 But the triangulation continued; and, when Farragut moved his fleet up the river on the 15th, charts prepared by the Coast Survey guided each of Porter's vessels to a position from which it could fire accurately at one of the forts while suffering minimum exposure to enemy guns.On the morning of the 18th, when the schooners began bombarding the forts, each gunner knew to the yard the distance from his mortar to the target.In the days that followed, whenever a vessel changed its position, an officer of the Coast Survey would immediately calculate the new distance for her gunners, enabling them to resume their extremely accurate fire.