MLB Extended Gameday Pitch Logs
Alan M. Nathan
August 6, 2007
I. How to
Download
(a)
Downloading
the data: Go to the web site http://gd2.mlb.com/components/game/mlb/. Click on the year, then on the month; on the next page click on the day; on the next page click on the specific game; on the next page click
on pdb; on
the next page click on pitchers. For
the
(b)
The
above steps take you to a page with a bunch of links that are of the form zzzzzz.xml,
where zzzzzz
is a six-digit code for a specific pitcher (see section III). For the above game, click on 122201.xml, which will get you to the
pitch logs of Paul Shuey, who pitched to two batters
in the 7th inning.
(c)
You
will then see a lot of numbers on the screen. Use whatever tools you have with
your browser (e.g., "save page as") to save it as 122201.xml in some convenient folder.
(d)
Now
launch Excel. From the File menu,
open the file you just saved. An Open
XML box will pop up. Check the As an XML list box,
then click OK, and the file will
load. You should see columns A through AK (37 columns total) filled and with
headers in the first row. Immediately save is as an Excel file.
II. What do the
columns mean?
Only
the columns that are not obvious from the context will be described. Before
proceeding, some definitions are needed. A coordinate system is used in which
the origin is at the point of home plate. The y-axis point from catcher to pitcher;
the z-axis points vertically up; the
x-axis points to the catcher's
right. Using this notation, the pitched baseball is traveling in the -y direction. Due to gravity, it will fall so that z generally decreases along the trajectory.
id: six-digit
code identifying the pitcher (see section III).
batter:
six-digit code identifying the batter (see section III).
pitcher:
same as id.
x, y: x and z (note the change of notation in that the parameter y refers to the z coordinate) location of pitch as
it crosses the front of home plate. The units are in camera pixels and are
therefore not very useful. I recommend ignoring these two parameters and
instead use p_x
and p_z,
defined below.
start_speed:
speed of ball in mph at the starting position (defined below).
end_speed: speed
of ball in mph as it crosses the front of home plate, located 1.417 ft from the
point of home plate (i.e., at the coordinate y=1.417). Note the end_speed is less than start_speed
due to the effect of air resistance.
sz_top, sz_bottom:: a line of constant z
(in ft) defining the lower and upper limits, respectively, of the strike zone.
That is, these are the height above home plate of the top and bottom of the strike zone. Currently, these parameters are set for each batter by the operator by visually
observing the image from the center-field camera.
pfx_x,pfx_z: The deviation (in inches) of the pitch trajectory from a straight-line in the x (horizontal) and z (vertical) directions between y=40 ft and the front edge of home plate, y=1.417 ft. It is important to note two things. First, the initial value is y=40 ft, regardless of the value of the initial value y0 (defined below). If the pitcher's release point had been used (approximately y=55 ft), then the deviation would have been nearly twice as large. Second, the effect of gravity has been removed from pfx_z, so that both parameters are the "break" of the pitch due to the Magnus force on a spinning baseball. Note that the online Gameday reports the quantity pfx, which is presumably the square root of pfx_x2+pfx_z2. Given our sign conventions, a positive value of pfx_x cooresponds a deviation to the catcher's right and a negative value to the catcher's left. Similarly, a postive value of of pfx_z is a pitch the drops less than it would from gravity alone (most pitches fall in this category), whereas a negative value is a pitch that drops more than from gravity alone (e.g., a "12-6" curveball).
p_x, p_z: location of pitch in the
x and z coordinates, respectively, as it crosses the front of home plate, in
units of ft. When you watch Gameday, this is the
location of the dot on the screen that appears for each pitch. It is computed
from the tracked trajectory. There is a one-to-one correlation between p_x and x and between p_z and y (see above description of x
and y).
x0,y0,z0, vx0,vy0,vz0,ax,ay,az: These parameters are the most important ones in the database, since all others are computed using these. The parameters represent the result of making a least-squares fit to the measured trajectory assuming constant acceleration, for each of the three dimensions. The first three parameters are the initial positions in ft, the next three are initial velocities in ft/s, the next three are the accelerations (assumed constant) in ft/s2. To calculate the full trajectory, use the formula x(t)=x0+vx0*t+0.5*ax*t2, and a similar formula for y(t) and z(t), where t is the time. Note that y0, which is the distance from the point of home plate where the tracking begins, is 50 ft for the file discussed above although earlier files used 55 ft or 40 ft. This point is the location where the start_speed is determined and from which the break parameters (see below) are calculated.. Note that start_speed is just the square root of
vx_02+vy_02+vz_02, converted to mph.
break_y,break_angle, break_length: These quantities
refer to a different definition of "break" than the
quantities pfx_x, pfx_z defined
above. They are arrived at as follows. A straight line is drawn
from the starting location x0,y0,z0 to the final location defined by p_x, p_z and
y=1.417 ft. Such a line determines a straight-line
trajectory from starting point to ending point. That trajectory
is then compared to the actual trajectory determined by the
constant acceleration fit to the data. The quantity break_length is
the largest deviation, in inches, of the actual from the
straight-line trajectory. The quantity
break_y is the y-distance
from home plate where the maximum deviation occurs. The
quantity break_angle is the direction of the deviation, with the
convention that a pitch that breaks away from or toward a RHH has
a negative or positive angle, respectively; a
break_angle of 0 is a pitch with no horizontal break and is
typical of a straight fastball. Most pitches will have a break_angle
between about -50o and +50o. An angle greater than 90o in absolute
value is almost impossible, since that would imply an upward
break, defeating gravity (the myth of the "rising
fastball"). Both break_length and
break_angle are shown on the Gameday screen. Also the break_angle is indicated with an
arrow that points straight down for 0o, down and to
the left for a positive angle, or down and to the right for a
negative angle.
III. Identifying pitchers and batters
If you go to the site with the specific game whose pitch logs you are downloading, http://gd2.mlb.com/components/game/mlb/year_2007/month_08/day_01/gid_2007_08_01_balmlb_bosmlb_1/,you will see a bunch of links. Click on players and you will see a players.xml file containing the names of the players on the rosters of each team and their associated 6-digit numerical code. You should "save page as", then load it into Excel and save it for future reference.