 Like a migratory bird, Jim holds a 'map' of the swampland in his mind, whilst also seeing the detail of grass, undergrowth and water.A flight in Ashley's biplane gives him a view of the landscape which confirms his mental map.Later, in the trenches, he seems to go out of himself and see the battle as a map - while he is present in the mud and heat of battle, part of his perception observes, detached, from above.