What is the point of having a 10-yard chain to measure first downs in football? the field is very well marked… all the way across every 5 yards and smaller lines every yard. why don’t they just have one pole where the first down line is? if the ball is at the 20 and 1/2 yard line, and the first down is at the 30 and 1/2… why is it more accurate to put a pole on the 20 and 1/2 and then stretch a chain? why can’t they just measure a half a yard from the 30? it seems like an inexact science either way you look at it… the field is really wide… you can’t eyeball it down to the inch with that pole when you are standing in the middle of the field… and if you could… why wouldn’t you just go eyeball where the ball is… the whole double pole system just seems superfluous to me… does anyone know more about this?
perhaps it’s best use is for when the ball doesn’t lay near a hash mark or yard line.
You pick up the chain on one of the long yard lines, run on to the field, lay it down at that spot, pull both ends tight, and you can measure if you’re at the 30yd 1 ft 6 in line or the 30 yd 1ft 5in line easier than you could with one pole.
No, it’s quite exact. The chain is exactly ten yards. The officials indicate where the ball is spotted on first down, so they know that the second pole is exactly ten yards away.
When it comes to a measurement, the official holds a link that crosses one of the yard lines. They go out onto the field and put that link exactly on that line. This ensures an accurate measurement. It’s also a lot quicker than measuring half a yard or whatever.
And even if it seems superfluous to you, why should it matter at all?
To get an exact end point of ten yards one must have the exact beginning point. Of course that’s not the case in football. And by rule, the ball is down exactly where the referees say it is, but that’s not really exact either.
The chain also ensures that the distance measured is always a constant. If you’re measuring 1 1/2 yards, then 6 3/4 yards, then 2 14/93 yards, it could be hard for everyone to be sure that the right distance is alway being measured.
And hash lines have width. From where in them do you start the measurement?
Constancy is a necessary part of any sport. Look at all the trouble baseball has gotten itself into over the past few years by allowing each umpire to set up his own strike zone.
One reason for the chain is to ensure that the first-down pylon gets put back in the right place should it (or the guy holding it) get knocked or chased out of position by a play.
When a team gets a fresh set of downs, the Head Linesman (I think) oversees the placement of the chains. He eyeballs the current spot of the ball, and orders one of the pylons to be placed there. The chain is then stretched taut, and the second pylon plopped down. The HL then attaches a small clip-on marker to the chain where it crosses a marked yard line (probably a specified edge of a marked yard line, as I believe they’re 4 inches wide).
Now, should the chain gang get disrupted for some reason, the chains can easily be put back pretty much exactly where they had been.
This is also really handy when doing on-field measurements; put the clip on the yard line once you get out there, and you’re good (subject to a cosine error if the chain isn’t precisely parallel to the sideline 
).
Now, it doesn’t take a genius to identify several sources of error in all of those steps - some of them potentially pretty large. So how on earth do they justify doing millimeter-perfect measurements when it comes time to decide if a first down has been obtained?
My theory: because you need a defined line, no matter how imprecisely it may have been obtained.
From the start of a set of downs, the official line-to-gain has been sitting there on the sidelines for all to see. You Must Get HERE For A First Down. Perhaps it’s only 9.5 yards from the initial spot; perhaps it’s more like 10.5 (although I think it’s rarely that far off). We don’t know for sure, and we’re only so inclined to care because it doesn’t really matter. The line has been drawn, and is known to all.
Eyeballing the initial placement of the chains from across the field is considered acceptable. Eyeballing the ultimate first-down-or-not decision is not - can you imaging the number of controversies that would result? It’s a practical decision, more than a scientifically correct one.