The Allied Expeditionary Air Force (AEAF) 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. 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. At Gold, frogmen discovered the shore between Asnelles and La Rivière was soft and could not support the weight of tanks. Twelve Armoured Vehicle Royal Engineers (AVREs) were fitted with bobbins to overcome this problem by deploying a roll of matting over the soft surface. The material would then be left in place to create a route for more conventional tanks.