Why does my GPS device show a different distance than the event?

If you have ever seen a discrepancy between your GPS device and the race distance, this is common.

How are Courses Measured?

Courses are measured using the shortest possible route by mapping a path that includes straight lines between the corners or s-curves of each turn in the course. If you run-the-tangents, you’re going to have faster times.

To ensure accuracy, two measurements are made that must be within 0.08% difference.

How GPS Watches Work

GPS noise is the biggest factor in watch accuracy. A GPS watch has an accuracy of around 16 feet, causing your actual position to record a longer path than the one you run. Even if you run a perfectly straight line, your watch will track you slightly to the left or right. Zoom in on your Strava map and you’ll see this GPS noise. Over the distance of a race, these small inaccuracies accumulate, leading to a longer distance showing on your watch compared to the official course measurement. The difference isn’t a reflection of a miscalculated course but rather a characteristic of GPS technology. The next time you find a discrepancy between your GPS watch and the race distance, remember it’s a common aspect of using GPS technology in running.