In levelling, the run is brought back to the mark it started from, or to another mark of known height, so the sum of all height differences should be zero. The leftover misclosure is compared with an allowance that grows with the square root of the distance run, often a few millimetres per root kilometre. A loop that fails to close is rerun, because an unchecked blunder would end up inside every datum at the station.
Loop closed at 1.5 millimetres, well inside tolerance, pack the rods and head back.
Also heard as
- loop closure
- closing the run
Related terms
Leveling
Precise measurement of height differences between bench marks and the gauge or staff using a level and rods.
Bench mark
A permanent, stable mark of known height near a tide station that preserves the station's datums.
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.