Quote Originally Posted by washburn1445 View Post
1. I can use a plumb line (something weighted) to measure the depth of the pool.
Yes, or a folding ruler, or a pool pole with a yard stick taped on.

2. The accuracy of the measurement for this approach is based on how small I make the squares?
Exactly. But if you get too small, it will probably be hard to maintain equal squares throughout the measuring process.

As an aside based on your statement of "when you next empty and fill", is there any reason I would need to completely empty the pool on a regular basis? I presumed I would only ever need to empty to repaint or resurface the pool.
Also, correct. That's sort of and if-and-when note: If you do happen to drain, that would be when you want to get an exact volume.