Map 2. Planning map for heavy howitzer barrage, Messines Ridge area, 7 June 1917. Ahead of a creeping barrage, medium and heavy howitzers laid down a lifting barrage. This map shows the planning of such a lifting barrage and how deep a barrage went by mid-1917. A map for the previous day is exactly the same. This barrage was fired on the day of the attack, but the previous day an identical barrage was fired to soften the Germans up even more than the preliminary bombardment, to inflict casualties, to test British preparations, and to confuse the Germans about when the attack was actually going to be launched.
Source: Rawlins Papers, RAI.