Lead Line



Quick Facts:

Used to estimate the depth of the ocean, the rope was lowered into the water until its lead-stumped end hit the sea floor. Depth was determined by what mark was visible to the leadsman.

Date: 3400 BCE - Present day

Sounding Lead, 7 lbs., marked in 25 fathom increments (156 feet)

Lead Line

Sounding Lead, 7 lbs., marked in 25 fathom increments (156 feet), The Mariners’ Museum.

Lead lines allowed ancient sailors to anticipate shallow water in order to prevent running aground. These lines served as a sort of ancient radar for these sailors, and were still used by the U.S. Coast and Geodetic Survey—the organization that began the National Oceanic and Atmospheric Administration (NOAA)—in the 1920s.

Lead lines, also known as sounding lines, were used in the process of sounding. Sailors would drop a lead line into the water until it hit the sea floor, then use the visible marks on the line to estimate the depth of the water. These lines are some of the oldest of all navigating tools, and were used even before the invention of the compass! The first evidence of man’s interest in the deep sea can be found in ancient Egyptian tomb paintings, such as the bas-relief carvings of Deir al-Bahri, which were commissioned under Queen Hatshepsut. These carvings depict a voyage to the land of Punt on the horn of Africa by the Red Sea. Included in this ancient work is a carving that shows one of the ancient sailors using a sounding pole to measure the depth of the Red Sea. The Egyptians used long sounding poles to measure depth as opposed to ropes, because their main waterway, the Nile River, was not very deep. As the ancient sailors began to cross the Mediterranean, it became easier to tie rocks to long ropes that could reach the depths of the Mediterranean Sea. By 100 BCE the ancient sailors were able to measure a mile down into the Mediterranean!

Sailors in the 1500s (CE) used lines to gather samples of the ocean floor as well. They would attach tallow, a wax-like substance, to the lead piece. The tallow would stick to the sand or shells on the seafloor, and pull it up from the bottom. Samples retrieved would help the mariner learn about where he was sailing, by providing various pieces of information from the sea-bed.

By 1600 CE in England, the lead line had developed even further. The English standardized the markings and measurements showing how deep the water was. The standard measurement, the fathom, is a word meaning the “span of outstretched arms.” It was standardized to a length of six feet: an average distance between a man’s outstretched arms as he held the lead line.

A lead line used in shallow water was usually 25 fathoms long, or 150 feet, while a line used in deep water measured 100 fathoms, or 600 feet. At certain points along the line, small bits of material, or “marks,” would be attached to make the line easier to see and to measure.

The table below shows the typical marks on a line:

2 fathoms2 strips of leather
3 fathoms3 strips of leather
5 fathomsA strip of white duck (fabric)
7 fathomsA piece of red bunting (fabric)
10 fathomsLeather square with a hole
13 fathomsA piece of blue serge (fabric)
15 fathomsA piece of white duck (fabric)
17 fathomsA piece of red bunting (fabric)
20 fathomsA piece of cord with two knots
Deeper than 20 fathomsUnmarked; known as “deeps”
  • King, Dean, John B. Hattendorf, and J. Worth Estes. A Sea of Words: A lexicon and Companion for Patrick O’Brian’s Seafaring Tales. New York: Henry Holt and Company, 1995.

  • Oleson, John Peter. “Testing the Waters: The Role of Sounding-Weights in Ancient Mediterranean Navigation,” in The Maritime World of Ancient Rome, edited by Robert L. Hohlfelder, 119-176. Ann Arbor: University of Michigan Press, 2008.

  • Theberge, Capt. Albert E. “Sounding Pole to Sea Beam.” Technical Papers 1989 ASPRS/ACSM Annual Convention Surveying and Cartography 5 (1989): 334-346.

  • NOAA. “Ocean Explorer: Sonar.” Last revised April 16, 2013. http://oceanexplorer.noaa.gov/technology/tools/sonar/sonar.html