Why Your GPS Watch Reads Longer Than the Race
Why your GPS watch shows more kilometers than the race markers, how a race course is measured and how to adjust your pace with simple math.
In this article
Your watch can show more kilometers than the race markers because an official course is measured along the shortest possible path and you do not run exactly along that line. Every wide turn, every overtake and every change of side adds meters that the measured course does not count. In a 10K, 100 extra meters on your watch is a 1% difference.
This article explains how a course is measured, why your watch piles up more distance, how much your pace changes depending on the difference and how to adjust your strategy to follow the race markers without losing your goal. The numeric examples are of our own design and are guidance.
How a race course is measured
According to the IAAF and AIMS document on course measurement, the approved method for measuring road races is a calibrated bicycle with a Jones counter, which records the revolutions of the front wheel. The bicycle is calibrated by riding over a stretch of known distance.
The key rule is this: the course is defined by the shortest possible path a runner could follow without being disqualified. The measurer rides it hugging the curves, using the full width of the road, up to about 0.3 meters from the curb.
On top of that, a safety margin is added. The so-called short course prevention factor is 0.1%: it is multiplied by 1.001 so that the course never comes up short. In a 10K that is 10 meters more; in a marathon it is about 42, so the measured course comes to around 42,237 meters.
Why your watch reads more
AIMS points out that, in practice, a runner cannot follow that ideal line. That explains most of the difference:
- Actual path: you swing wide on turns, dodge other runners and switch sides of the street. Every extra meter adds to your watch and not to the measured course.
- GPS signal: the watch estimates your position with a signal that is not perfect, and it can add or subtract small stretches depending on the surroundings.
- Watch processing: each watch filters and smooths the points differently, so two watches in the same race can show slightly different distances.
There is no single figure for everyone. In a race with many runners, the actual path can stray further from the shortest path than in one with few people.
The math: how much your pace changes
Let us look at an example of our own design. Imagine a 10K that you finish in an official time of 54:10 (3,250 seconds), which is 5:25 per kilometer by the markers. Your watch will not show the same:
| Distance on the watch | Extra meters | Difference | Pace on the watch |
|---|---|---|---|
| 10.00 km | 0 | 0% | 5:25 |
| 10.05 km | 50 | 0.5% | 5:23 |
| 10.10 km | 100 | 1.0% | 5:22 |
| 10.15 km | 150 | 1.5% | 5:20 |
| 10.20 km | 200 | 2.0% | 5:19 |
The practical conclusion: with a 1% difference, your watch shows about 3 seconds per kilometer faster than the official pace. With 2%, about 6.
How to adjust your strategy with the markers
The solution is not to ignore the watch, but to measure its drift with the first markers and correct for it.
- Press the lap button each time you pass a kilometer marker, instead of relying on the automatic lap.
- Work out your factor: the watch distance divided by the marker distance. If at the kilometer 5 marker your watch shows 5.06 km, your factor is 5.06 / 5 = 1.012, which is 1.2% extra.
- Expect the same at the end: 10 x 1.012 = 10.12 km in the 10K, or 21.0975 x 1.012 = 21.35 km in a half marathon.
- Correct your watch pace: official pace divided by the factor. For a goal of 5:25 per kilometer, 325 / 1.012 = 321 seconds, which is about 5:21 on your watch.
In the example, if you pass kilometer 5 in 27:05, your official pace is 5:25 per kilometer (1,625 seconds divided by 5), but the watch, with 5.06 km, would show 5:21.
What to do in the race
- Trust the race markers for time and splits. The watch is for instantaneous pace.
- If your watch beeps before you reach the marker, do not worry: it is normal. Not speeding up to match is the key.
- If your goal is a time, check your split at the halfway point, as explained in how to run a 10K in 50 minutes or half marathon in 2 hours.
- Before the race, compare your watch against a known distance from training, such as a track or a measured stretch, to see how much it drifts.
- Train with the same watch you will use in the race. That way your paces are comparable.
When the difference is very large
If the drift is much bigger than 2%, or changes a lot from one stretch to another, several causes may be at play: a weakened signal in a specific stretch, a misplaced marker or another reason. There is no way of knowing without more data. A simple guideline: compare with the next marker. If the difference evens out, it is most likely watch noise.
Whatever the cause, the time that counts is the official one, and the distance that counts is that of the markers.
How to fit it with your predictions
If you use a time from another race to work out your goal, check that the watch and the markers measured the same thing. A result recorded with a watch that showed 10.12 km in a 10K is a time over 10 kilometers, not over 10.12. You can recalculate with the pace calculator and time predictor and review the methods in predict your race time.
Common mistakes
- Speeding up so that the watch shows the kilometer when the marker has not appeared yet.
- Using the watch pace as the official pace in your prediction math.
- Changing watch or settings right before the race.
- Not pressing lap at the markers and losing the reference for your splits.
- Blaming the watch for everything: part of the difference is your path.