Bricks, Masonry, Asphalt and Concrete - Finding levels

I'm having difficulty while digging out a trench for a structure in judging
levels, especially as I'm working with a circle 20 feet in diameter.
A spirit level isn't that easy to use when placed on roughly dug out earth.
I seem to remember reading somewhere that if you get a flexible see thro
pipe and fill it with water, run it across the siteand then raise both ends,
the water finds the same level both sides and you can judge the horizontal
level?
Kinda difficult to explain but does anyone know what I mean....?
Thanks

The generic term is 'water level'. In your case differential leveling with a water level would be a new form of misery.

A more or less standard way of excavating a footing or trench by hand is with static string line and a builders square.

A static line can be erected about the perimeter of any plain geometric form though a series of fixed points. In a relatively level plain stakes, masons or builders line, and a line level will do.

It is common practice to drive wooden stakes plumb, and notch them, after establishing level before affixing the line. It is also common practice to bow the line though curves and recheck the line on either side. Steel form stakes are sometimes used in leu of wood.

Regardless of the leveling and measuring devices used, the same devices are used throughout the layout process so that the source errors remain constant.

For short distance work plumb and level are assumed to be 90 degrees in opposition to each other.

A builders square is a site built 'L' of suitable material, (typically 1x3s or 1x4s). The body and tongue are squared to each other using the 3, 4, 5, (or divisions or multiples thereof), rule of right triangles. The body and tongue lengths are made to suit. One or more angle braces are affixed between the tongue and body for stabilization.

In trenching use the 'L' is inverted. A torpedo level is affixed (taped or screwed) to either the body or tongue (the most convenient); the respective reading taken, and the bodies length is adjusted by sawing so that the tongue touches the line when the body has achieved the proper depth.

During layout and trenching a 45 caliber revolver is loaded and holstered. An unauthorized person that moves a stake, the line, or uses the square for other purposes is shot. Layout helpers that incessantly talk are shot also.

If you would rather use a water level: purchase 3/8" clear plastic tubing to suit, and corks. Degas the water by boiling for 10 minutes. Fill the tubing by syphoning; then cork the ends. A few drops of food coloring may help when taking readings. Air bubbles in the line should be removed (re-cork the ends after each use).

To take a reading a length of tubing at either end is held plumb, and the measurement to the depression in the water is noted. Afterward subtract the least measurement from the greatest.

Electronic water levels (static target and free end) [a noisy; irritating rendition of submerging tubing in a jar of water held at a fixed point] suffer some of the same problems as tubing. They require a battery also.

If string line is out, purchase a lazer level, attach it to a rod, and shoot it at a fixed target. An old fashion way imployed two rods and a pail of water. One rod is staked. The pail is nearly filled with water, and is set between the fixed and moveable rod. The pail is leveled by noting the waters distance from the brim. The operator sites from the moveable rod over the brim of the bucket to the opposite rod. Then both rods are marked with the assistance of a helper. A more 'modern' version used a level site on one rod and a target on the other.

Thanks 2000 for a very full and useful answer.
I will study it and hopefully build a structure that will not have a Pisa type lean!
I am working alone so thankfully - as this is the UK - not need a gun to shoot those who interfere with my stakes.
Best Regards!