Robot umpires are coming to MLB. Here's how they work

The system calculates balls and strikes at the midpoint of the plate—eight and a half inches from both the front and the back. This differs from the traditional rule book definition, which treats the zone as a three-dimensional cube. While big league umpires are historically impressive, calling roughly 94% of pitches correctly according to UmpScorecards, the move to ABS aims to bridge that final gap in accuracy. By standardizing the zone across the league, the system hopes to ensure that a strike is a strike, regardless of who is calling the game.

Where has ABS been tested?