Rolling Down the Road
Factoring in the Extras
GPS can be extremely accurate: This was clearly illustrated in 2006 in Beirut, Lebanon, when I was measuring the marathon and saw the destruction inflicted in the Israel-Lebanese war, where the outbound section of the highway was taken out by a missile strike, but the inbound section left intact. Military GPS is that accurate, but that is a far cry from the units we wear on our wrists for running, as I have found in years of course measuring – and there are obvious reasons of safety why military standards of GPS are not provided to sport or even driving units.
If you download the data from your GPS running watch, you get a vast amount of information that appears to be highly accurate. Take for example my two screenshots below left of the Athens Marathon, which would suggest the course is 575m too long. However, closer inspection reveals that the runner is apparently running just off the side of the road, and that one right turn has me creating a completely new road before running down the side of the park. Now keep in mind this is me running as one of the team that actually measured this route for the 2004 Olympic Games, so I know where I actually ran, and I know how to take the very shortest but still legal route on the road – yet my watch gave me 575 metres over the distance. In another run( below right), this time in Durban, I am apparently weaving all over Currie Road, and in some places it would appear I am running through people’ s front lounges! These are only minor examples of the
By Norrie Williamson
Going the( Right) Distance
I often see runners complaining about the accuracy of course measurement when they finish races and their GPS watch says the course was slightly long, but there are many factors at play here that runners do not understand – and the courses usually are accurately measured.
inaccuracy of GPS watches, and just one of the reasons they will always read slightly longer than the measured line of the race.
Rolling Down the Road
Now consider the accuracy of the bicycle course measurement device. The counter is attached to the front wheel and rotates with the wheel such that there is an increase of 23 or 27 digits per complete rotation of the wheel. Assuming a standard 700c( 29er) wheel, this means each digit on the counter equates to 90-100mm on the ground. Since we can read to a portion of a digit, we have an effective reading on the distance the tyre went, to around 50mm( 5 centimetres) accuracy. However, because the tyre is filled with air, it will expand in the heat, contract in the rain or cold, and flatten more with a heavy person while riding higher with lighter people. Even the nature of the road surface and the tyre tread will change the digit reading, and for these reasons we ride the bike over a known 300m distance four times before and after measuring any road race, so that we have an accurate figure of how many digits on the counter equate to 1km on the ground.
So now consider the actual measurement: IAAF Rule 240 requires a race to be measured using the bicycle method to define the shortest possible line in the portion of the road that the runner is allowed to run in. Let’ s be very clear: This means that the pavement cannot be used, but it does allow a race to restrict runners to using only one side of the road, or even one lane of a multi-lane road. While there would normally be marshals, cones or fencing to indicate this, there is no rule requiring this, but it would have to be specified to the runners.
With this in mind, we measure adhering to the width of road available and taking the shortest possible line, ridden 300mm from a curb, fence or coned line, or 200mm from an open edge. Where a race is measured by two measurers, the maximum‘ tolerance’ between the two measurers is 8 metres per 10km. That’ s an accuracy of 0.08 %. However, as a standard practice, and to protect against minor error of say, cutting the 300mm too close on corners, every measurement has a 1m / km safety factor built into the distance. In effect, this means that a 10km race is actually measured as 10.010km as standard. A marathon is then actually‘ not less than’ 42.2372km( 42.195 + 0.042195). This also protects the runner in cases of records, as the rule is that when verifying the distance, no safety factor is used and the distance needs only be 42.195km or more.
Factoring in the Extras
So where does the‘ extra’ distance on GPS watches come from that runners complain about after races? Here are a few possible reasons:
• Runners start their watches at the gun, but may be between 10m to 100m( Comrades) behind the start line, and this gets added to the distance.
• Wearing the watch on one arm means corners in the other direction add distance as the watch is further out from the measured radius( about 1m).
• Similarly, if there are other runners between you and the curb it adds distance.
• Trips across the road to water tables, toilets or to greet friends, etc.
• Not stopping the watch immediately at the finish line.
• The extra safety distance built into the measurement( 42m in a marathon).
• Any other aspect that keeps the runner off the shortest possible running line.( Note: A blue line on the route is not the shortest line, but an indication of direction, as it is
Images: Norrie Williamson, Jetline Action Photo
34 ISSUE 91 FEBRUARY 2017 / www. modernathlete. co. za