Does being inside a car or train affect GPS accuracy?

The short answer

Being inside a car or train weakens a GPS signal to some degree, but it rarely stops a phone from getting a position at all. Wherever the vehicle is metal, the signal simply does not get through, the same as it would not through a metal wall. That leaves the windows as the only path in, and most window glass lets a GPS signal through with only a small loss. Some does not: a fair number of modern windscreens, and some side and rear windows, carry a very thin, invisible coating added for reasons that have nothing to do with GPS, and that coating can cut a real amount of signal on its way through.

Why the metal body is not the part to worry about

A GPS receiver needs a working path to several satellites at once, and solid metal is about as effective a barrier to that signal as anything gets — a car roof, the doors, and the frame around each window all block it completely rather than just weakening it. None of that is unusual or a design flaw; it is the same reason a phone loses signal walking deep into a parking structure or a building with no windows nearby. What keeps a car or a train from being that same dead zone is that it is not solid metal all the way round. A window is a gap in the metal, and a signal that would be blocked everywhere else has exactly that gap to come through.

The coating most people have never heard of

Many vehicle windscreens, and increasingly some other windows, carry a low-emissivity coating — a metal-oxide layer only a few atoms thick, applied to help the glass hold cabin heat in winter and reflect it back out in summer. It is invisible to the eye and has nothing to do with tracking a route; it exists purely for climate comfort. But a coating built from metal oxide interacts with radio signals the way metal does in general, just far more weakly than a solid sheet of it would — attenuating a signal passing through rather than blocking it outright. It is common enough that some vehicles have a small, deliberately uncoated patch etched into the windscreen near the mirror, specifically so a toll transponder or a built-in GPS antenna has one clear spot to work through. A phone sitting on a seat or in a pocket has no such dedicated clear patch reserved for it — it gets whatever signal manages to pass through wherever in the car it happens to be.

Why speed is not the issue

It is easy to assume a fast-moving vehicle gives GPS less to work with, but speed itself is not what causes any of this. A receiver calculates position from satellite signals arriving at a given instant, and that calculation does not care whether the antenna is standing still or moving at highway speed — GPS was built from the start to track things moving far faster than a car or a train ever does. A separate effect can make a track drawn from a fast vehicle look like it cuts corners on tight turns, because logged points end up spaced further apart along the ground at higher speed, but that is a question of how often positions are recorded, not of the positions themselves being less accurate. The enclosure and the glass are what change accuracy inside a vehicle; the speedometer reading has nothing to do with it.

Trains carry the same two issues, plus one of their own

A train carriage follows the same logic as a car: a steel shell with windows as the only real opening, and some train windows carry a coating of their own, sometimes for the same thermal reasons and sometimes for reasons specific to that operator's rolling stock. What a train adds on top is more frequent, complete loss of signal rather than just a weakened one — a cutting, an overpass, or a tunnel blocks the sky outright for as long as it lasts, the same way any tunnel does for a car. That is a different, binary kind of gap from the steady, partial loss caused by coated glass, and it is worth telling the two apart: one thins out a track a little the whole way through, the other removes a stretch of it entirely, and the line simply reconnects once the sky is visible again.

What actually helps, if it matters to you

None of this is usually worth engineering around for an ordinary drive or a commute, but a few things make a real difference if a cleaner track matters for a specific trip.

• Keep the phone near a window rather than in a door pocket, a bag, or the centre console, where more metal sits between the antenna and any opening at all.
• A visible clear patch etched into a windscreen near the mirror is a hint that the rest of the glass carries a coating — a phone propped closer to that patch, or simply nearer any uncoated window, has an easier path.
• There is little to do about a coated window itself. The coating is baked into the glass, not something a case or a pouch is involved in.

Where TrailCam fits in

Driving, transit and train are three of TrailCam's 11 built-in activity types, alongside hiking, cycling and the rest, each with its own colour on the map. The GPS recording underneath runs the same way no matter which one is picked, which means the same windows-and-coating effects apply whether a route is filed as a drive or a train ride. TrailCam is free to download.