One of the real problems I have, when putting posts in the ground is insuring they are true verticals. Most of the time I kluge together something, either with boards or rope and a level to get the post within working vertical. Even then it require two people, something I don't sometimes have. And sometimes once I was done, I'd discover that it was not really true and have to go back and do it over.

I saw this trick in a video, and had to slap my forehead.

It uses 3-4 ratchet straps tie downs. The convenience is that you can make simple adjustments by using the tie down own hardware. While this would work best on dirt and lawn, so you could drive stakes in to anchor the straps, you could use concrete bricks with added weight as long as you made sure not to let them slide.

I'd like to put a horizonal support above my raised beds to secure rope lines to allow my climbing veggies to grow, so I may try this next Spring.

On a historic note, I used a similar principle in the Army when I was young to erect radio antennas using the relationship of a triangle's hypotenuse to its sides. The radio antenna had three guide wires (ropes). We would erect it first with people on each guide rope and true it up. Then when we would lower it for transport, we'd record the distance from the base the stakes were, then mark the length of the rope to the stakes from the antenna. To erect it at a new location, lay the complete antenna on the ground, measure out the distances of the stake, tie off two of the three ropes, then holding the last rope, we could pull and have the antenna rise knowing it wasn't going to fall. One person could raise an antenna quite quickly this way.