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Bench mark / datum

- a fixed reference point of known elevation left while undertaking a survey. Has to be permanent, so they are often found at the base of buildings. Usual form is the "war dept arrow" with a horizontal line above. See ordnance survey. Apprentice engineers used to be asked to go and borrow a portable bench mark, the equivalent of apprentice carpenters being sent to look for a left handed hammer.
A datum is any fixed level to which other levels can be referred, ‘dations are quite different being a quantity surveying abreviation for foundations.