Have a look at the Gun position he shows. He does say drawn from memory, but he wasn't there at the battle, just that evening and later when the battlefield was revisited. So rather than drawn from memory it should be drawn from information supplied. The most interesting thing about Noggs map, for me, is the note at the bottom, South of the road, 'lots of dead found here'. that he would have personally witnessed, its also east of Durnfords last stand.