Why does a GPS track zigzag on a straight trail?

The short answer

Every GPS point carries a small amount of position error, typically a few metres, even with a clear view of the sky. On a curve that error is easy to miss. On a straight stretch it is the only thing left to see, because there is no real direction change for it to hide inside. Each point lands slightly left or right of where you actually stood, and a track is drawn by connecting those points in the order they were recorded — not by fitting a smooth line through them. String together a run of small, independent left-right errors and the result is a visible zigzag sitting on top of a path that, in reality, never wavered.

A GPS point is a location, not a dot painted on a line

It helps to be precise about what a receiver actually hands over at each fix: a best estimate of latitude and longitude at that moment, worked out from the timing of signals arriving from several satellites at once. That estimate is a small cloud of uncertainty collapsed down to a single coordinate, not a guaranteed exact position. Walk in a genuinely straight line and the true position moves in a straight line too — but each individual estimate still carries its own small, mostly random offset from that true position. Nothing later on removes those offsets, because a track is simply the sequence of estimates as recorded, joined together in order.

That is also why the effect reads as a zigzag rather than as a track that is merely a bit thick or blurry. A wide, fuzzy line would suggest the points scattered randomly in every direction with no pattern between neighbours. What actually happens is closer to alternating: a point lands left, the next lands closer to centre or slightly right, the one after that left again. Connect them with straight segments in that order and you get a saw-tooth, not a smear — the same handful of metres of error, just made visible by the shape of how the points are strung together.

Why it shows up more at walking pace than from a bike

The size of the error does not change much with speed — it comes from the receiver and the satellite geometry, not from how fast you are moving. What changes is how far apart consecutive points are. Walking slowly, two points recorded a few seconds apart might be separated by only a few metres of real travel — a distance similar in size to the position error itself, so a small sideways offset is a large fraction of the gap between points and reads as an obvious kink. Covering the same few seconds on a bike or descending fast on foot puts far more real distance between the same two points, so the same few metres of sideways error becomes a small fraction of a much longer segment and barely bends the line at all. The error is not bigger on a slow walk; it is just a much larger share of a much shorter step.

What makes it worse in specific places

Open sky with a clear view in every direction is the easiest case for a receiver, and that is where zigzag is smallest. It gets worse anywhere a signal can bounce before it arrives — off a rock wall, a stretch of water, dense tree canopy, or buildings close to the trail — because a reflected signal arrives slightly later than a direct one and the receiver has no clean way to tell the two apart. That effect, usually called multipath, nudges individual points sideways by more than the ordinary open-sky error, and it does not affect every point evenly, which is exactly the kind of point-to-point inconsistency that turns into a visible zigzag once connected. It is the same underlying reason accuracy suffers under tree cover and near tall buildings, covered in their own guides — a straight trail through a corridor of trees is close to the worst case for this specific pattern.

How this differs from a single spike

A spike is one point flying a genuinely long way from where it should be — sometimes tens of metres or more — usually from a single bad fix, and it is easy to spot as one obvious jump out and back. A zigzag is the opposite kind of pattern: many points, each off by only a small, ordinary amount, none of them individually looking wrong, but strung together in a way that reads as a wobble rather than a straight line. Both come from the same underlying source, ordinary GPS position error, but a spike is one outlier standing out from an otherwise clean line, while a zigzag is the normal, everyday version of that same error showing up everywhere at once on a stretch with nothing else to hide it in.

Does it actually change the numbers that matter

Barely, in most cases. Because the sideways offsets alternate roughly left and right rather than pushing consistently outward, a zigzag adds a small amount to total distance — each kink is a slightly longer path than the straight line it is sitting on top of — but the effect is usually a small fraction of a percent over a full hike, not something that visibly changes a distance total. It has essentially no effect on elevation gain, which is calculated from altitude readings rather than the lateral position of a point. The zigzag is mostly a visual thing: it looks more dramatic zoomed into a screen than it is in the underlying numbers.

Things that trip people up

• Assuming a zigzagging trail on the map means the phone lost GPS or the app is malfunctioning — it usually means the opposite: the receiver is recording plenty of points, and ordinary position error is just visible because the real path had no turns to absorb it into.
• Expecting the zigzag to be worst on the fastest-moving parts of a hike — it is usually the reverse, since slow, closely spaced points make the same error a bigger share of the distance between them.
• Worrying that a visibly wobbly track means the distance total is significantly wrong — the sideways offsets mostly cancel each other out over a full hike and add only a small amount to the total.
• Blaming a specific stretch of trail for the wobble when the real cause is what is next to it — tree cover, a canyon wall, or buildings close to the path — rather than anything about the trail itself.

Where TrailCam fits in

TrailCam records a GPS point in the background for the whole length of a hike and draws the route back exactly as it was recorded when you replay it on the map, wobble and all, rather than hiding what the receiver actually logged. A GPX export carries the same raw points, so if a stretch looks zigzaggy on the map it will look the same in the exported file, which is also the version any other GPX-reading app or site will show. Export is part of the optional TrailCam Pro upgrade; the app itself is free to download.