Why a loop hike doesn't end exactly where it started

The short answer

A GPS receiver does not read your position off some fixed underlying grid the way a survey marker would. Each time it calculates a position, it works that number out fresh from the satellites in view at that moment, and that calculation carries a small, independent margin of error — typically a few metres. Stand in the exact same spot twice, once at the start of a loop and once at the end, and the two readings are two separate calculations, so there is no reason they land on the identical coordinate. The gap between a loop's start and end point is that ordinary margin, showing up as a visible seam because the two points happen to be compared directly.

Why the same physical spot can produce two different coordinates

Every GPS fix is built from the timing of signals arriving from whichever satellites happen to be overhead, and that arrangement of satellites is never quite the same from one moment to the next — they are constantly moving. An hour or two into a loop, the satellites used to calculate your position are not the same ones, in the same positions, as when you started. Small differences in signal delay, atmospheric conditions and which satellites are in view all feed into the calculation, and the result is a position that wanders by a few metres around your true location rather than locking onto it exactly. That wander is symmetric — it is not that the device drifts steadily off in one direction, so the loop does not spiral away from its start. It simply lands near the same spot without ever being obligated to land on the same coordinate twice.

Why this shows up on a loop and gets ignored on an out-and-back

An out-and-back hike has the same underlying GPS wander running the whole way, but nobody compares its start point to its end point directly, because the two are not supposed to be the same place. A loop invites the comparison by design — you end where you began — so the same few metres of ordinary GPS noise that go unnoticed everywhere else in the track become visible right at the point where a reader expects a perfect match.

The rest of the loop has exactly the same noise sitting in it. It just is not being checked against anything, so it never produces a visible seam the way the start and end point do.

What would make the gap unusually large

A gap of a few metres is the ordinary case and is not worth a second look. A gap that stretches to tens of metres, or that puts the end point on the wrong side of a building or a stand of trees, usually points to something specific rather than routine GPS noise — most often a weak signal at one end of the walk, from dense tree cover, being close to tall buildings, or a slow satellite lock right after starting the recording. That last one is common enough on its own: the first fix or two after a recording starts can be rougher than the fixes that follow once the receiver has settled, which pulls the very first recorded point away from where the walk actually began.

Things that trip people up

• Assuming the app lost track of the route somewhere along the way — a gap only at the start and end, with the rest of the loop looking clean, points to ordinary endpoint noise, not a broken recording.
• Trying to manually snap the end point back onto the start point before sharing a track — this edits the recording to look like something it was not; leaving the small seam alone is more honest than tidying it away.
• Reading a large gap as proof the whole track is inaccurate — a rough patch at one end usually stays local to that end, rather than meaning every point along the way is equally off.
• Expecting a shorter loop to close more tightly than a longer one — the size of the gap comes from GPS conditions at that particular spot, not from how far you walked to get back to it.

Where TrailCam fits in

TrailCam records a route from the GPS positions an iPhone reports as you walk, without smoothing or snapping the start and end of a loop together, so what you see afterwards is the track as it was actually recorded. Distance, pace and elevation gain are calculated from that same recorded line. The app is free to download.