The technological limits of surveillance during the American Civil War dictated that commanders often decided where to deploy their troops based largely on what they could see. We know that Confederate general Robert E. Lee was virtually blind at Gettysburg, as his formerly brilliant cavalry leader J.E.B. Stuart failed to inform him of Federal positions, and Confederate scouts’ reconnaissance was poor. The Confederates’ field positions, generally on lower ground than Yankee positions, further put Lee at a disadvantage. A striking contrast in visual perception came when Union Gen. Gouvernour K. Warren spotted Confederate troops from Little Round Top and called in reinforcements just in time to save the Federal line.

More: A Cutting-Edge Second Look at the Battle of Gettysburg

The interactive map looks quite good. It’s important to remember that next week is the 150th anniversaries of the twin events that said the Confederacy was doomed - Meade’s trouncing of Lee at Gettysburg and the fall of Vicksburg to Grant.