Meaning of lead line

lead' line"

Pronunciation: (led), [key]
— Naut. Naut.
  1. a line by which a lead is lowered into the water to take soundings: in deep-sea practice, divided into levels one fathom apart, variously treated as marks and deeps.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.