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.
