The battle began with the detonation of 19 mines beneath the German front position, which devastated them and left large craters. A creeping barrage, deep began, protected the British troops as they secured the ridge with support from tanks, cavalry patrols and aircraft. The effect of the British mines, barrages and bombardments was improved by advances in artillery survey, flash spotting and centralised control of artillery from the Second Army headquarters. British attacks from 8 to 14 June advanced the front line beyond the former German (Chord Position, Oosttaverne Line to the British). The battle was a prelude to the much larger Third Battle of Ypres, the preliminary bombardment for which began on 11 July 1917.