Atlas Obscura has an overview (pun intended) on aerial Reconnaissance Photos of WWI Battlefields:
Aerial photography was a relatively new technique at the outset of World War I. Over the next four years, it became an invaluable reconnaissance tool. Cameras were developed for aerial use, but the process was still not easy. To note enemy positions accurately, clues in the resulting images, like shadows and soil displacement, had to be interpreted as well.
As the book The Great War Seen From the Air recounts: “They took photographs from an open cockpit. The exposure had to be set manually and after every shot the glass negative had to be replaced and stored away. Meanwhile, the observer photographer combed the skies for enemy planes looking for observation planes to shoot down.”