian student, looked at the map over Syria. In an area that was otherwise dark, clear outlines glowed — rectangles, paths, loops. They were military bases, drawn by the running routes of the soldiers stationed there, along with patrol paths and supply routes in Iraq and Afghanistan. The US Department of Defense subsequently opened a review of its rules on GPS devices.
The point of the story is not that somebody was careless. The people involved were trained, security-conscious, and had every reason to be careful. No single recording gave anything away. Only the quantity did.
Why many walks are more than one walk repeated
A single route is a place. Many routes are a habit — and habits say something about the future that a place cannot.
Applied to a dog owner: from one shared walk, somebody learns that you live somewhere around here. From twelve, they learn that on weekdays you leave the house around 6:40 for twenty-five minutes, that the evening walk falls in the early evening and runs longer, that Sundays add two hours — and that nothing at all was recorded in the week of the 14th to the 21st, so you were probably away.
None of that is written down anywhere. It follows from timestamps that look entirely harmless on their own. That is exactly what makes aggregation a problem you cannot feel while sharing a single walk: every post is unobjectionable by itself, and the picture grows with each one anyway.
What helps — and what does not
The most common workaround is to start recording a few streets further along. That helps a little against the address. It does not help against the pattern, because the pattern lives in the times of day and in the regularity, not in the starting point. Share a walk every weekday at 6:40 and you are sharing your routine, no matter where the line begins.
- What actually works: do not share publicly. A private account ends the aggregation problem completely, because there is no audience to piece anything together. Everything else is a matter of degree.
- Do not share every walk. A pattern needs repetition. Keeping half of them to yourself does not halve the risk — it makes the inference considerably blurrier, especially if it is not always the same half.
- Check whether the time of day is published too, and how precisely. “Three days ago” is not the same as “Tuesday, 6:41”.
- Trimming the start is still right — it just solves a different problem. How it works and where its limits are is on the Home Zone page.
- Remember the gaps. Two weeks without a recording is a statement to anyone reading along, not the absence of one.
What Barnaby does here, and what it does not
Barnaby trims the start and end of a shared route before anyone else sees it, and that protection is on before you change any setting. That addresses the where.
It does not address the when. A publicly shared walk carries a point in time, and many points in time make a pattern — with us too. We would rather write that down honestly than claim a protection that does not exist. If you do not want to give away the pattern, set your account to private; the recording, the statistics and the map all work exactly the same, just without an audience.
Sources
- ABC News (Australia), 29 January 2018: Strava has published details about secret military bases, and an Australian was the first to know — origin of the discovery, name and age of Nathan Ruser.
- NPR, 29 January 2018: Pentagon Reviews GPS Policies After Soldiers Strava Tracks Are Seemingly Exposed — the US Department of Defense response.