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Fathometer

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In 1925, the Submarine Signal Company of Boston developed the first fathometer, known as the 312 fathometer, to chart the depth of water while a ship was moving. The fathometer proved more precise and easier to use than previous sounding methods, making it a vital tool for surveying the oceans.

Hydrographic surveys had been conducted since the early 1800s along American coastal waters. Most began onshore, through the use of leadlines dropped overboard to determine depth. Leadlines were less reliable, however, when measuring offshore depths. Ocean currents could compromise the survey’s accuracy, as both the vessel and the leadline could drift. Human error was also a problem. Crewmen misread the depth values on the leadline, and recorders inaccurately transcribed data called out by the crewmen reading the depth values.

Echo-sounding, first developed in the early 1920s, proved to be the critical advance in the field. An operator sent signals to the ocean floor, and listened on headphones for a returning echo. Using a variable-control mechanism, the operator would determine the depth. Surveyors from the United States Coast and Geodetic Survey used this Sonic Depth Finder to take deep-water soundings from Norfolk, Virginia, to San Diego, California. But the technology had clear limits. Operators needed to be highly skilled to deploy signals and recognize the echo. The instruments themselves could be unreliable, and they did not sound in less than 100 fathoms.

In 1925, the United States Coast and Geodetic Survey obtained the first fathometer, built by the Submarine Signal Company, which had pioneered the development of shipboard sonar and underwater telegraphy systems in the 1910s. Their 312 Fathometer obtained deep-water depth soundings through use of a continuously rotating white light. An operator read depths by monitoring the position of the light at the moment that he heard an echo in his headphones. Unlike previous systems, the 312 Fathometer could be operated while the vessel was moving. Soon, this technique would be replaced by a red-light method. A rotating neon tube flashed next to the depth scale when the echo arrived.

1933 marked a further advance in this technology with the development of the Dorsey fathometer. What made the Dorsey fundamental for hydrographic work was its incorporation of a transmitter and a receiver into a single mounted unit called a transceiver. It had an operating range of 3 to 900 feet. By 1939, it could record depths automatically, thanks to the incorporation of Hughes Veslekari’s graph-recording instrument.

In 1940, the 808 fathometer was released. It became the standard device for shallow to intermediate surveying for the next twenty-five years. By the mid-1960s, digital technologies were invented for surveying the ocean floor, leading to computerized data collection systems. Accuracy increased in the 1970s with the creation of single-beam frequency systems, which, by the 1980s, were bolstered by the deployment of dual frequency beams. They combined a narrow high-frequency beam for precision and a wider low-frequency beam to sample the surrounding area.