Sheridan believed that understanding the topography of the Shenandoah Valley would give him a strategic and tactical advantage. By the summer of 1864, Meigs was recognized as one of the best mapmakers in the Union Army. At his base in Harpers Ferry, West Virginia, he spent hours being tutored by Meigs on the layout of the valley. From Meigs, Sheridan learned that the long ridges in the Shenandoah were not the great protective barriers Union commanders had assumed. The ridges had a number of gaps. Confederate forces could ride north and south, screened by the ridges, and then turn through the gaps to initiate flank attacks on Union forces. Sheridan resolved to protect his flanks from these attacks, and take advantage of the gaps himself.