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The Coast and Geodetic Survey Schooner Matchless.
In the early 1800s, schooners were often used as survey ships, especially in coastal waters. Sailors would cast out a line marked in measured spots and attached to a lead weight. They could measure the depths of the water according to where the water’s surface hit the marked line once the lead weight landed on the seabed.
In the image on the left, a surveyor is casting a lead line, a method of collecting depth data in the early days of surveying. In the image on the right, surveyors are measuring a three-point fix with sextants. These optical navigation tools are used to measure the angle between the horizon and a celestial body, or between two points.
In open water, surveyors used a combination of dead reckoning and the position of celestial bodies — such as the stars, moon, and sun — as well as the horizon, to calculate the schooner’s position. Closer to shore, surveyors used the three-point fix technique. This technique used angular measurements from three reference points, such as prominent rocks, to determine a position.
The dead reckoning formula: Distance = speed x time
Before the invention of navigation technology, mariners could use dead reckoning to calculate their approximate position at sea by using the vessel’s previous position, or fix; the direction they were sailing into, or head; and the distance traveled.
In the image on the left, a mariner is manually plotting a three-point fix with a metallic three-arm protractor to determine the vessel’s position. The tool allows the mariner to plot the observed, measured angles between three fixed objects. The center of the protractor is the vessel’s position. The image on the right shows the geometry of the three-point fix, often measured with a sextant, a tool used to measure the angle between two points. The diagram shows three fixed reference points along the shoreline used to determine the navigator’s position.
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