Depth sounding, often simply called sounding, is measuring the depth of a body of water. Data taken from soundings are used in bathymetry to make maps of the floor of a body of water, and were traditionally shown on nautical charts in fathoms and feet.

What does sounding lead mean?

: a mass of lead at the end of a sounding line.

What is the sounding lead used for?

A sounding lead can be used as a primary depth-finding device, or as a backup for electronics. It is typically attached to a light line which is measured off with bands, tags, knots, or paint markings every fathom or so (known as a 'leadline').