Field notes 5 MIN READ
Why your marathon reads 42.6 km
The course was measured to 42.195. Your watch says more. Both are right, and the reason is worth knowing before you print either number.
The course was certified at 42.195 km. Your watch said 42.61. You did not run further than everyone else.
GPS error only adds
A GPS fix is accurate to a handful of metres, and the error moves around. Over a straight kilometre your recorded position wanders either side of where you actually were.
Here is the asymmetry: distance is the sum of the gaps between consecutive points. A wobble to the left and back adds two short segments where the truth was one straight one. Error to the left adds length. Error to the right also adds length. There is no wobble that subtracts.
Over 42 km, a few metres of jitter a second becomes a few hundred metres of phantom distance.
Better receivers reduce it. Tall buildings and tunnels make it far worse — which is why big city marathons produce the largest discrepancies, and why a canyon of towers around a turnaround is the worst case there is.
And you did run further
The other half is real.
A certified course is measured along the shortest legal path — the racing line, hugging every corner. Nobody runs that. You run wide around a bend to pass someone, you cross to the shaded side, you take the outside of a hairpin because the inside is a scrum at the water station.
Every one of those is real distance that the measurement does not include. On a course with many turns, running well can save you a hundred metres or more, and running badly costs the same.
So your 42.61 is partly instrument error and partly the truth about your line.
Which number to print
We do not decide this for you. The headline and the numbers underneath are yours to type.
There is a case for the official distance: it is what the event was, and it is what the medal says. There is a case for your recording: it is what you did, on the day, on your line.
There is no case for a number you did not get either way.
What the line is drawn from
The shape is your recording, always. The tangents you failed to take are visible if you know where to look, and the wobble is what makes it yours rather than a map of the course.
Questions people actually ask
- Should the print say 42.195 or what my watch recorded?
- Whichever is true for you. The official distance is what the event was; your watch distance is what you ran. Both are defensible and you can type either.
- Why is my GPS distance always longer, never shorter?
- Because GPS error adds length in both directions but a route only accumulates it one way. Every wobble to the left and back is extra distance, never less.
- Does running the tangents actually help?
- Yes, measurably. A course is measured along the shortest legal line, and every metre you spend on the outside of a bend is a metre nobody measured.
- Does the printed line use my recorded route?
- Yes. The shape is your recording, wobble and all. It is the only version that is evidence you were there.
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