Today we set up batter boards and put out marking strings to prepare for constructing a round deck.
We wanted 2 strings that marked the center lines, crossing each other at 90 degrees. We set up batter boards and ran strings between them. Then we measured out a 3/4/5 triangle from the crossing. We looked at the error, adjusted the strings, and repeated until we thought it was good enough.
The strings were not at the same height, so measuring the crossing was tricky. Measuring against the middle of the string is hard, because the string moves so easily.
Each time we adjusted the strings, the crossing point moved, so we had to measure all 3 legs of the triangle each time. If you get measure the hypotenuse and find it's off by some distance d, you have to move the ends of the string by some value that's larger than d, but you don't know how far. You just have to adjust, measure, and repeat.
In this case, the deck is 30' diameter. We started with legs of 6'/8'/10', then realized we had enough room for 12'/16'/20'.
If I was in my workshop building with wood, I would use a carpenter's square or factory corner of a piece of plywood, but at this scale that doesn't work.
I get that 3/4/5 is a good way of checking a right angle, but it's a painful way to create one. Is there a good way to create a right angle with strings, getting it right the first time?