Keeping an eye on the track of events

In April, the Association of UK Course Measurers declared that from 2013 to 2015 the course for the Greater Manchester Marathon in the United Kingdom had been 380 meters short, and the times of the 24,000 runners would be invalidated.

Although it's well-known that a full marathon course is 42.195 km long, how do organizers ensure that the length of each race conforms to the regulations and avoid the error that blighted events in Manchester?

Zhao Pu, one of just three B-Class marathon measurers from the Chinese mainland accredited by the International Association of Athletics Federations, has measured more than 100 marathon courses.