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Correct answer is a rotary laser level, but they are a few hundred, so might be a hire job. Cheaper solution is a builders line with a hanging spirit level.
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A water level .
You only really need one end to measure with - I just chucked the other into a bottle of water, which was used as the datum.
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A taught string and a line level will work fine.
https://www.screwfix.com/p/stabila-builders-line-level-3-80mm-/5494X
Put the level at one end of the string but not in the middle to prevent sagging.
Currently digging down in to my garden in preparation to build a deck. Is there something I can use to check my levels against the house on the other side of a 5m patio?
The garden's about 12m long so I feel like a level and string isn't going to cut it for accuracy.