How precisely do they resolve first downs in football?

This probably a dumb question, but…

In football, when the ball is close to the first down point and it’s too hard to call out on the field, they drag the 10 yard posts out onto the field.

Now, here’s my question: Do the referees just run out there, hoping to run as perpendicular to the football field as they can, and measure it? It seems like a tiny bit of tug in either direction could throw it off.

Or, probably more likely, but I don’t know for sure - at the end of every down, do they round off the ball position so that it’s always exactly on one specific yard line? In that case, do they drag the poles out, align the rear one with the tick mark for the official start yard line, and take it from there? In that case, it would seem that you’d be able to judge whether the ball was over just by comparing it to the 10th tick mark away from the line of scrimmage.

The chains have two poles, one on each end. In between those two poles is a marker (probably a spike) that can move along the chain. They put that marker directly onto one of the yardage markers (the white lines.) The chains are then pulled tight, moving outward from there.

Paraphrasing from David Feldman’s book, DO PENGUINS HAVE KNEES (Harper Collins 1991), quoting the NFL’S

When the first down is achieved, the back stake of the chain is established at the point-of-possesion (the responsibility of the head linesman official), then a metal clip is clamped to the chain at the back-end of the yardline wthing the chain range, closest to the point-of-possesion.

Example, after a kickoff return, the point-of-possession is established by the head linesman at the defense’s 23.75 yard line. The back stake is placed there, and the “clip” is clamped on the chain at the back-end of the defense’s 25 yard line. Whenever a measurement is required, the chain gang runs out and aligns the “clip” to the back end of the defense’s 25 yard line (which conveniently runs the entire width of the field). Then the chain is stretched.

Essentially that is what they are doing. They mark the chain at the yard line (tick mark[?]) and when that mark is lined up they pull the chain tight. Khadaji was saying the same thing. I just thought it may be clearer to use a quote from you to tie it in.

The clip used is a wheel that turns to tell you which yard line the chain is clipped to. If I remember right it is only has numbers for the 5s (5 15 25 35 & 45) since the chain will always be crossing one of these lines.