In reply to Jim.

This took me a while to work out the mechanics. I was trying to figure out how the guy at the 1st end get's his level to the right place to start with, because if he raises or lowers, the level is going to stay the same.

I assume they start with both ends at the reference point and move them up and down together to set the reference level (3') in your example?. Then 1 guy walks to the other end with 1 of the ends of the tube to transfer the measurement?

Otherwise I envision a lot of yelling across the jobsite. Higher, No lower, No Higher No lower....