Below is a contemporary illustration of the Union Grounds by Cincinnatian and content producer for the Reds HOF and Museum, Cam Miller.
QueenCityHistory
The Illustration is missing the roof over the seating along the left field line, but he did include the often-mentioned gate in right field.
It appears he's going off of the 550ft square estimate on the Union Terminal Historical marker. My original estimates of 360ft - 400ft square came from:
- A) The fact I had assumed the park was bound by streets, which we've since discovered weren't really there during the time the Union Grounds were still standing.
- B) BR Reference stating that the foul polls were only 150ft beyond the bases (240ft from homeplate).
At this point, I don't think anyone really knows the true dimensions of the field. It is entirely possible that a photo or accurate illustration of the park is out there somewhere sitting in a safe or an old chest in someone's attic but, until such a thing is made available to the public, I don't think we'll ever really know the true dimensions and full nature of the park.