Field notes 5 MIN READ
GPS drift, and why your line wobbles
The spike through the building, the wander in the tunnel, the strange loop at the summit. Where drift comes from and what we do about it.
Look closely at any recording and it is not a clean line. Where does the noise come from?
Reflections
The most visible errors are not random. A receiver works out its position from how long signals take to arrive from several satellites. If a signal bounces off a building before reaching you, it took longer, so the receiver computes a distance that is too long.
Do that with several satellites at once and the position lands somewhere plausible and wrong — often several tens of metres away, sometimes on the other side of a building.
This is why city centres are worst, why a spike through a block of flats is common, and why the same corner produces the same error every week.
Geometry
Even with clean signals, accuracy depends on where the satellites are. Spread across the sky, the intersection is tight. Clustered in one part of it, the intersection is a long thin region and your position slides along it.
In a valley, a forest, or beside a cliff, half the sky is missing. The line wanders.
Standing still
The worst-looking artefact comes from not moving. At a light, at a café stop, at the top of a climb while you get your breath, the receiver keeps fixing. Each fix lands a few metres from the last. Over five minutes that draws a small scribble.
This is also where phantom elevation comes from: those same fixes drift vertically too.
What we do
We simplify. A route is reduced to the number of points a sheet of paper can actually resolve. This removes the smallest jitter as a side effect, because sub-millimetre detail cannot be drawn.
We do not straighten. No snapping to roads, no smoothing of corners. The route on the paper is the route in the file.
That is a deliberate choice and it costs something: a cleaned-up line is prettier. It is also a drawing of a map rather than a drawing of your ride, and the difference is the only reason the object means anything.
The scribble at the summit is where you stopped to look at it.
Questions people actually ask
- Should the wobble be smoothed out of a print?
- Barely. We simplify to remove points a sheet of paper cannot resolve, which quietly removes the smallest jitter, but we do not straighten anything. A cleaned-up route is a drawing of a map, not of your ride.
- Why do I get a spike straight through a building?
- A reflected signal. The receiver sees a satellite bounced off a facade, computes a distance that is too long, and places you where those wrong distances intersect — which can be indoors.
- Does the wobble show at poster size?
- Some of it. On a 50×70 print a 200 km ride is drawn at roughly one millimetre per kilometre, so most jitter falls below the paper's resolution. A big spike survives, because it is large.
- Can drift be corrected afterwards?
- Partly. Snapping to a road network removes drift and also removes the evidence — you get the road you were on rather than the ride you had.
YOURS, FROM A FILE, IN 90 SECONDS
One effort. One sheet of paper.
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