Why does a GPS track cut corners on a fast bike descent?

The short answer

A recorded track is a series of straight lines drawn between GPS points, not a curve traced continuously along the road. Through most of a ride the points sit close enough together that the straight segments between them are invisible, and the line looks like a smooth curve. Carry speed into a tight, short-radius bend and the points recorded through it can end up spaced further apart than the bend itself is wide, so the straight line connecting them cuts across the inside of the turn rather than following the road. It is a drawing artefact from where the points happened to land, not a sign the recording missed anything or that the app is less accurate on a bike.

Point spacing scales with speed, not with the road

On iPhone, a moving GPS recording settles into something close to a new point every second or two, driven by whether you are moving rather than by a fixed timer. That interval does not change because the road ahead happens to bend — it is a function of time and motion, not of the shape of the terrain. What does change is how much ground gets covered in that interval, and that scales directly with how fast you are moving.

A hiking pace of a little over a metre a second means each recorded interval covers a bit more than a metre of ground — small next to almost any bend a trail takes. A bike descending at speed can be moving five to ten times faster, so the same one-or-two-second interval between points now covers several metres to several times that. The points themselves are not less accurate at speed; there are simply fewer of them, spread further apart, across whatever the road did in that stretch of time.

Why a fast downhill hairpin is the one place it shows up clearest

Two things line up at exactly the wrong moment on a descending switchback. Speed tends to be highest right where the corner is tightest, since a rider carries momentum from the straight into the bend rather than slowing to a walking pace for it. And the bend itself is often the shortest-radius turn on the whole route, because a hairpin exists specifically to lose height quickly in a small footprint. Wide point spacing and a short-radius bend landing in the same few seconds is what turns an ordinary, invisible drawing artefact into something visible enough to notice on the map afterwards.

The same effect exists on foot, on a tight bend taken at a jog or a fast scramble downhill, but it rarely gets far enough to notice, because walking speeds keep the points close enough together that even a sharp turn has several points recorded through it.

What this does, and does not, change about your ride

A shaved corner on the map is a local effect confined to the length of that one bend, not a sign that the ride as a whole was recorded poorly. Total distance for a ride is barely touched by it — cutting a few metres off the inside of one hairpin is a rounding error against a route of any real length, the same way a squared-off switchback is a rounding error on a hike. Elevation gain is unaffected for the same reason it is unaffected by point density anywhere else: it is built from the altitude at each recorded point, not from how faithfully the line between points traces the road.

What it does change is how the map looks at that one spot, and only there. Zoom in on the corner and the line will look flattened; zoom out to the whole ride and it disappears into a route that measures and climbs the same as the road you actually rode.

Things that trip people up

• Assuming a cut corner means the phone lost signal in the turn — a lost fix leaves a gap or a jump in the track, not a straightened bend. Corner-cutting comes from points that were all recorded correctly, just spaced further apart by speed.
• Worrying that cycling is tracked less accurately than hiking — the recording works the same way for both; a bike ride just covers more ground between points because it moves faster, not because anything about the tracking changed.
• Expecting a denser recording to fix it — point spacing is a background negotiation between the app and the phone's location system, not a setting to turn up, and it is already tuned to record more while you are moving rather than less.
• Reading a shaved hairpin as evidence the whole ride's distance is short — the effect is confined to that one bend and is too small, over a full ride, to move the total in any way worth worrying about.

Where TrailCam fits in

TrailCam records position the same way for a bike ride as for a hike, whichever of its activity types you pick — continuously in the background, with no separate setting for cycling versus walking. A route exports afterwards as GPX 1.1 or CSV, the same file whether the ride included a flat towpath or a hairpin descent, as part of the optional TrailCam Pro upgrade. The app itself is free to download.