I used a line level to see if I have my new pole barn site level. The fill dirt is 26' long and I had 10" of drop. I put the line level in the center of the string, which I had very tight. I also turned the line level around and it read the same each way. How accurate can I expect to get with a line level? Can I assume I'm within an inch either way in 26'?
When I did my site I used a 16' 2x6 and a 24" level - it was within an 1" on the 24x36 area. I had one known high spot - ledge that I used a a reference point, then everything else needed to be equal to that high point.
By using a few peices of scrap 2x4 flat spread out around the center and perimeter this is a quick and easy way to see where it's high and low.
I don't know, it's strongly dependent of the operator.
Go buy 30 feet of 1/2 clear PVC tube, it's cheap, and do the water level. Don't use a garden hose with clear PVC fitted to the ends, you can't see the air bubbles. If you get all the air from the line, you can get within an 1/8 inch.