 The Allied Expeditionary Air Force undertook over 3 @,@ 200 photo reconnaissance sorties from April 1944 until the start of the invasion . <unk> 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 <unk> <unk> <unk> prepared detailed harbour maps , including depth <unk> . At Gold , <unk> 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 <unk> to overcome this problem by deploying a roll of <unk> over the soft surface . The material would then be left in place to create a route for more conventional tanks . 
