Surprise was essential for creating maximum disruption, so Bruchmüller adopted the Pulkowski Method, for bombardments without the customary registration fire. The position of each gun was surveyed. Knowing the muzzle velocity of the gun and taking into account variables like air temperature, wind velocity and direction, and using tables provided by mathematicians, it was possible to fire fairly accurately at targets on the gunnery maps. The Germans concealed their attack preparations but their initial target data had to be precise. (The British had fired from the map in their assault at Cambrai on 20 November 1917.)