Dominating Tennis

How does Hawk-Eye work in tennis, and how accurate is it

See how the court measures the ball. Know its error bar.

Key facts

  1. 1

    Hawk-Eye uses about ten high-speed cameras running at about 340 frames per second.

    CNBC on Sony's Hawk-Eye (2023)

  2. 2

    Hawk-Eye claims a mean error of about 3.6 mm for its ball tracking.

    Hawk-Eye Innovations

  3. 3

    The ITF tolerance an electronic line-calling system must meet is 5 mm.

    ITF

  4. 4

    By 2025, electronic line calling was making the calls across the ATP tour.

    NBC Sports

  5. 5

    Hawk-Eye was used in cricket from 2000 and for tennis line-call challenges from 2006.

  6. 6

    Hawk-Eye was invented by Paul Hawkins, and the company was later sold to Sony.

Why it works

One camera flattens the world. It knows the direction to the ball, not the distance. A second camera at a different angle draws a second direction, and the two lines cross in one place. Add more cameras and the crossing gets tighter, and it survives one camera being blocked by a player.

Stitch those crossings together frame by frame and you have the ball's flight. Everything else, the bounce, the speed, the curve, is arithmetic on that line. The cameras never see the ball touch the ground. The mark you see on the replay is where the flight says it must have landed.

So a replay is a model, not a photograph. When it shows a ball 2 mm out, the error is of the same order as the margin, so the replay can't prove 2 mm either way. What makes the ball out is a rule: the system's call is the call. That's fair, because the system is more consistent than a person.

The line to say on court

“Hawk-Eye doesn't see the ball land. It computes where it must have.”

Drill: Beat the system

15 min · alone
  1. Watch any match with electronic line calling. On each close call, say in or out from the live view before the graphic appears.
  2. Note the margin the graphic shows, and mark any call under 5 mm.
  3. After ten calls, count how many you matched and how many fell inside the tolerance.
Number to beat6 of 10 calls matched to the system

More drills like this: watch like an analyst

Faults and fixes

Reading the replay as a photo
The bounce mark is computed from the flight. The ball never appears in it.
FixThink "model" every time you see a replay.
Quoting one accuracy number
Different sources give 2.2, 2.6 and 3.6 mm, measured by different methods.
FixQuote the range and the 5 mm tolerance.
Assuming tracking gives spin as reliably as position
Spin measured from cameras is less reliable than position.
FixTreat broadcast spin numbers as estimates.

Questions players ask

How accurate is Hawk-Eye in tennis?

Hawk-Eye claims a mean error of about 3.6 mm, and independent tests put it at about 2.6 mm, though sources differ by method. The ITF requires a system to be within 5 mm.

Does Hawk-Eye actually see the ball bounce?

No. It computes the bounce from the ball's flight path. The mark on the replay is a reconstruction.

How many cameras does Hawk-Eye use?

About ten high-speed cameras, running at about 340 frames per second.

Can Hawk-Eye be wrong on a very close call?

On a call of a couple of millimetres, the error is about the same size as the margin, so the replay can't prove it either way. The rules make the system's call final.

What this doesn't cover

The bounce mark is inferred from the flight, not seen, and the drawn ellipse is a model of a squashed ball. Spin from cameras is less reliable than position. Accuracy claims differ by source and method, so trust the range, not one number. The system is consistent, not certain.

Sources

  • Owens, Harris & Stennett (2003), IEEE
  • Hawk-Eye Innovations
  • CNBC on Sony's Hawk-Eye (2023)
  • NBC Sports on electronic line calling by 2025
  • ITF: electronic line-calling tolerance

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