Ever noticed a discrepancy between your GPS watch reading and the official race distance? This is a common occurrence in running events, and understanding the reason behind it can reduce confusion and aid in training.

A Marathon is Not 26.2 Miles

A marathon is officially defined using the metric system at 42.195 km (42,195 meters). That distance is about 100 feet longer than 26.2 miles.

How Race Courses Are Measured

Road running race courses are measured using the shortest possible route that a runner can take without deviating from the course. This involves mapping out a path that includes straight lines between the corners or s-curves of each turn in the course. Savvy athletes are usually aware of this and aim to "run the tangents".

Images courtesy of USATF, the entity that certifies road race measurements in the USA.

Races are also measured by fallible humans, so to ensure that human error does not accumulate too much, two measurements must be made that agree to each other within 0.08%, the shortest is used with an added Short Course Prevention Factor of 0.1%. The short course prevention factor adds about 140 feet to a marathon if it was perfectly measured*.

*The Short Course Prevention Factor protects the measurement from accidentally being too short. It will always add less than 140 feet because human error in the measurements will always measure a path that is not the exact shortest possible route.

If you're keeping score, that's 240 feet so far beyone 26.2 miles. Without accounting for any human error, we're already at 26.25 miles for a marathon. Depending on how many decimal places your watch shows, it may already be rounding to 26.3 miles.

The GPS Watch Factor

So you think you ran the tangents perfectly, but your watch still reads somewhere between 26.22 and 26.50 (or even higher), so what gives? It's because GPS noise is the biggest factor in watch accuracy.

While GPS watches are invaluable for training and tracking, they have a limitation in precision. Typically, a GPS watch has an accuracy of around 16 feet. This means it can slightly vary from your actual position, causing the watch to record a longer path than the one you're running. Even if you run a perfectly straight line, your watch is always going to make a mark slightly to the left or slightly to the right of you. If you've ever zoomed in on your strava map, you've seen this GPS noise. Over the distance of a race, these small inaccuracies can accumulate, leading to a longer distance showing on your watch compared to the official course measurement.

My actual strava map showing GPS noise on an an out and back run when I followed a straight line.

Understanding the Distance Difference

This difference isn't a reflection of a miscalculated course but rather a characteristic of GPS technology. The course is carefully measured, but your GPS watch might not capture its exact length due to the inherent limitations of the GPS system.

For the Runners

As a runner, it's helpful to know that while your GPS watch is an excellent tool for tracking your runs, it may not always align perfectly with the official race distances. This awareness can help you better understand your performance and the data from your device.

Conclusion

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. Your focus on your performance and the enjoyment of the race remains paramount, with the watch being a helpful, though not infallible, tool in your running journey.