Chip timing vs GPS tracking: what each one measures, and why they disagree

When someone races, two sets of numbers are recorded about them, and the two won't agree. One comes from the chip on the bib, the other from the GPS in a phone. Here's what each one actually measures, why they differ, and which to watch when.

What the chip measures

The chip in the bib produces timestamps: when the runner crosses a timing mat, the system records the exact time. That's all it does, and it's exactly why you can trust it.

You get official, verified times at the start, at a handful of intermediate points and at the finish, and both net time (chip to chip) and gun time (from the starting pistol) come from them. Between the mats you get nothing; as far as the system is concerned, the runner doesn't exist there. The chip works with the official distance, so a marathon is 42.195 km even if the runner covered more.

What GPS measures

The phone (or watch) checks where it is every few seconds and joins the points up. That gives you a continuous position, current pace and a predicted finish, and you can see when someone stops. It can't give you an official time, though: a GPS trace isn't a result. The distance it measures is the path actually covered, measurement error included.

Why the distances disagree

This is the question people ask most. There are three causes, and all of them are normal.

1. The official distance is measured along the shortest legal line. A course is certified on the tightest possible path: the inside of every bend. A runner in a field, weaving between aid stations, is bound to cover more than that. On a marathon 150–400 m extra is entirely ordinary.

2. GPS errs too, and mostly upwards. Receiver error is largest on bends and between tall buildings, and it tends to push the trace outward, so the line bulges and the measured distance grows.

3. A watch and a phone don't agree either. They're two devices with different antennas and sampling rates, and 100–300 m apart on the same run is common.

The thing to remember: the chip gives you the official time, and GPS shows how they got there. You don't have to pick one over the other.

Why the pace differs

Because it's divided by a different distance. Pace from official splits uses the official distance; GPS uses its own. If the runner covered 42.6 km instead of 42.195, their GPS pace looks several seconds per kilometre faster than the official one, even though it's the same run.

That's why our tracking matches the signal to the route: distance is counted along the course rather than by adding up raw GPS points. It keeps the "where are they" a supporter sees close to the official splits.

Which one to watch, and when

As a spectator you get the most out of using both: the chip tells you how they officially stand, and GPS tells you where they are now and when to head to your spot on the course.

If one of them stops updating, we have a separate piece on what to do about it.

Frequently asked questions