Doyle --

Regarding your statement,
"The difference at Gettysburg was that the 11th Corp were in position and had chosen their ground well."

Gettysburg was a meeting engagement in which units came off the road and into battle wherever needed. The position defended by the Federal XI Corps was simply the best that could be made of a bad situation. Had the battle developed in a more orderly manner, that line outside Gettysburg would never have been selected for defense against Ewell's advance.