The Smithsonian magazine online has a map featured that shows the battlefield of Gettysburg, reconstructed 19th century maps and photographs. At decisive moments in the battle, you can go to a panorama view to observe the battlefield from the perspective of the commanding officers on each side. You can see what information the generals acted on--what they could see, and more importantly what they could not see from their positions in the field.
Link here.
Link here.