Differential levelling with an optical or digital level and graduated rods, run between the tidal bench marks and the gauge's reference point or tide staff. It ties water level measurements to permanent marks so datums can be recovered and checked for movement. Runs are made at installation, at regular intervals and at removal, and each run is closed back on its start to measure the error.
Leveling's due at the station this month, run all five marks plus the radar reference point.
Also heard as
- levelling
- running levels
- differential leveling
- level run
Related terms
Bench mark
A permanent, stable mark of known height near a tide station that preserves the station's datums.
Closing the loop
Finishing a level run back on the starting bench mark to measure how much error crept in.
Tide staff
A graduated board fixed vertically in the water and read by eye to check a gauge against datum.
Brass cap
Surveyor slang for a bench mark disk, the stamped metal cap set in concrete or rock.
NAVD
North American Vertical Datum: the geodetic height reference used for land elevations across the United States.