"Perfect" is doing some heavy lifting here. The string is always a non-straight catenary curve, unless infinite force is used to pull an indestructable string.
A laser beam* across the room will show the defect in the string straightness. It's more than good enough to fool human eyes, which are not good at judging slow gradients (such as all the touristy "mystical anti-gravity locations" where balls roll apparently uphill). Therefore, the snap-line is good enough. But not perfect.
* Gravity of course still affects the laser beam's straightness, but on a level good enough to fool electron microscopes, so we can give that a pass.
Yes it will be a catenary but it is not a problem if the purpose is to mark the ground.
If the purpose is also to measure the distance between the 'pegs' and one uses the length of the cable in between, then it can be a problem. That's why survey chains are expensive.
If we get real picky, no physical method will really be accurate because straight line is a mathematical abstraction. It can only be approximated in the physical world, much like a circle.
Light paths come closest, although they 'bend', they bend in a way that is 'straight' with respect to space-time.
You can usually put enough tension on the string to make any droop negligible. But yes modern laser levels are a better if less tactile option in some cases.
"Perfect" is doing some heavy lifting here. The string is always a non-straight catenary curve, unless infinite force is used to pull an indestructable string.
A laser beam* across the room will show the defect in the string straightness. It's more than good enough to fool human eyes, which are not good at judging slow gradients (such as all the touristy "mystical anti-gravity locations" where balls roll apparently uphill). Therefore, the snap-line is good enough. But not perfect.
* Gravity of course still affects the laser beam's straightness, but on a level good enough to fool electron microscopes, so we can give that a pass.
Yes it will be a catenary but it is not a problem if the purpose is to mark the ground.
If the purpose is also to measure the distance between the 'pegs' and one uses the length of the cable in between, then it can be a problem. That's why survey chains are expensive.
If we get real picky, no physical method will really be accurate because straight line is a mathematical abstraction. It can only be approximated in the physical world, much like a circle.
Light paths come closest, although they 'bend', they bend in a way that is 'straight' with respect to space-time.
You can usually put enough tension on the string to make any droop negligible. But yes modern laser levels are a better if less tactile option in some cases.