The following article, which appeared in Road Race Management Newsletter and was taken from the Credit Union Cherry Blossom 10 Mile Run site, helps explain why sometimes a GPS might not agree perfectly with the calibrated and verified course.
Among the phone calls and emails a race director inevitably receives
from runners after an event complaining about stale bagels, long
bathroom lines and T-shirt shortages are sure to be a few (and
sometimes a lot) claiming that your course was the wrong length.
Usually, they state that the course was too long (funny how no one ever
seems to complain when a course is short, isn’t it?), and some will go
so far as to request/demand that their time be adjusted, particularly
in the case of a marathon when a Boston qualifier is on the line.
Virtually every one of these will come from runners who wore a GPS
device during the race and found out the distance was anywhere from a
few hundredths to several tenths of a mile off. What can a race director do with these claims, aside from hitting
the “Delete” button on your email? Well, there are several reasons you
can provide that should convince all but the most irrational runner
that, to flip Shakespeare on his head, “the fault lies not with us, but
in the stars.” More accurately, it’s in the artificial heavenly object known as
satellites that are cause for much of the error. GPS units calculate
distance based on triangulation of readings taken from a series of
fixed orbiting units, but the degree of accuracy depends on several
GPS watches typically worn by runners, costing several hundred
dollars, can’t achieve the results obtained by survey- or
military-grade units, which sometimes use two base units that can read
the satellite signals at a higher degree of precision. Even the best
commercially-available GPS unit is only accurate to about 12 feet at
any given time, and can be hundreds of feet off in accuracy. Most units will indicate what their current accuracy is, and it can vary from 12 feet to 350 feet or more.
GPS units must have a clear view of at least three satellites to get
a reading, and the more they can acquire, the more accurate they are.
However, trees, buildings, and even a runner’s body can interrupt the
signal, making it less accurate at any time.
Further, they only check their position periodically, not
constantly. Some units check every second, some every 20 seconds. The
user can sometimes set the unit to check at certain time or distance
intervals, but if it has lost contact with the satellites, it can't
tell where it is, so it misses that checkpoint. So, if someone is
running quickly, they may make a few turns while the unit doesn't have
contact, so that section will be measured incorrectly.
The other part of the equation is the way the runner ran the course
versus the way it was measured. A certified course is measured along
the Shortest Possible Route (SPR), a line that cuts all the tangents
just one foot from the curb or road edge. Very few elite runners, with
an unimpeded road available to them, tend to run that tightly. For
those farther back in the pack, the crowd of runners around them makes
this almost impossible, and possibly not worth the extra effort it
would require to weave through the field to follow the SPR. Also,
runners may start their watches before reaching the actual starting line
and stop them after the finish.
These two factors are the primary cause for readings that don’t
agree with the actual course distance. Tests performed by members of
USATF’s Road Running Technical Council have found that runners usually
will get a reading indicating the course is 1 percent long. (Several
threads on the topic are available on the RRTC Bulletin Board at http://measure.infopop.cc/eve).
Strictly speaking, all certified courses are long, since a 0.01
percent Short Course Prevention Factor is added to ensure they don’t
come up short and fail validation in case of a record, that is probably
not enough to explain the longer readings obtained by runners’ GPS
units. The RRTC has not fashioned an official statement on GPS
measurements, save to state that they are not accurate enough for
course certification, but increasing requests from race directors may
lead to one being created and voted on at the next USATF Annual
Of course, it’s crucial that you make sure your course was set up
and run as certified. It’s a mistake to rely on memory when locating
critical points like the start and finish, and it’s equally important
to make sure runners didn’t inadvertently go off course due to poor
marking or course monitors’ errors.
In short, the best response to runners claiming your course was long
is to tell them their GPS unit isn’t quite as accurate as they think
(something many won’t believe or want to hear), and that they probably
didn’t run the course as tightly as it was measured. Stating this
beforehand, on the race website on the course information page, should
go a long way toward reducing those post-race calls and emails.
Oh yeah, and get fresher bagels and a few more PortaJohns, too.