This afternoon, I tuned in for the last two games of the Wimbledon semifinal between Andy Murray and Jo-Willy Tsonga. Andy Murray won the match, making him the first British male finalist at Wimbledon in 74 years (no pressure, Andy!).
The winning point was a bit strange, however. Murray hit an aggressive shot that was a tough call for the umpire. The crowd exploded, but then quickly realized the ump called it out. No matter. Murray put a finger in the air and challenged the shot. Under tennis replay rules, players can challenge in or out calls, and the disputes are decided by the Hawk-Eye ball tracking system. Hawk-Eye uses high frame rate cameras to map the trajectory of the ball. The Hawk-Eye trajectory showed the ball nicking the outside of the line, and Murray was through to the finals!
I asked my buddy Tony, a staunch replay opponent, whether he would have felt bad if Murray had lost the match in the absence of replay review. He said no, because he guessed the tennis replay system faces precision issues. This is his standard “blades of grass” complaint. On extremely close plays, there is no way to tell whether a ball (or player, in other sports) is in or out, because it comes down to the deformation of the ball on impact, individual blades of grass, and lack of precision in painted lines.
I set about to prove him wrong, under the premise that a computer-driven system like Hawk-Eye would have definable precision. In one way, I was right. Hawk-Eye’s average error is 3.6 mm. But if that’s the case, why is it used to make calls that are well within it’s margin of error? Continue reading