Why an out-and-back hike never measures exactly double

The short answer

A recorded track is the literal sequence of points your phone logged while you moved, not the fixed line of the trail itself. Walk the same path twice — once out, once back — and you produce two separate recordings of it, each shaped by exactly where your feet went and exactly which points the GPS happened to log at that moment. From a zoomed-out map the two legs look like the same line. Up close they are two different paths that happen to run alongside each other.

That alone is enough to keep the two legs from measuring identically, before anything about GPS accuracy even comes into it.

Two passes, two different paths

You rarely walk the exact same line twice

On a wide track you might drift to one side going out and the other coming back. A rocky step, a puddle, a patch of mud — the line you actually walk bends around whatever was in the way at that moment, and it rarely bends the same way twice. None of this is far off the marked trail, but a GPS track measures the line you walked, not the line on the map, and small drift adds up over a few kilometres.

A detour on one leg, not the other

A short walk out to a viewpoint, backtracking a few steps for a photo, a wrong turn corrected a minute later — any of that adds real distance to whichever leg it happened on. The other leg never walked it, so the two totals were never going to match.

Why GPS noise does not cancel between the legs

Every recorded point carries a small amount of ordinary positional wobble, and distance is calculated by summing the straight-line gap between one point and the next. A wobble to one side and back always makes that sum longer, never shorter, because a jagged path between two points is at least as long as the straight line between them — this is covered in more detail in why a GPS track disagrees between two apps. The important part here is that this wobble is independent on each leg: different satellites in view, a different angle to the sky, a different stretch of sky visible through the canopy. There is no reason for it to land the same way twice, so it does not cancel out between the outbound and the return.

What "double the one-way distance" quietly assumes

Taking half the round-trip total and calling it "the one-way distance" is a convenient shortcut, not a measurement. It assumes the two legs were identical, which is exactly the assumption a recorded track keeps failing to confirm. The round-trip total is the one number that was actually measured from end to end; a one-way figure produced by halving it is an estimate sitting on top of that measurement, not a second measurement in its own right.

What the mismatch does and does not tell you

A gap of a few per cent between the two legs is ordinary and is not a sign that the track is faulty or the app got something wrong. It is only worth a second look when the difference is large enough that a stretch of one leg looks obviously off on the map — a straight jump where the trail does not go, for instance — and even then the usual cause is a single stray point, not the out-and-back structure of the walk itself.

TrailCam records the whole route continuously in the background, including any short detour or backtrack along the way, so the distance reported for each leg is the sum of the points actually logged on that leg, not an assumption that the return mirrors the outbound. It is free to download.