Stories and curiosities
Why your GPS watch shows a different distance from the official course length
· 6 min read
In short
- A GPS receiver is accurate only to approximately 5 to 10 metres, while the frequency at which it measures and stores positions depends on the device and its settings. Over a long run, small deviations affect the final distance.
- An official course is measured with a calibrated bicycle fitted with a Jones counter, following the shortest possible route, known as the tangent—the exact line a perfect runner would take.
- In a real race, runners take wider lines through bends and weave around other people, so they actually cover more than 42.195 km even when the course is certified at precisely that distance.
- Road running world records are therefore not recognised on the basis of GPS data, but only when the course has a certified length.
You complete a marathon and your watch beeps at 42.6 or even 43 kilometres instead of 42.195 km. You immediately wonder whether the course was measured incorrectly. The usual explanation is a combination of the runner’s own path through turns and GPS inaccuracy, rather than an error in the certified course.
How an official course is measured
Certified courses are measured using a calibrated bicycle fitted with a device called a Jones counter, which counts rotations of the front wheel. It was developed in the 1970s by father and son Alan and Clain Jones, who improved on an earlier calibrated-bicycle method used by the British cycling federation. The device has since been used to measure almost every Olympic marathon course, and since 2004 it has also been required for road running world records.
The tangent: the shortest possible line
The crucial difference from GPS lies in how bends are measured. An official course follows the tangent, meaning the shortest possible line through each bend—the exact route a perfect runner would take without having to avoid anyone. In a real race, runners take wider lines around bends and weave around other people and aid stations, so they actually cover more than the shortest possible tangent distance of the course.
Why GPS is inaccurate
A typical GPS receiver is accurate only to approximately 5 to 10 metres and records positions at a frequency determined by the device and its settings, rather than completely continuously. It fills the gaps between readings with straight lines. Over a long run, these small deviations gradually affect the final distance and may either increase or reduce it. In practice, watches often appear to record a longer distance in cities or open countryside, partly because of the route the runner actually takes.
Why records are not recognised from GPS data
Because accurate course length is essential for comparable results, World Athletics recognises road running world records only on certified courses measured using an approved method, such as a Jones counter—never from a runner’s GPS recording. World Athletics has officially ratified road running world records since 2004. Learn more about the requirements in Records in running and sport.
What event organisers should take away
If you organise your own race, it is worth having the course certified or at least measured as accurately as possible. The race information should explain to participants that a difference from the distance shown by their watches is normal. RFID race timing measures when competitors pass points on the course, not distance. The two should not be confused, even though they are related. Learn more in How to choose a timing system for your event.
Frequently asked questions
Why does a GPS watch show a longer distance than the official course length?
Because it measures the route you actually ran, including wider turns and weaving around other competitors, rather than the shortest possible line used to certify the course. Inaccuracy in the GPS signal adds another source of variation.
How accurate is GPS for running?
A typical receiver is accurate only to approximately 5 to 10 metres and records your position only once every few seconds rather than continuously. Over a long run, these small deviations can accumulate into hundreds of metres.
What is a Jones counter?
It is a device that counts the revolutions of the front wheel on a calibrated bicycle. It was developed in the 1970s by Alan Jones and his son Clain. The device records fine mechanical increments proportional to the rotation of the front wheel; after calibration over a known distance, those increments are converted into distance. It has since been used to measure and certify running courses, including Olympic marathon courses.
What is the tangent when measuring a course?
It is the shortest possible line through a bend—the route a perfect runner would take without having to avoid anyone. A certified course is measured along this line, with an allowance on the safe side so the course is deliberately measured to ensure it is not shorter than the stated distance. It is typically shorter than the route actually run by most competitors.