Ball Tracking in Cricket DRS: How Hawk-Eye Works and Its Limits
How ball-tracking technology works in cricket's Decision Review System — the Hawk-Eye system, projection accuracy, umpire's call zones, and what the tech can and cannot determine.
What Is Ball-Tracking in Cricket DRS?
Ball tracking is the computer-based technology used in cricket's Decision Review System (DRS) to determine whether a delivery would have hit the stumps after striking a batter's pad. The most widely used system is Hawk-Eye, which uses multiple calibrated cameras — typically six to eight — positioned around the ground to triangulate the ball's three-dimensional position at every frame of its flight. These position data points are used to model the ball's trajectory both before and after impact.
How Hawk-Eye Works
Hawk-Eye cameras capture images at 340 frames per second. The ball's position in each frame is identified using computer vision algorithms. From a sequence of positions along the ball's pre-impact trajectory, the system calculates a mathematical model of the ball's flight, accounting for the seam angle, spin rate, and atmospheric conditions. It then projects where the ball would have gone without the pad obstruction — producing the coloured line shown on television that continues to the stumps.
Umpire's Call Zone
Because no technology can perfectly predict what an unlocalised object would have done after an impact, there is always a margin of uncertainty in the projection. The ICC acknowledges this through the 'umpire's call' zone — an area defined as: (a) clipping the edge of the stumps or bails at an angle that makes it genuinely uncertain whether impact occurred, or (b) the projected ball path crossing the stumps by less than half a stump diameter at the point of the stumps' height. Within this zone, the on-field umpire's original call stands.
Limits of Ball Tracking
Ball-tracking systems model flight curves using the ball's trajectory before impact, but the ball may deviate significantly after hitting the pad: spinning deliveries can grip on the pad and turn differently post-impact, seam deliveries can deflect off padding seams unexpectedly. The projection assumes the ball continues on its modelled pre-impact path — which is an approximation, not a certainty. This is why the umpire's call zone exists and why contentious LBW reviews remain genuinely debatable even with technology.
Frequently asked questions
How accurate is Hawk-Eye ball tracking?
Hawk-Eye's manufacturers claim accuracy within 5mm. Independent studies have estimated 3-7mm accuracy for the projection model, with greater uncertainty for deliveries that have spun or seamed significantly before impact.
Why does 'umpire's call' exist in DRS?
Umpire's call acknowledges that even the best ball-tracking technology has a margin of uncertainty around the stump edges. Within this uncertainty zone, it would be inappropriate to overturn an on-field decision that was within the zone of doubt.