A fixed monument, usually a stamped metal disk set in bedrock, a massive structure or a deep rod, whose height is known relative to the tide station's datums. A water level station normally has a network of several marks, tied together and to the gauge by levelling. If a gauge or pier is destroyed, the bench marks let a new gauge be set to exactly the same datums, which is why their stability and documentation matter more than the gauge itself.
Pier's gone after the storm but three bench marks survived, so we can re-establish the datum.
Tide and geodetic survey documents write bench mark as two words; the one-word form usually comes from outside the trade.
Also heard as
- benchmark
- tidal bench mark
- BM
- primary bench mark
- PBM
Related terms
Leveling
Precise measurement of height differences between bench marks and the gauge or staff using a level and rods.
Brass cap
Surveyor slang for a bench mark disk, the stamped metal cap set in concrete or rock.
Recovery
Finding an existing bench mark in the field, checking its condition and reporting it as found, damaged or lost.
Station datum
An arbitrary fixed zero set below the lowest expected water at a station, used to record raw water levels.
Tide staff
A graduated board fixed vertically in the water and read by eye to check a gauge against datum.