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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.