The simplest sounding tool: a weighted line marked at known intervals and lowered until the lead touches bottom. A hollow in the base of the lead is armed with tallow to bring up a seabed sample. Surveyors use calibrated lead lines to check echo sounders alongside a pier, to confirm least depths on piles and masts that sonar can miss, and to settle arguments when the electronics disagree.
Lead line says 4.3 at the pier corner and the sounder says 4.4, close enough.
A lead line stretches with use, so crews calibrate it against a steel tape before trusting it as a check.
Also heard as
- leadline
- hand lead
- sounding line
Related terms
Bar check
A singlebeam calibration that hangs a plate on marked lines under the transducer and compares the sounder with each known depth.
Sounding
A single measured depth at a known position, or a depth figure printed on a chart.
Least depth
The shallowest depth over a wreck, rock or other feature, which is the value that gets charted.
Droggy
Royal Navy slang for a hydrographic surveyor, from a clipped form of hydrographer.