So as I said the pool area did not appear to be as level as he said. It appeared he overdug the high side when eyeballing it. So I build a water level to check.
Basic theory of a water level: Still water in a container exposed to atmosphere will always be level. The same applies to water in a tube if both ends of the tube are exposed to atmosphere. On the condition that the tube contains no air bubbles, and both ends are exposed to air, and the line isn't being moved, the water will settle to the same height on both ends of the tube. It does not matter how far apart the ends are or how the tube runs between the points. The middle of the tube can even be higher than the ends! So long as the three requriements are met (no air bubbles in tube, exposed to air, line not moving) the water level on both ends will be the same.
First I bought some hose. Size (diameter) isn't important, but I wouldn't go much smaller than this or filling becomes a pain. HD didn't have longer than 20', so I had to buy a coupler.
Air bubbles the size of the line will screw up water levels, so it's important to fill it with zero air. I use a bucket and immerse one end, feeding the line in. Watch the water level in the line, you can feed it too quickly and it will stop feeding. Just make sure no air bubbles get added.
Once the entire line is submerged, place a finger over each end and pull the line out. A helper may be handy to help uncoil the hose. The finger is to ensure that until the ends are level water doesn't drain out of the line.
Once the hose is out, line up the ends. Congrats, you have now created a water level!
Unfortunately unless you're trying to level a beam, water in a hose by itself isn't very useful. You need something to attach the hose to that will keep it a fixed distance off the ground. For two people, two yardsticks would work great, but as I was by myself and had no yardsticks, I used empty juice jugs. Note the water in the tubes comes to the same height on both jugs.
If the water level is too high, simply lower one end and let some water flow out, then raise it again.
Water level is now lower. We need some reference marks. A sharpie would be good, but I couldn't find it so I put tape on both sides with the top of the tape marking the top of the water.
Demonstration, raising one side on a 4x4 (so 3.5").
A note on water levels: The water moves up on the low side by one-half the height difference, and moves down on the high side by one-half the height distance. The total water height difference gives you the total height difference. E.g. 1.75" up on one side and 1.75" down on the other, so we have 1.75 - (-1.75) = 1.75" + 1.75" = 3.5".
(Negative since the water is below
the reference mark)
You are ready to measure! Go move your jugs to the two locations you want to measure.
Be sure to keep the ends at the same height as you move. Set both jugs (stands) down at one spot, then, placing a finger over the tube end to seal it, raise one end and move to the other side of the pool, and set it down before removing your finger. Otherwise water is likely to leak out of the tube.
Another quirk I noticed is that depending on the location, the water on BOTH ends can move up or down from the initial creation of the level. That is fine, but it complicates the math slightly. For example, here is the measurements left to right on my pool. Note both sides are above the inital reference mark I made.
So, let's do the math. First off, note I used the "wrong" side of a framing square, so each small tick is 1/12" of an inch, not the standard 1/8" or 1/16" inch. On the low side (high water), the water is 8/12" (0.67"), and on the high side (low water) it's 3/12" (0.25"). As with the 2x4 example, it's the difference that matters, so take 0.67 - 0.25 = 0.42". This is less than 1", so the pool area is within spec left to right. Yay!
Unfortunately things looked unlevel front to back, so let's look at that. First, a shot of the water level in action.
High side the water is down by almost 1 11/12" (1.92")
Low side the water is up by 2 10/12 (2.83").
Again the total height difference is the total water difference, but since one side is below the reference mark and the other side is above the reference mark we will
add the readings instead of subtract. So 1.93" + 2.83" = 4.76" of unlevel! This obviously won't work for a pool.
So, bottom line. I
was right. The pool looked unlevel and it was indeed unlevel. They overdug the high side. Now, this isn't terrible for two reasons. Firstly, this is a tight packing sand/small rock mix, and I doubt it would settle much if we moved some from the high side to the low side (yes I know that's not recommended, and certainly wouldn't work for regular dirt, but I think it has a good chance of working with this stuff). Secondly, the high side is just the sand/gravel mix, so it's easy to dig away to match the height of the low side.
The other thing is I didn't measure out exactly the distance, and they dug wider than the pool, and this is a smallish pocket on the low side. But in any case, it's still way more than 1" off.
I'll have to talk to them when they come back and figure out what went wrong with their transit level, and make sure we're all on the same page (again) before they finish out the digging.