This question actually encompasses the way all artillery was aimed during the conflict. From what I can gather, the equipment and troop movements of ww1 were heavily hampered by communication systems. I know telephones existed but in a very rudimentary kind of way. So what I'm asking is, how did the troops on the front line tell the artillery where to shoot?
Was it all based on a grid system? That's the only way i could think it would work. And how was the location of targets relayed to the artillery miles behind the line? I guess its just hard for me to imagine how to accurately aim a gun when you are miles away from the target and you have spotty, at best, communication with the front lines. Courriers would take a long time to get back to the artillery, and by the time they did targets could have changed position.
This is based on 1923 regulations of Finnish artillery, the regulations were based on mostly Russian but also German and French WWI artillery methods. The American and British terminology is probably different, as well as firing methods.
In principle, the battery commander would be the spotter. The artillery spotter would be somewhere between the target and guns where he can see both and use trigonometry to establish the battery-target line. The spotter would give the battery-wide firing solutions or to the battery via telephone line. Each gun in the battery had known compensation. In practice the firing solution would be initially very rough guess: distance based on map, direction by theodolite used by spotter. The proper solution for target was found by bracketing with a single gun. Once the solution was found for a certain target, it was possible to extrapolate to other targets.
During the WWI the methods were developed and the roles of battery commander and forward spotter were separated. Instead spotter always doing the calculations for firing solution, the regulations mention the battery plane (artillery board for battery) as an alternative method. The plane was large piece of coordinate paper where the known points (observation spot, battery, main targets) were marked and calculations could be done. Both the battery and spotter(s) had their own copies of plane. That would be probably known as battery grid. However, regulations warn against using effective fire using only the board and the observer should always establish a bracket with bracketing fire.
When the element of surprise would be canceled by bracketing the offensive fire could be directed using only the board. The wind and weather could be compensated by calibration shots to well known targets. The initial targets of offensive fire were usually behind the front lines so the errors could be compensated before firing targets closer to own troops.