From Dan Brooks and SABR member Alan Nathan at Baseball Prospectus on April 5, 2017:
According to Dave Cameron and recently confirmed in a blog post by Tom Tango, MLB has changed the meaning of start_speed, a pitch-by-pitch parameter in the MLB Components Data (“Gameday”) Files. This brief post summarizes the history of the start_speed parameter, includes a cautionary note to new pitch-tracking researchers, and describes a method for estimating release point (extension) by taking advantage of Gameday’s parameter switch.
The parameter start_speed has, for the better part of 10 years, coded for the velocity at a fixed distance 50 feet from home plate. Although 50 feet is much too close to home plate to actually be a realistic guess at a pitcher release point, this distance was initially chosen to reasonably match the velocities reported by scout’s radar guns. Several websites (including BP and BrooksBaseball.net) quickly realized that 55 feet was actually a better estimate for pitcher release point, and so have used that as convention for much of the PITCHf/x era. Due to technical limitations of the PITCHf/x system, it was not possible to record the actual release point of the pitch, which limited the ability of the system to determine the actual speed at release.
Trackman Doppler Radar, which serves as the pitch tracking hardware for the new MLB Statcast system, has the advantage of being able to measure the actual release point of the ball—and the speed at that point—with excellent fidelity.
Read the full article here: http://www.baseballprospectus.com/article.php?articleid=31529
Originally published: April 6, 2017. Last Updated: April 6, 2017.