 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 . 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 . 
 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 <a href='uninflect' title='1'><mark>were</mark></a> 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 <a href='uninflect' title='2'><mark>were</mark></a> 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 <a href='None' title='3'><mark>the</mark></a><a href='negative_concord' title='4'><mark>weight</mark></a> of tanks . Twelve Armoured Vehicle Royal Engineers ( AVREs ) <a href='uninflect' title='5'><mark>were</mark></a> 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 . 
The Allied Expeditionary Air Force undertook ova 3 @,@ 200 photo reconnaissance sorties from April 1944 untill the start of da invasion . Photos of the coastline was taken at extremely low altitude to show the invaders tdaterrain , obstacles on the beach , and defensive structures such as bunkers and gun emplacements . Inland terrain , bridges , troop emplacements , and buildings was 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 thdahore between Asnelles and La Rivière was soft and could not support no weight of tanks . Twelve Armoured Vehicle Royal Engineers ( AVREs ) was fitted with bobbins to overcome this problem by deploying a roll of matting oovathe soft surface . The material would then be left in place to create a route fa more conventional tanks .