The Allied Expeditionary Air Force undertook over 3,200 photo-reconnaissance sorties from April 1944 until the start of the invasion. Photos of the coastline were taken at extremely low altitude to show the invaders the terrain, obstacles on the beach, and defensive structures such as bunkers and gun emplacements. To avoid alerting the Germans as to the location of the invasion, this work had to be undertaken over the entire European coastline. Inland terrain, bridges, troop emplacements, and buildings were also photographed, in many cases from several angles, to give the Allies as much information as possible. Members of Combined Operations Pilotage Parties clandestinely prepared detailed harbour maps, including depth soundings.