Aircraft
The fundamental purpose of the aircraft in trench warfare was reconnaissance and artillery observation. Aerial reconnaissance was so significant in exposing movements, it has been said the trench stalemate was a product of it. The role of the fighter was to protect friendly reconnaissance aircraft and destroy those of the enemy, or at least deny them the freedom of friendly airspace. This involved achieving air superiority over the battlefield by destroying the enemy's fighters as well.
Spotter aircraft would monitor the fall of shells during registration of the artillery. Reconnaissance aircraft would map trench lines, first with hand-drawn diagrams, later with photography, monitor enemy troop movements, and locate enemy artillery batteries so that they could be destroyed with counter-battery fire.
Read more about this topic: Trench Warfare, Weapons of World War I Trench Warfare