Before seeing the link I was imagining a beam visible in the air spanning from sideline marker to sideline marker. It takes an amazing amount of power for a beam to be visible in the air. Projecting a laser onto the grass is a entirely different kettle of fish. Hence my asking for a link.
It’s actually the outer edge of one post to the inner edge of the other. If the ball lay such that the nose of the football is just touching the 20 yard line, the chains will be set so that the posts are both right on the inside edge of the 20 and 30 yard lines. This means the outer edge of the upfield post will indicate the 1st down spot, and the inner edge of the downfield post will indicate the line to gain.
so it sort of looks like this, if o is the posts, … is chain, and | | represents the stripes.
|o.|…|…|…|o |
What about the spot itself? The official gauges where the ball was when the running went down, after which the runner falls forward (or backwards) and usually with a pile of other players around. Then the official runs in from 5 to 10 yards away, puts his foot down where he thinks the spot should be, and then places the ball there.
How easy would it be to be off by six inches either way, assuming you gathered with 100% accuracy where the ball should be placed?
The general practice, unless it’s close to a first down, is to purposefully be off by as much as a foot or more, because of the advantages to having the ball spotted on a yard line when enforcing penalties or re-spotting after an incomplete pass.
Are the line judges in a union?
The game officials are in a union, The NFL Referee’s Association. Many are also members of the National Association of Sports Officials (NASO), their state association, USA Football, and/or Collegiate Football Officiating, but these are not unions.
The chain crew is contracted by the home team for NFL games and may or may not be officials at some level. AFAIK, there is no union for chain crew members.
This always reminds me of an old AP article
"By Amy Beth Hanson
Associated Press Writer
HELENA, Mont. – For the past three years, high school football players who played at Vigilante Stadium had a tougher challenge than they realized: the first-down chains that are supposed to measure 10-yards actually are 10-yards, 6 inches. “It’s just one of those things that you take for granted that it’s accurate,” said school activities director Jim Opitz.
The inaccurate markers, purchased new three years ago, have been used at home games for the city's two high school football teams, Helena Capital and Helena High. The error wasn't noticed until the second half of the season-opening high school football game between Helena Capital and Missoula Hellgate last month. Hellgate started a possession on its 20-yardline, and ended up going for it on fourth-and-short. After the play, the nose of the ball was on the 30-yardline, but a measurement with the chains showed Hellgate was still a few inches short of a first down.
Game officials awarded possession to Capital.
"The Hellgate coaches were beside themselves, which is understandable," Capital coach Mark Samson said. "They called a timeout. "When I got to the huddle, kids were asking, 'Coach, how can we go 10-yards and not get a first down?"'
A simple question, really.
The officials guessed that the maintenance crew hadn't lined the field correctly. They told Samson they were giving the ball to Hellgate. "Everyone just kind of ignored it after that," Samson said. But that wasn't the end of it. Dave Powell, facilities manager for Helena schools and in charge of chalking the lines on the field, didn't like that his crew was being blamed.
"When I heard they were running my crew around, double checking the fields, I was a little bit upset, because the first thing I thought they should be double checking was the chains," Powell said. "Those guys are used to going 10-yards. They didn't work out and train themselves to go 10-yards, 6 inches," he added. "In a way it's pretty laughable."
The new entry in the official's rule book states: "The game officials shall check the measuring device for accuracy prior to the start of the game." And that's being taken quite seriously these days." :smack: http://www.hornetfootball.org/documents/lighter_side_news.htm (Best link I could find)Not only that, but when they run it out from the sideline to the middle of the field, how could they place it precisely where it was on the field when it was at the sideline?
It seems more like it’s about the need for something objective/official even if it’s inaccurate and somewhat random.
There is a clip on the chain that is aligned with the back edge of the intervening 5-yard line. When the chains come out the guy in the middle (3 guys on a chain gang) lines up the clip to the line, the chains are extended, and the measurement on the sideline is exactly the same as the one on the field. It’s certainly not rocket science.
The thing that makes it so amusing to me is that they are using fairly precise equipment to measure a spot that is generally very imprecise.
Oh, that makes sense. Thanks.
I mean - the whole process is still an estimation, but it does eliminate one variable.
No problem. I figured you didn’t know because I know you haven’t seen too many first downs.
Now, now, no Cleveland jokes to hijack the thread. 
Besides, that’s kinda like shooting fish in a barrel, ain’t it?
Isn’t there an official who stands on the sideline at the spot?
There are officials at each sideline. The official first-down chains are only on one sideline.
When a play ends, the sideline officials will judge where the dead ball spot is and mark it with their foot. One of the officials in the middle will then place the ball between the hash marks, aligning it with the sideline official. The sideline official will then back off the field and set the chains. All this eyeballing and walking about introduces variance in where the chains are in relation to the ball (not to mention where the ball is spotted in comparison to where it became dead). This is why it’s convenient to sell a dead-ball spot on a whole yardline if possible. That way the stripes and hash marks can be used as a guide.