Hawk-Eye ball tracking: how a bounce mark is reconstructed

Where the system came from

Hawk-Eye was developed in 2001 by Paul Hawkins at Roke Manor Research in Romsey, Hampshire, and first appeared that summer as a broadcast tool in Channel 4's cricket coverage. The company was later bought by Sony. Its first officiating role was in tennis: the International Tennis Federation approved it for line calling after testing, and the US Open became the first Grand Slam to use it for player challenges in 2006. The same core idea, several cameras feeding one reconstructed path, now sits behind cricket's LBW projections, football's camera-based goal-line systems and the offside lines used in video review.

How the tracking works

Each camera sees the ball as a small blob in its own two-dimensional picture. Because the cameras' positions and lenses have been calibrated against the court markings, the blob's position in one picture defines a line through space; where lines from two or more cameras intersect is the ball's position at that instant. Repeat this frame by frame and you have a string of points, and a flight model is fitted through them. A tennis installation typically uses ten cameras around the court. The bounce is the point where the fitted path meets the surface, adjusted for how the ball compresses and skids, and the graphic shows the footprint that model produces.

The number of cameras matters less for precision than for coverage. A player, umpire or ball kid standing between a camera and the ball removes that camera from the calculation for those frames. The system copes as long as enough others still see the ball, which is one reason the cameras are mounted high and spread around the court rather than clustered at one end.

What it can resolve

In tennis the question is narrow and physical: did any part of the ball's footprint touch the line? The graphic answers yes or no and there is no margin category, unlike cricket's umpire's call. That suits a challenge system, because the outcome is unambiguous even when the underlying estimate is not. Under the challenge protocol players were given a limited number of incorrect challenges per set, so a player has to weigh confidence against the remaining allowance, which keeps frivolous stoppages down without a referee having to police them.

Hawk-Eye Live goes further and removes the line judges altogether, calling every line in real time with a recorded voice. It was first used at the Next Gen ATP Finals in 2017; the US Open used it on outer courts in 2020, the Australian Open across all courts in 2021, and Wimbledon announced that it would replace line judges with electronic calling from 2025. The trade-off is plain: fewer disputed calls and no challenge theatre, at the cost of the visible human check that a line judge provided and the jobs that went with it.

What it cannot

Every position the system outputs is an estimate with some error around it. The figure most often quoted is the manufacturer's stated mean error of about 3.6 mm in tennis. The ITF's approval process tests candidate systems against its own reference measurements, and the exact tolerance sits in the ITF's published technical documents rather than on any broadcast graphic. If the precise number matters to you, that document is where to check, not a commentary box.

On clay the physical mark is available, and the French Open kept human line judges and the chair umpire's inspection of the mark longer than the other three majors. That is a defensible trade: a visible mark is direct evidence, whereas a reconstruction is inference. It is not a clean win for the mark, though. A skidding ball leaves an elongated smear whose relevant edge is itself a judgement, and marks from earlier points can sit nearby.

Cricket adds a harder problem. After the ball strikes the pad nothing is measured; the path to the stumps is a projection from the flight measured before impact, and the shorter the measured flight the weaker the projection. The umpire's call margin in DRS exists in part to acknowledge that, and the DRS sheet works through it.

Alternatives and how to compare them

Hawk-Eye is not the only provider. FoxTenn has been used at some ATP and WTA events and combines high-speed cameras with laser scanners along the lines. In cricket, Virtual Eye, from Animation Research in New Zealand, provides tracking in several countries. What qualifies a system for officiating is the governing body's approval, not the brand. Comparing providers fairly is difficult because each publishes its own accuracy claims under its own test conditions, and the honest position is that all of them are far more consistent than a human line judge, and none of them is a photograph.

The rule of thumb that falls out of the table is this: trust the system most where it measures and least where it projects, and treat the absence of a margin on screen as a design decision rather than a claim of perfect accuracy.