WW1 Army Map Artillery Grid System (with examples of Shoots)
The map below is also contained in my book, For God England and Ethel and gives two examples of artillery 'shoots' carried out by 6 Squadron in its time serving on the Western Front and the way in which enemy positions (or targets) could be recorded by the observer in an aircraft using a common mapping grid system and transmitted by wireless to the artillery on the ground - to an accuracy of plus/minus five yards. By flying between the battery and the target, the observer in the aircraft could then watch where the shells fell and transmit aiming corrections by means of a clock code system until the target is destroyed. There are many internet sites that explain the principles of the Clock Code, so I won't go into too much detail here.
If you imagine the target as being a bullseye at the centre of eight concentric circles (Y - 10 yds, Z - 25 yds, A - 50 yds, B - 100 yds, C - 200 yds, D - 300 yds, E - 400 yds and F - 500 yds) then the observer would transmit the position of the shell burst in relation to the bullseye with the appropriate letter followed by the angle as depicted by the hours on a clock. For example, if the shell burst 70 yards to the west of the target, the observer would transmit to the battery his Squadron's call sign (J - 6 Squadron) followed by his personal number within the squadron (eg 11) followed by the battery call sign (eg AR2) followed by the correction (in this case B9) - the complete message being J11AR2B9.