You need to keep in mind the distances here. For calibrating your rifle you're shooting at targets several hundred yards away, making it much harder to tell if the area is clear. This also isn't happening on a football field, the area around the trenches is going to be scored with secondary trench lines, foxholes, communications trenches, big craters from artillery, etc. And the people in there aren't exactly trying to make themselves known, they are going to keep low and you may not even be visible if they are in a little offshoot of a trench or something. The secondary lines are also going to be relatively densely populated. Odds are if you try to shoot at a target a couple hundred yards behind the trench lines, the round is going to be within a dozen yards of someone's head. You probably wouldn't hit anyone, but if someone stands up at the wrong moment... an even bigger danger is if your shot goes high. If your shoot goes above the rock you're aiming for, it will keep going further back into the lines. Given that this would be 30 caliber ammo and there's no body armor, that bullet would still be going fast enough to kill someone 1.5 miles out, maybe 2 miles. Someone very familiar with the area and with good situational awareness might be able to guarantee there's no one between them and the target, but not for a mile out.

The other big factor is what your buddies in the trench will think of this arrangement. If they know it's you shooting over their heads they will be absolutely pissed, and probably face some sort of discipline from officers. If they DON'T know it's you shooting in their general direction (which they probably won't unless you make sure to tell everyone in advance) it will probably prompt return fire. If they can tell from the sound or muzzle flash that the shot came from their trenches, they may think a sniper infiltrated their lines, which will merit a particularly aggressive response.

I wouldn't be surprised if someone, somewhere during the war did this, but it certainly wouldn't be condoned.

> Why can't you have the situational awareness that nobody is there just by looking with your eyes?

The entire setup of an army, all its communication mechanisms, and systems of control and safety measures, are focused on telling you about what is happening forward along the axis.

Trying to figure out if it's safe to fire backwards is going to be massively error-prone as nobody is set up to tell you this and asking it is going to cause massive confusion.

Also, you can hit things that you can't see, with ricochet and fall of shot.

Hmm, I'm not militarily experienced obviously, so I'm not going to push this any more. I guess my last statement is that I believe you're correct, but it still seems like the creation of an additional system to safely designate a calibration target would be beneficial, especially to the side. Giving away your position sounds more error prone.

Other comments about what calibrating actually means seem to have filled in that missing intuition though.

Are your questions also coming from a standpoint of modern tech. such that a sniper could accurately measure range using a modern rangefinder?

This was circa the 1916-1918 time-frame. Modern range finding equipment did not exist, and what did exist at the time very likely required someone carrying a rather large range finding target over to the enemy lines to use to determine range (with the obvious negative result to that poor individual).

With no easy way to determine range (distance to target) beyond eyeball estimates, the only real way to sight in the sniper rifle was by taking test shots at a target and watching where one's bullet actually lands as compared to that target.

As mentioned in TFA, the preferred target for sighting was a puddle of water.