Statcast defines different types of batted balls based on launch angle (LA) http://m.mlb.com/glossary/statcast/launch-angle. They call under 10 degrees a grounder, 10-25 a liner, 25 to 50 a fly ball and over 50 a pop-up. Those are not new terms, of course, those definitions have existed forever. Merriam Webster uses this definition for a line drive https://www.merriam-webster.com/dictionary/line%20drive.
To evaluate the boundaries I first looked at some characteristics of batted ball types using the boundaries that Statcast uses: https://imgur.com/a/wlWbsNE
The categories were BABIP, ISO and BA (used BA instead of BABIP to include homers) dependency on EV. You can see that “grounders” under 10 degrees have a BABIP of around .280, a very low ISO and a steady positive relationship of EV and BA. Liners have a very high BABIP, a high ISO of .435 and a relatively low impact of different EVs. On fly balls (25-50 degrees) you have a very low BABIP, a very high ISO and you have the “donut hole” where you have the bloopers on very low EVs, mostly outs at medium EVs (80-95) and then again (extra base)-hits at high EVs.
To test the existing boundaries I now did the same tests with other boundaries.
Under zero degrees the BABIP is mostly under .200 except for very hard hit balls where it is around .300. From 0 to 5 and 5 to 10 that changes, at low EVs the BA on contact is low and at medium and hard contact it gets pretty high (around .500 and higher). That means those batted balls around 5 degrees behave like grounders at low EVs and like liners at medium and high EVs.
At 20-25 degrees the BA on contact is .682 on soft contact, just .251 on medium contact and around .700 on hard contact which is about the same as fly balls. That means balls hit at those angles behave like a liner on soft contact and like a fly ball on medium and hard contact.
I also compared the range of 5-20 with the 10-25 range and an alternative range of 5-20 https://imgur.com/a/qjaVrIP
What you can see is that the 5-20 range matches more closely with the “core line drive range” of 10-20 than the 10-25 range.
Overall it is not totally clear what is better. Neither range is perfect as both edge ranges (5-10 and 20-25) are more velocity dependent than the “core range”. The lower edge behaves like a grounder on soft contact and the upper range behaves more like a fly ball on medium and hard contact showing the famous donut hole.
IMO there are a lot of reasons to either narrow liners to the 10-20 range or alternatively use 5-20 if you want to keep the same angle range because soft contact is only 20% of all MLB contact. That means on 80% of all contact 5-20 behaves like a liner and on 20% like a grounder. The 20-25 range, however, behaves like a fly-ball 80% of the time and just 20% like a line drive.
So the changed ranges would be either:
You could also introduce additional batted ball types to make it more precise as Andrew Perpetua did here but I think the easier solution would be to cut off the upper 5 degrees of the original range because that range behaves like a FB on most batted balls.