The Astros are getting rid of Tal's Hill, the exceptionally weird incline/flagpole combination in play deep in center field at Minute Maid park. My girlfriend, who is not a baseball fan, was stunned to find that baseball parks don't all have the same dimensions, and it truly is one of the strangest and most wonderful aspects of the sport. I don't think I liked Tal's Hill – as Emma Baccellieri wrote at Baseball Prospectus, it was a quirk manufactured to be a quirk, and a dangerous one at that – but it was the most extreme example of a general tendency that I greatly value. The difference between it and the Green Monster is one of degree and origin, not category.
But Tal's Hill is gone, and it's been mourned sufficiently elsewhere. I wanted to look at the practical impact its removal will have on Minute Maid Park, since Tal's Hill was not just a monument but an actual, in-play piece of the park that changed what happened during games played in Houston.
First, we have to decide on the scope of our inquiry. Based on the reports on the upcoming renovations, it seems like the fences will be built along the edge of the hill, where the warning track currently runs, putting them roughly 409 feet from home plate at their deepest point. Using Google Earth and some tenth-grade trigonometry skills, I set the boundaries for the balls that were impacted by Tal's Hill at a lateral angle of -7 degrees to 10 degrees and at least 400 feet from home, then used Baseball Savant and Statcast to find all balls that fit those parameters. 400 feet is a little bit short, but I wanted to err toward being overinclusive. Here's the shortest hit in the sample that had a video clip associated with it, a 401-foot triple by Evan Gattis:
You can see that it doesn't exactly make the Hill proper, but I think it's fair to say moving the fences in to that secondary warning track would make that hit play differently.
With those boundaries set, we get 64 batted balls of this type in 2015 and 2016 at Minute Maid, and 2,001 in the rest of the league (excluding some weird things like fan interference). The results of these batted balls in Houston are, unsurprisingly, very different from those elsewhere:
Way fewer home runs, way more triples, and way more outs. With the fences in their new position, Houston is going to look like almost every other park, and a ball hit more than 400 feet to dead center is either going to clear the fence or bounce off it, instead of being caught or pinballing around.
Minute Maid Park, in its former composition, was a nearly neutral offensive park. Per FanGraphs, in 2015, it ranked 21st leaguewide with a park factor of 98 (where 100 is a league-average park). It looks like almost all of that (relatively minor) offensive suppression came on strikeouts, however, where Houston had a park factor of 103, tied for the largest in the league. On the park factors that relate to Tal's Hill, Minute Maid was already above average, with a home run park factor of 105 (9th), a triple park factor of 108 (8th), and a double park factor of 100 (10th). Moving in the fences in deep center – one of the only places where Minute Maid takes away hits rather than adds them – is likely going to push Houston's overall park factor over 100.
Horizontal lines show average run values of hits of that type; vertical bars show average run values of batted balls to deep center in Houston and the rest of baseball.
So while Minute Maid Park is becoming more traditional in its layout, it's likely going to change from a roughly average park to an offense-friendly one. There's a reason to think this change might not be motivated only by aesthetic concerns on the part of the Astros, or at the very least, that they might stand to benefit from the shift.
As I mentioned above, there were 64 balls hit to Tal's Hill in 2015 and 2016, but they weren't distributed evenly. Astros hitters were responsible for 37 of them, while Astros pitchers only gave up 27, roughly a 60/40 split. Expanding the scope to include away games shows that Astros pitchers gave up 62 deep hits while Astros hitters had 74, and that difference is the sixth-largest in the majors. If this trend continues, or if Houston tries to play it up by acquiring certain pitchers and hitters, their offense will be helped more than their run prevention will be hurt by the lack of Tal's Hill, and batted balls that used to become outs, doubles, and triples turning into home runs.
In 2015–16, the Astros starting rotation had a 47.1 percent ground ball rate, the seventh-highest in the majors. Most prominently, Dallas Keuchel was the owner of a 59.5 percent ground ball rate in that two-year period (the third-highest among pitchers with at least 200 IP), but Lance McCullers (50.5 percent, 67th-highest) has also demonstrated some ground ball tendencies.
While the bullpen as a whole has been less inclined toward ground balls, two of Houston's top relievers are Will Harris, who had a 58.0 percent ground ball rate in 2016, and Luke Gregerson, who induced ground balls 60 percent of the time. Both will be with the Astros through 2017, and Harris for much longer.
On the other side of the ball, Houston's offense had the fourth-highest ratio of fly balls to grounders from 2015–16. That said, the players most responsible for the trend – Colby Rasmus, who had the seventh-highest ratio over that span among players with at least 500 PAs, and Luis Valbuena, who had the 21st-highest – aren't guaranteed to be back with Houston next year. Still, the Astros acquired both those players in the last few years, perhaps already trying to play to their park's inflation of home runs and demonstrating both the ability and willingness to take advantage of their dimensions going forward.
This kind of thing isn't new. Teams with parks that lean in one direction or the other keep that in mind when building their roster, though they range from the successful (the Red Sox and Chris Young, or any of the other players picked for their ability to bounce balls off the Green Monster) to the unsuccessful (the Rockies and every one of their failed pitching experiments). Tal's Hill wasn't the kind of feature Houston could plan for, and its removal will almost certainly turn their park from a neutral one into an offense-friendly bandbox. That's certainly something Houston can try to take advantage of, and based on their roster over the past couple of years, they might be planning to already.
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Henry Druschel is a Contributing Editor at Beyond the Box Score. You can follow him on Twitter at @henrydruschel.