Why does a GPS track have a random spike on the map?

The short answer

A spike is almost always one recorded point — occasionally a short run of two or three — that landed further from the true path than its neighbours did, while everything around it stayed normal. The point itself is not drawn as an isolated dot; the app connects it to the point before and the point after with a straight line, the same as every other point in the track. One point fifty or a hundred metres off the trail, joined to two points that are right on it, is what turns a single bad fix into a sharp, unmistakable triangle on the map.

It almost never means the walk went wrong, and it almost never means the recording app made a mistake. It means that, for one fix out of thousands, the phone had a worse view of the sky than it did for the fixes on either side of it.

What actually produced the bad point

Positioning works by comparing signals arriving from several satellites at once, and a good fix depends on seeing enough of them, spread across enough of the sky, arriving by a clean and direct path. Almost everything that makes a single fix briefly worse is a short-lived change to one of those three things:

• Reflection off one nearby surface — a parked vehicle, a rock face, a metal roof, still water — arriving at the phone a fraction of a second after the direct signal and confusing the calculation for that one fix. This is the same distortion, multipath, that makes GPS wander under a forest canopy, just concentrated onto a single point instead of spread across many.
• A satellite briefly dipping behind a ridge, a building, or even your own body as you turn, changing which satellites the receiver is using and, for a moment, leaving the remaining ones bunched into a narrow strip of sky rather than spread wide across it.
• A fix recalculating right after the phone briefly lost the sky altogether — under a bridge, inside a short tunnel, in a stairwell — and settling back to normal a point or two later.

None of these conditions last. The next fix, a second or a few seconds later, is usually back to normal, which is exactly why a spike looks like a narrow triangle rather than a sustained detour: one point misplaced, surrounded on both sides by points that are right.

Why one bad point makes such a visible mess

Ordinary GPS wobble — the kind you get under trees or between buildings — spreads a few metres of error across every point, so the line looks fuzzy but stays roughly on course. A spike is the opposite shape of error: almost all the good points sit right on the trail, and one point carries the entire error by itself. Drawn as a line, a small error shared by everyone barely registers, while the same amount of error dumped onto a single point is instantly obvious.

Vertical position is also inherently noisier than horizontal position, for reasons that come down to satellite geometry rather than anything an app does. A spike that displaces a point sideways by a modest distance can carry an elevation change several times larger in proportion, which is why a spike on the map is often paired with a sudden, brief "climb" or "drop" that reverses again on the very next point.

What happens to your distance and elevation numbers

Distance is the sum of the gaps between consecutive points, so a single point that lands a hundred metres off the trail and back adds roughly two hundred metres of distance that nobody actually walked, in an instant. Left completely unchecked, that would make spikes a real problem rather than a cosmetic one.

In practice most recording apps check a fix against the ones immediately around it rather than accepting every point at face value. A point that would require covering that distance at a speed no hiker reaches is treated as suspect and weighed accordingly, rather than folded straight into the running total. That is why a visible spike on the map does not automatically translate into a visibly wrong distance or elevation figure — the map can show a raw point that the totals underneath did not fully count.

What to do about it

• Nothing, for an isolated spike. It is normal, it is rare relative to the thousands of good points around it, and there is no setting that stops the sky from being briefly worse for one fix out of many.
• If a spike genuinely bothers you on a route you plan to keep or share, a GPX file is plain text, and the point responsible can be found by its coordinates and timestamp and removed the same way you would trim out any other unwanted stretch of a track.
• Getting a fix out in the open before you set off gives the receiver a clean starting estimate, which mainly helps the first minute or two of a walk rather than preventing an isolated spike somewhere in the middle of it.
• Judge a walk by the whole track, not by one point. A single narrow spike surrounded by a sensible line on both sides is a recording behaving normally under a brief, ordinary obstruction, not a fault worth chasing down.

TrailCam records your route in the background for the whole walk and exports it afterwards as GPX 1.1 or CSV, so a stray point like this is something you can see and, if it matters to you, edit — not something buried inside a total you have to take on faith. It is free to download.