Navigation: Depth

One of the first and most valuable inventions in navigation was the lead line. In the 13th century the lead line was used for measuring the depth of water and determining the nature of the sea floor. This line was weighted with lead and had graduated markings to determine sea depth. The lead was coated with tallow, grease or wax to bring up samples of the bottom. A method of navigating from one depth to another based upon the condition of the bottom developed, with sailing directions from the 14th century reading:

”Ye shall go north until ye sound in 72 fathoms in fair grey sand. Then go north until ye come into soundings of ooze, and then go your course east northeast.”

72 fathoms is 432 feet! That’s a long lead line!

These days sailors no longer use soundings alone as an accurate form of navigation. Instead soundings are marked on navigational charts as a reference to be used along with accurate compass bearings, triangulation and a GPS. All mariners’ charts show the depth of the ocean floor below sea level. When a boat is near the shoreline your depth sounding can verify your position when compared to the chart’s listed depth at your plotted fix.


The depth gauge on the boat uses echolocation, just like the RADAR equipment.

The word fathom comes from the old English word faethm, meaning, “Two arms outstretched to embrace.” The fathom measurement came from sailors who used the distance of their outstretched arms from fingertip to fingertip to measure the rope used for lead lines.

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