Ballpark figure

[HaraKiriBlade]

I know it means a rough figure, and I also know it alludes to a baseball field. But what about baseball field that made the term mean what it means?

Thank you very much,

HKB

[mykwyner]

I'm guessing, but the sense I get is that a hit baseball is either in the ballpark or out of it. If the ball is in the park, then it's playable. A ballpark figure is one that is inside the ballpark, therefore within the range of usefulestimates.