Depth Gauge

An instrument displaying water depth or the measured distance beneath a vessel from an acoustic transducer or other sensor.

Most small-craft depth instruments are echo sounders that time a sound pulse reflected from the seabed. The display may be offset to show depth below the transducer, below the keel or an estimated total depth.

The offset, units and datum must be known. Aerated water, soft bottoms, weed, heel, speed, multiple echoes and transducer position can produce false or lost readings, so charted depth and position still require cross-checking.