Why does a GPS track go straight through a tunnel?

The short answer

A GPS receiver needs a clear path to several satellites to work out where it is. Rock, concrete and several storeys of parking structure block that path completely, rather than just weakening it, so for the length of the tunnel the phone is not receiving anything to calculate a position from. No position means no point gets logged. The moment satellites are visible again at the far end, the receiver locks on and logging resumes — and whatever drew the map connects that last point before the tunnel straight to the first point after it, because a straight line is the only honest way to join two points with nothing recorded in between.

This is a different failure from tree cover

Under a forest canopy, a signal still gets through, just weaker and sometimes bounced off a branch first — the receiver keeps producing a position, and the visible symptom is a line that wanders a little either side of the true path. A tunnel or an underground parking garage is a harder case: the sky is not partly obscured, it is entirely gone. There is no weak, noisy signal to work with, because there is no signal reaching the receiver at all. The two situations look similar in a search — "why is my GPS track wrong here" — but they produce opposite visible results: a wandering, slightly-too-long line under trees, versus a perfectly straight, slightly-too-short line through a tunnel.

A short underpass or a single bridge is usually a non-event by comparison, because the sky reopens again within a few seconds and the receiver barely notices the gap. It is length and depth that matter — a two-second dip under an overpass is nothing like ten minutes under a mountain.

What happens for the length of the blackout

A phone can sometimes place itself very roughly using Wi-Fi networks or cell towers it can see, but that is a city-block-scale estimate at best, nowhere near precise enough for a trail-quality track, and it disappears too once you are deep enough underground or far enough from a cell signal to lose that as well. For a genuine tunnel or garage, the practical reality is simply a gap: no new points, no updated distance, no updated elevation, for as long as the blackout lasts.

The two points on either side of that gap are exactly as accurate as any other point in the file — nothing about the recording itself is damaged. What is missing is only the stretch in between, and that absence is what turns into a straight line once something tries to draw the route as a single connected path.

What it does to distance and elevation

Distance across the gap gets measured as the straight-line hop between the two known points, which is shorter than the actual curved or winding path through the tunnel almost every time — so total distance for that stretch reads slightly short rather than slightly long, the opposite of what tree cover usually does to a number. Elevation can show a small, odd jump right at the two edges of the blackout, because the first fix after a signal reappears is often the least precise one the receiver produces all day — multipath off the tunnel mouth and the surrounding terrain settles down again within a point or two.

None of this needs correcting by hand for the walk or ride to still make sense. It is a known, physical gap in a known place, not a sign that anything recorded before or after it is unreliable.

Where TrailCam fits in

TrailCam keeps recording in the background the whole time, whether that means an open ridge, a forest, or a stretch with no signal of any kind, and it does not need cell service to do it — GPS recording works without a data connection, and only the map tiles depend on being online. Coming out the far side of a tunnel, it simply picks the fix back up and carries on. The finished route exports as GPX 1.1 or CSV, for a single walk or your whole history at once, as part of the optional TrailCam Pro upgrade; the app itself is free to download.