A marathon course is measured to a defined standard by a qualified measurer, using a method chosen for repeatability rather than convenience. Certification is what makes results comparable between cities.
The calibrated bicycle method
Measurement uses a counter fitted to a bicycle wheel, recording revolutions along the route. The counter is calibrated on a precisely surveyed straight before and after the ride.
Calibration is repeated because tyre pressure and temperature change during the day, and an uncorrected drift would introduce error across the whole distance.
The method is preferred over satellite positioning because it is repeatable to a much finer tolerance and is not affected by buildings or tree cover.
The shortest possible route
The measurer rides the shortest legal line a runner could take, cutting each corner to the inside and taking the tangent between bends.
This matters because a course measured along the centre of the road would be longer than the distance a runner actually covers.
Marking the shortest line on the road during the race is not required, but the measured route determines where barriers and cones must be placed.
Why courses are deliberately long
Measurers add a small allowance to the calibration so the course is marginally longer than the nominal distance rather than shorter.
The reasoning is asymmetric consequence. A course slightly long produces slightly slower times; a course slightly short invalidates every performance on it.
The allowance is small enough to be irrelevant to the outcome and large enough to absorb measurement uncertainty.
Conditions attached to record eligibility
Record-eligible courses must satisfy limits on how much the start is higher than the finish, and on how far apart the two points are.
The elevation rule prevents a net downhill course from producing assisted times, and the separation rule prevents a course aligned with a prevailing wind.
Point-to-point courses can still be certified for the race itself; the restrictions apply to what the times may be used to claim.
What can invalidate a race afterwards
Courses are verified again after major races, and a lead vehicle taking a wrong turn has occasionally shortened a route already certified.
Roadworks, diversions and last-minute security changes can alter the route between measurement and race day, which is why organisers re-check before the start.
When a discrepancy is found, results stand as a race outcome but not as marks for record or qualification purposes.
