(Q) How are home run distances measured?
(A) Imprecisely. The distances announced at games are ballpark estimates of where shots would have landed unimpeded. To figure that, says Robert Adair, author of The Physics of Baseball, "you'd need to know the speed and angle at which the ball left the bat, the temperature, the barometric pressure, the altitude—and even then you couldn't be exact."
Since baseball doesn't officially track dinger distance, the estimates are left to the clubs. Most use charts drawn up by IBM in 1987 for its Tale of the Tape promotion; teams with parks built since '87 have created their own charts. (Before '87 tater lengths were measured even more haphazardly. Mickey Mantle's famed 565-foot shot against the Senators at Griffith Stadium was determined by a Yankees publicist, who supposedly paced off the length.) The charts provide distance estimates for line drives, medium shots or towering flies, all of which, of course, are subjective calls. According to the Busch Stadium charts, a Mark McGwire jack to the first row of section 383 of the leftfield upper deck would register as 423 feet if towering, 437 feet if medium, and 453 feet if a line drive. "It's accurate to within 10 feet," says St. Louis player development assistant John Vuch, who plots distances at Busch. "We've been using it for 14 years, so if it's off, at least it's off consistently."