Since you have ancestors so heavily involved in Day Two of the Battle of Gettysburg, let me suggest a book to answers your questions. If you can only afford to spend $10 or less on a book about Gettysburg, here's where you spend it --
http://www.amazon.com/Gettysburg-Second-Day-Harry-Pfanz/dp/080781749X

Pfanz was a park historian at Gettysburg for many years. Not only does he place your ancestors exact position at any given moment, Pfanz also explains what Federal units were shooting at them, at least the ones your ancestors could see nearby.

For what it's worth, my parents submitted to persistent pleas to revise their vacation plans in Washington DC and drive to Gettysburg for one day. It snowed during the night, and the battlefield was covered in white. I took lots of pictures at Gettysburg that morning, including one of the Florida monument in the snow.

When the pictures were developed, I took them to high school to show friends. Someone saw the name 'FLORIDA' on the monument surrounded by white and asked if I enjoyed my trip to the beach.