Can We Trust
Track & Field Records?

How
accurate are they?

By Daniel Engber

Beat
this

The
Jamaican sprinter Asafa Powell set a new world record
for the 100-meter dash on June 14, with a time of 9.77 seconds.  The previous record, set by American Tim
Montgomery, was 9.78 seconds—just one-hundredth of a second slower. How
accurate are the devices used to time these events?

They
can record to the ten-thousandth of a second, although that level of precision
is not commonly used. That's because every track (and every lane) differs in length
by a minuscule amount, so two runners who race at exactly the same speed might
cross the finish line with times that differ by a tiny fraction of a second. In
the 1972 Olympics, for example,
two swimmers finished a 400-meter race within two-thousandths of a
second—or a few millimeters—of each other.  As it turned out, each lane of the
swimming pool had been constructed to a precision of just 10 millimeters, so
there was no way to determine the winner.

At
short events (including the 100-meter, 200-meter, and 400-meter races), times
are measured to the thousandth of a second and reported to the hundredth.
(Exact times are always rounded up to the next highest hundredth of a second.)
In longer races, official times are often less precise: Marathon times are given to the whole second. And any race
that's 800 meters or longer can be hand-timed with a stopwatch, according to international
rules.

To
set a world record in the 100-meter and 200-meter races, a runner's performance
must be recorded with a photofinish
system. The best systems use a digital camera positioned on the stripe that marks the finish
line. This camera has a very narrow field of view and records only the thin
slice of space directly above the line. As runners cross the line and into its
field of view, the camera takes a rapid series of time-stamped pictures. These
create a composite image (like this one)
that
shows when each athlete passed through the slice and over the line.

In
track events, the race is over as soon as the runner's
"body"—meaning his or her torso, not including arms, legs, or
head—crosses the finish line. (Other sports use more specific markers: In
speed-skating, it's the front of a skate.) Before an official time can be
posted, timing personnel must review the photofinish to determine where each
"body" begins.

Officials
can get a quicker and less accurate reading by placing light beams across the
finish line. Times are recorded as soon as an athlete passes through the beams,
although there's no way to tell whether it was a hand, a head, or an abdomen
that broke the plane of the finish line. As a result, these times don't always
match up with those gleaned from the photofinishes. When Powell first completed
his record-breaking race, his time showed up as 9.78 seconds—which
would only have tied the previous record. But after a review of the digital
images, his time was corrected to 9.77. (Initial readings are often padded with
some extra time, so officials never have to adjust up—and take away a
world record.)

How
does the clock get started? International rules hold that it must begin to tick
within .001 seconds of the official gun. The "gun" can be either a real
gun (at least .32 caliber for outdoor events and .22 for indoor) or a
"silent gun," without real bullets, that triggers the emission of a
clicking sound from speakers positioned just behind each runner. The flash of
the gun—which is fired near the finish line—provides an
instantaneous signal to the photofinish camera, which then starts its clock.

Explainer
thanks Giles Norton of Lynx System Developers, Inc., and Imre Matrahazi and
István Gyulai of the International Association of Athletics Federations.