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