Squaring the lines on field... no marks

I would do it like this:

Pull a string line the length of the eighty yards, now go back to where your start is and measure back up the line 3 feet. Now pull perpendicular to the end line with a string and measure 4 feet. The corner will be square when your measurement between the 3' and 4' mark is 5'.

You can google "carpenter 3 4 5 rule" to get a better description. I have no idea what you are talking about "diesel it", so your on your own there.

As a part time coach and full time math teacher I fully second this approach. Use the 45' and 60' as the legs of the triangle and 75' as the hypotenuse (longside). Repeat for the other corners.

If you have enough rope/string and 53 1/3 yards of width (160 feet) I recommend using some pythagorean theorem to get the following measurements: 80 yards/240 feet long, 53 1/3 yards/160 feet wide and a diagonal from one corner to the other corner of 96 yards 5 inches or 228 ft 5 inches, sort of mirror that on the opposite side of the diagonal and that should be the squarest field you get.

After you have these measurements with the side lines and end lines, measure every 5 yards or 10 yards on both sides with a string going all the way across from one end to the next.

how do you paint your game field? I paint all the fields around the high school I coach at and we have strings. 2 that are 120 yards with notches every 5 yards. And 4 that are 53.3 yards (the width of a football field)

Why cant you just use those? Or make some your self.
Make as accurate of a rectangle as possible. find out where your 50 yard line is....or 40 yard line (if you only have 80 yards). You are going to be right on if not pretty darn close. In an area that is 120 yds x 53.3 yds a few feet here or there is hardly noticed....esp if it is just for a practice field.

The more width lines you add in your rectangle the more accurate it will be.

Not if you use a T square for the corners.

But how much? It's an 80 yd. practice field. If it's off a little who cares? If the OP has the money to hire a surveyor, great. If he has the time and inclination to do pythagorean theorems and such, OK. If he wants to get a practice lined quickly and without too much muss and fuss it doesn't have to be a major engineering problem if that's the way he prefers to go.

Put it in the corner of the end line, perpendicular and extend a line out about 10 yds. Then extend the line from there.

Hell yeah a laser level would be more fun! Why didn't you mention that in the first place?

ummmmmmm..... not trying to be a cork but...... Survey it, calculate it, triangulate it; which ever way works for you.

Then once you have the corners, drive a 3 foot piece of rebar into the ground 2 inches below grade. Then drive a 1 foot piece of PVC pipe around the rebar, and cap the the PVC pipe 1 inch below grade.

From then on, what ever you spent in money/effort to set the field up will be repaid in time the next 3284 years with a $15 childs metal detector and an old screw driver.

The 3x4x5 triangle method is as old as Pythagoras. You can extrapolate this over any distance, simply keep the multiple based on the 3x4x5 base. IE 9x16x25, 30x40x50, 60x80x100, ect...

Set the base corner and work your math off of that corner. It doesn't matter if the field is cork eyed from the surrounding area. Don't think that the field should marry off the road/school/bleachers/trees..... simply set where you need start and work from there.

So far as your 80 yard limit, that won't hinder your calculations. If your line runs long, so what. In fact running your line an extra 25+ feet can help you verify that you figured correctly.

If you are still leery of your math, just paint it to begin with and see how it looks before you drop the diesel to it.

And to the diesel issue, if the canister works, then diesel will work in it. So far as the diesel itself, there ain't a better grass/weed killer made. You ain't going to be putting enough down to have an environmental effect ( that would take tens of thousands of gallons) even if you sprayed it every day.

Just a FYI

We finished yesterday... our first corner we eyeballed the angle with a square cone base... the outgoing end was maybe .25 of an inch wider than the corner... thought.. no big deal..

Boy was I wrong... at the width of 160' that .25 inch grew to nearly 40 feet!

Actually it's a total biznatch. If you don't have a solid point to start from its a massive task. Maybe if you gave them the four goal posts it would be really interesting to see if they can use the problem solving. That would have been a lot more fun than running through dozens of trig proofs.

Crazy to add to this....our game field was redone this summer, and we didnt have a season yet. Soccer plays its first home game later today....The company who fixed our field did a great job, and promised they would come back and reset the corners. Our AD called and texted them continually over the last couple of months. Bottom line, they have not came back and set our corners on our new game field. Like I said, there hasnt been a game of any kind on it yet, soccer has first go at it today, we dont play until later this month. So I take my father in law and wife out there yesterday afternoon. My father in law has an engineers mind, is great at Math and figuring things out.....me not so much. Anyway, we find the middle of the field from goal post to goal post. We measure out to corners, we are trying to find the corners on the football field, so the soccer coach can use that to set the soccer field....he is using the the whole 3/4/5 rule and the pythagoreum therum, and it looks good.....we get what we thing is the 4 corners set, measure the width of the field.....perfect 160 feet..... measure the length of the first sideline and we are 10 inches too short. I just dont see how we could have gotten it any better. We were very careful... I am at a loss on what to do.

I had a HC make our play field by that 3-4-5 rule using string and a spray can like a compass and pencil. But for a practice field, how much precision do you need? I'd just eyeball it. And do you really need to line every line? How many will you actually use?

hahahahahahaha

What an interesting project for the trigonometry class!