One Time,
One Place.
Highly accurate military intelligence was gathered and illustrated in trench maps. How accurate they are can be seen by comparing them to the aerial photographs. The trenches shown in bold are German; showing the enemy positions was the purpose of such a map. The British trenches were usually not shown. In this case they can be seen - faintly drawn - opposing the enemy line.

It is possible to measure how close the battle lines are. A side of one of the numbered squares is 1000 yards long. The sides of these squares are divided into intervals of fifty yards.

Although the date of the map (left - click to enlarge) is 1917, this was the position of both sides at the end of the 2nd battle of Ypres. This battle line remained static for years. [SOURCE : In Flanders Fields Museum]
Official Trench Map of Ypres Salient
Left. A Map/diagram showing the ground taken by the Germans in the gas attack
Below. The Official War Office map/diagram showing the positions of both sides on the day in question.(Museum of the Northumberland Fusiliers)
Below. A copy of the actual document of the 6th Battalion Diary for the 26th of April, with an officer's sketch map indicating the start position of the Battalion in the attack that afternoon. (Public Records Office)
April 26th, 1915
Mousetrap Farm