From Wikipedia

Four Years Of Thunder

Episode Two

The initial campaigns of 1914 proved that cavalry could no longer provide the reconnaissance expected by their generals, in the face of the greatly increased firepower of Twentieth century armies. It was quickly realised, on the other hand, that aircraft could at least locate the enemy – even if early air reconnaissance was hampered by the newness of the techniques involved. Early scepticism and low expectations quickly turned to unrealistic demands beyond the capabilities of the primitive aircraft available. Even so, air reconnaissance played a critical role in the "war of movement" of 1914, especially in helping the Allies halt the German invasion of France. On August 22, 1914, British Captain L.E.O. Charlton and Lieutenant V.H.N. Wadham reported German General Alexander von Kluck’s army was preparing to surround the BEF, contradicting all other intelligence. The British High Command listened to the report and started a withdrawal toward Mons, saving the lives of 100,000 soldiers. Later, during the First Battle of Marne, observation planes discovered weak points and exposed flanks in the German lines, allowing the allies to take advantage of them. The Germans' great air "coup" of 1914 (at least according to contemporary propaganda) was at the Battle of Tannenberg in East Prussia where an unexpected Russian attack was reported by Lts. Canter and Mertens, resulting in the Russians' being forced to withdraw.

Late in 1914 the lines between the Germans invading France and the Allies stretched from the North Sea to the Alps. The initial “war of movement” largely ceased, and the front became static. Three main functions of short range reconnaissance squadrons had emerged by March 1915.

The first was photographic reconnaissance - building up a complete mosaic map of the enemy trench system. The first air cameras used glass plates (“Kodak” cellulose film had been invented, but did not have sufficient resolution).

Artillery “spotting” – enabled the ranging of artillery on targets invisible to the gunners. Radio telephony had not been invented, so communication was a problem. By March 1915, a two seater on “artillery observation” duties was typically equipped with a primitive radio transmitter transmitting the clicks of a Morse key, but had no receiver. The artillery battery signalled to the aircraft by laying strips of white cloth on the ground in prearranged patterns. These duties were shared with the observation balloon, tethered to the ground. Balloonists could communicate directly with their batteries by field telephone, but were obviously far less flexible in locating targets and reporting the fall of shot.

“Contact patrol” work (called Infanteriefliegerdienst by the Germans) attempted to follow the course of a battle by communicating with advancing infantry while flying over the battlefield. The technology of the period did not permit radio contact, and methods of signalling were necessarily crude, and included dropping messages from the aircraft. Soldiers were naturally reluctant to reveal their positions to aircraft, as it was difficult for them to distinguish between friend and foe.