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Historical C&GS | Nautical Charting | Navigation

navigation - positioning the survey vessel at sea

The Coast and Geodetic Survey has always been concerned with assuring that its surveying vessels have been navigated to the highest standards of accuracy.  For most of the first one hundred years of its existence, all determinations of ship position were made by visual means.  Deep ocean surveys and oceanographic studies required using classical astronomic means to position the vessel.  However, for the inshore hydrographic surveys that were the basis of coastal and harbor charts, the primary means of navigation were by the use of the three-point sextant fix or by means of shore observers simultaneously obtaining azimuths with theodolites to the survey vessel.

Both of these methods were quite labor intensive and required the utmost coordination and teamwork between the various observers to assure success.   Surveying by the three-point sextant fix method required two individuals simultaneously observing two angles between three known points with a common center point.  Their readings would be time-coordinated with a depth reading, recorded by a recorder, and plotted on the boatsheet with a three-arm protractor.  If the plotted position fell to the right or left of the desired survey line, a course correction would be called to the boat coxswain.  The second of these methods, the azimuth-azimuth method, was used with shore-based theodolite operators.  Upon signal from the ship, all theodolite observers would read an angle to the surveying ship.  These angles would be plotted at a later time and the actual ship position determined.  Although quite accurate, this method had the disadvantage that the ship personnel were never sure of their position until after the fact.  This method was mainly used in the early days and fell into disuse as the three-point fix method became better understood and more widely used.

The three-point fix  required that wooden signals be built over previously surveyed points.  These signals would be constructed of varying design and color schemes to assure easy identification.   On flat low-lying coasts, tall signals were built that could be seen from far offshore.  If the survey ship had to work further offshore, signals were placed on buoys or small vessels that were positioned at the limit of visibility of the shore signals.  If another string of signals was placed further offshore, their position would be determined by taut-wire and sun azimuth traverses.

Beginning in the 1920's, a new means of navigation was developed by the Coast and Geodetic Survey.  This was Radio-Acoustic- Ranging which depended on sound signals passing through the water to receiving hydrophones which keyed radio signals back to the surveying ship.  This was the very first system to eliminatethe need for visual means of navigation and was a first step on the road to electronic navigation systems.  This system was used for about twenty years until replaced by electronic systems developed as an outgrowth of radar development.  In 1945 the C&GS tested Shoran, a system similar to the "Gee"system used by World War II bomber commands, for hydrographic surveying.  For the next fifty years, various models of shore-based electronic surveying systems controlled offshore surveys.  These systems transmitted  medium to high frequency signals and all required shore antennas.  They all required installation of at least two fixed stations and a shipboard mobile unit for surveying in the range-range mode and at least three fixed shore stations and a shipboard receiver for operating in the hyperbolic mode.  Accuracies of 10 to 20 meters on the surface of the Earth were attained with these navigation systems.

In the early 1970's super-high-frequency (SHF) navigation systems with accuracies in the few-meter range became available for survey use.  They were line-of-sight systems that were compact, lightweight (except for the batteries required to keep them operating at remote sites), and easily installed.  Use of SHF systems led to the demise of the three-point-sextant fix for inshore and harbor surveys. Today's hydrographic surveyor uses the Global Positioning System (DGPS) in the differential mode (DGPS) to fix the position of his craft to accuracies of a few meters in virtually all locations within the coastal waters of the United States.  The surveyor no longer has to set foot ashore to build visual signals or install electronic navigation systems.  This has been a major step forward in increasing the overall efficiency of hydrographic surveying operations.  The use of DGPS for surveying was pioneered within the Office of NOAA Corps Operations and the Office of Coast Survey, both descendants of the Coast and Geodetic Survey.

The Navigation Section of the Coast and Geodetic Survey album is comprised of close to 200 images detailing the evolution of the precise navigation methods required for hydrographic surveying.  Come join the coast surveyors as they find their way at sea.




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Last Updated:
October 1, 2009