Modern Athlete Magazine Issue 128, March 2020 | Page 31

In another run, this
time in Durban (left),
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 inaccuracy
of GPS watches,
and just one of the
reasons they will
nearly 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 impossible for the line to be painted 300mm from the curb,
etc, due to the width of the painting vehicle)
• The error of GPS reading as shown above.
Tests throughout the world have found that even the most competitive runners who
run the shortest possible line and start and stop their watches on the official lines,
typically only get the distance on the watch to register 8m per km compared to
the measured distance. So, their watches will show 10.08km on the finish line of a
10km, and in a marathon, the GPS will show a reading of 42.533km as a minimum.
This is 10 times the inaccuracy of the bicycle measurement, before you even add
distance from the start line at the gun, and the remainder is probably how much
you ran off the true measured line!
Any marathon with a large field that reads around 42.8km is probably correct in
measured distance, so please don’t go appealing to officials about a long course
if they say the course has been officially measured. Above 43km between start to
finish lines is where there may be a measurement error. In fact, any marathon that
shows on a GPS as less than 42.6km is probably short! And for all the reasons
above, it is typical for a GPS in Comrades to register over 2km longer than the
measured distance! So by all means, use the technology, but before crying foul,
take the time to understand it.
GETTING TO GRIPS WITH GPS
The Global Positioning System (GPS) is
a US-owned utility that provides users
with positioning, navigation, and timing
services. The US Air Force develops,
maintains and operates the space and
control segments, with the space segment
consisting of a constellation of satellites
transmitting radio signals to users. The
US is committed to maintaining the
availability of at least 24 operational GPS
satellites, 95% of the time. To ensure this
commitment, the US Air Force has been
flying 31 operational GPS satellites for the past few years.
Like the Internet, GPS is an essential element of the global information
infrastructure. The free, open, and dependable nature of GPS has led to the
development of hundreds of applications affecting every aspect of modern life,
and GPS technology is now in everything from cell phones and wristwatches to
bulldozers, shipping containers and ATM’s, and is a vital component of many
industries, including farming, construction, mining, surveying, transportation,
banking, communication, package delivery, environmental protection,
emergency services, weather forecasting and earthquake monitoring
How Accurate is GPS?
It depends. GPS satellites broadcast their signals in space with a certain
accuracy, but what you receive depends on additional factors, including
satellite geometry, signal blockage, atmospheric conditions, and receiver
design features/quality. For example, GPS-enabled smartphones are typically
accurate to within a 4.9m radius under open sky. However, the accuracy of
smartphones and sport watch units can worsen near buildings, bridges, and
trees, due to satellite signal blockage or signals being deflected off buildings or
walls. This is why a GPS can sometimes show you to be in the wrong place.
Is Military GPS More Accurate than Civilian GPS?
The user range error (URE) of the GPS signals in space is actually the same for
the civilian and military GPS services. However, most of today’s civilian devices
use only one GPS frequency, while military receivers use two. Using two
GPS frequencies improves accuracy by correcting signal distortions caused
by Earth’s atmosphere, so high-end users boost GPS accuracy with dual-
frequency receivers and/or augmentation systems, which can enable real-time
positioning to within a few centimetres, and long-term measurements at the
millimetre level. This dual-frequency GPS equipment is commercially available
for civilian use, but its cost and size has limited it to professional applications,
and thus is not generally available in sport watches or smartphones.
ABOUT THE AUTHOR: Norrie represented Scotland and Great Britain in
numerous ultra-distance events, then emigrated and represented South Africa
in triathlon. He is an IAAF-accredited coach and course measurer and is the
official Old Mutual Virtual Coach. He has authored two books (Everyman’s
Guide to Distance Running and Every Beginner’s Guide to Walking & Running),
and counts 21 Comrades medals amongst his more than 150 ultra-marathon
medals. More info at www.coachnorrie.co.za.
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