A basic ground level instrument like this is pretty ‘agricultural’ compared to laser levels with a fiber-glass, Aluminum tripod and direct reading rod. Surveying tools like theodolites and automatic levels are sophisticated equipment that can measure accurately to 1.5mm. What is proposed here is a rough ground level, a rule of thumb but can still be useful. For instance I once had a steep property and wanted to place a tank on a high ridge which would be filled by a small spring and then used to irrigate an acre of blackcurrants. I used the method below to work out the height (220 metres / 721 feet) and to work out what type of pressur piping I woulld need.
To make a rough ground level, take a piece of board about 12 inches square and perfectly level, and fix same to a leg or peg from a piece of timber about 3 feet 6 inches to 4 foot long, with a cross piece exactly 3 feet or 3 feet 6 inches from the top. This will then ensure your sight is even if the stand is always driven to cross piece.
Lay a spirit level on top and make dead level across corners. When this is done sight along your spirit level to your staff, which should be marked plainly in feet and inches (or cms) up to about six feet.
Wherever your line of sight hits the staff you of course would have to allow the height of your sight board and level.