> You've got a water level, with all its tubing, on a table. The two ends
> are held up against a wall and you mark where the water is.
>
> Next you take all the tubing on the table and drop it on the floor. The
> water level just got lower (on both ends), right?
>
> So how do you account for that effect when you're marking multiple posts
> (on a grade) from one reference point?
>
The level stays the same as long as the ends don't move - the pressure
difference isn't enough to compress the water or expand the tubing enough to
be detectable.
As I posted before, once you carry the second end of the tubing to the point
where you're trying to establish level, you DO need to make sure that you
move the tube up or down at the reference mark (the point to which you're
leveling all the other points) to return the water level to the reference
elevation.
Or else mark the water level on both ends of the tubing and make sure all
measurements are made with the water at that level.
This is really much harder to explain clearly than to do ........
Karl
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