Measuring Water Depth
In 1857, sailors used a lead line to determine water depth. A lead line had a heavy piece of lead attached to a rope with markings. A sailor would throw the lead line and the lead would sink to the sea-floor bottom. Sailors would count the line marks on the rope to measure the water depth. Depths were measured in fathoms (one fathom equals six feet). No ship wants to run aground on sharp rocks or in shallow waters. This 1857 watercolor sketch by James Madison Alden shows a crew from the Coast Survey Brig FAUNTLEROY “sounding,” or measuring the water depth of Strawberry Harbor, Rosario Straits, Washington. (Photo credit: NOAA Photo Library)
“Let Her Fly!”
For many years, sailors used the tried and true method of measuring depth-lead lines. In a 1931 picture, a sailor winds up the lead line, swinging it higher and higher to “heave the lead” far out ahead of the boat. When the boat motored up to the lead line’s position in the water, the sailor would identify the water depth measurements on the lead line. Certain marks on the lead line identified the water depth. (Photo credit: NOAA Photo Library)
The left picture shows a picture of the many soundings an underwater survey crew would take in one area. The number of soundings (water depth measurements) was very important, since the crew had to make sure their information was accurate. This information would be interpreted and recorded on a nautical chart (right picture). Mariners needed to know when they were in deep waters and when the water became shallow (sometimes very quickly). On the right picture, look at how the water depth changes from 1 to 5 fathoms, becomes deeper at 10 to 11 fathoms, then drops quickly into 20 to 37 fathoms, and rises again to 9 to 11 fathoms. Sometimes, ships could only sail in a very narrow channel of deep water for safe passage. (Photo Credit: Coast Survey)
The image on the left is a 17th century drawing showing Netherland surveyors surveying Dutch dykes and canals.
On the right is a 2002 photo of a National Geodetic Survey crew surveying southern Louisiana coast.
Surveying the coast has changed dramatically from 17th century standards. Today, coastal surveyors use Global Positioning System technology to survey land and coastal waters.