Coast and Geodetic Survey (Retired)
12934 Desert Glen Drive
Sun City West, AZ 85375-4825
a limited amount of Geodetic Astronomy was accomplished during
the Hassler period with a few latitudes, azimuths, differences
of longitude by the chronometer method and at least one longitude
being observed. However, it was not until 1847 following Sears
C. Walker's development of the telegraphic method for determining
differences of longitude that all the quantities were of equal
latitudes were observed using the Horrebow-Talcott method following
its adoption in 1851 to the present. Bache introduced the method
in 1846 and the first complete set of observations was obtained
by Assistant T.J. Lee at Thompson, MA in the same year. In the
1970's, in response to contentions that the Sterneck method
was more accurate, observations at more than 30 stations showed
that both procedures gave essentially the same results.
1847-1922 longitudes were determined by the telegraphic method
whenever possible. After that time radio signals were employed.
were observed by a variety of procedures, although the direction
method was, by far, used the most often for primary determinations.
In the direction method, observations on Polaris or whatever
star was used, were taken as if it is simply another signal,
following the same measuring sequence as for triangulation.
A chronometer was set for estimated local time and the times
of measurement were corrected later for the difference with
true local time determined from observations on time stars or
radio signals, obtained prior to and immediately following the
azimuth observations. About 1975, digital clocks were adapted
to receive the standard radio signal directly. Repeating theodolites
were occasionally used for azimuth observations following the
same patterns as used for angulation.
and longitude observations were made from 1847-88 with large
transit instruments made by Troughton and Simms of London that
could be used as both meridian and zenith telescopes. Slightly
smaller similar transits built by the C&GS Instrument Division
were employed after that time until 1914, when Bamberg broken
telescopes were introduced. About 1960 the Wild T-4, another
broken telescope instrument, replaced the Bambergs and in the
1970's the Kern DKM-3A, a true universal theodolite was introduced.
Astronomic azimuths were generally observed using regular theodolites
except in the higher latitudes where any of the three astro
instruments employed after 1914 could be substituted. Determining
differences of longitude remained a problem for many years after
1847 because the telegraph lines required were not always available,
especially in the western part of the country and Alaska, and
the chronometric method continued to be used. As a matter of
fact, most of the longitude bases for the several local datums
in Alaska resulted from chronometric observations. On the other
hand, telegraph lines were sometimes extended to places specifically
to determine astronomic longitudes. One such case was at Lake
Tahoe in 1893, as part of the delineation of the California-Nevada
boundary, where it was necessary to string telegraph lines about
5 miles airline, all uphill from Genoa, NV.
on, astronomic latitudes and azimuths, because of the simpler
nature of the observations, were obtained more frequently. The
primary reason for the observations was to obtain deflections
of the vertical at the points by taking the difference between
the observed and geodetic azimuths and backing out the Laplace
equation to compute the equivalent difference in longitude.
When the astronomic longitude also is observed, the point is
identified as a Laplace station although technically only the
longitude and azimuth is required.
due course, Laplace stations were regularly spaced throughout
the country. The U.S. network was one of the few anywhere whose
orientation was rigorously controlled by Laplace azimuths at
prescribed intervals. This policy began about 1910 for the work
still in progress and continued in the establishments of the
North American datums of 1927 and 1983 (NAD27 and NAD83).
end results of the method described above for obtaining deflections
of the vertical were generally for analysis purposes only. However,
there was at least one prominent instance otherwise where the
method was employed to obtain such information needed to reduce
the PASADENA base to the mountain line Michelson used in his
experiments on the speed of light in 1922-23.
azimuths as observed also were used to control the first-order
taped traverses observed during the 1917-27 period. It was recognized
that the Laplace corrections would be very small in that part
of the country where the traverses were measured and that the
observed angles could easily absorb the differences. In 1956
a program was initiated to determine astro-geodetic deflections
along the 35th parallel at about 30km.(18mi) intervals as part
of an international study on the shape of the earth. Most of
the observations were completed, some made as part of the Transcontinental
Traverse (TCT) project where more than 1,300 astronomic positions
and azimuths were measured between 1961-76.
1974, a plan was drawn up to upgrade the network for NAD83 that
included the measurement of several hundred new base lines,
astronomic azimuths and required positions plus astronomic positions
for about 100 points, mostly base line stations where steep-slope
lines (in excess of 5) were involved. The purpose of the latter
was to determine deflections of the vertical for use in correcting
the observed angles. A maximum correction of 5" was found in
the TETON base triangulation, Grand Tetons, WY, an amount certainly
among the largest discovered to date.
1960 almost all geodetic astronomy was accomplished by the C&GS.
About that time, the Defense Mapping Agency (DMA) as part of
the missile and satellite programs began observing astronomic
positions at sites of particular interest to them. Later, DMA
measured several legs of the TCT including the required astronomic
positions and azimuths. Once the Global Positioning System (GPS)
became operational in the mid 1980's, geodetic astronomy along
with the classical methods for determining geodetic positions
was obsolete. Little or no astronomic work has been done since
... First of Wood
triangulation was developed utilizing the higher elevations
for station sites, for the obvious reasons. Whenever possible
to do so, sturdy triangular-shaped wooden stands, about 4 ft.
high, plumbed over the point, generally with a platform for
the observer, were used to hold the theodolite. On a few occasions
in the 19th century and after about 1965, similar stands of
metal were sometimes employed.
ground setups were ideal, it was nevertheless not unusual to
elevate the instruments further in order to clear various obstructions,
to extend the lines of sight, to minimize refraction conditions
and similar, and this was done even in the earliest time, despite
the heavy weight of the theodolites. As an example: The scaffolding
and tripod at both ends of the EPPING base, ME in 1857 rose
43 ft. above the surface marks, and the pole signals extended
10 ft. higher. For almost 100 years, the structures were made
of wood, in a few cases, the actual tree itself was used and
in very rare instances, of masonry construction. Whenever possible,
the stands holding the personnel were separate from the instrument
sparsely settled, wide-open spaces of 19th century and early
20th century America didn't lend itself to the European practice
of utilizing church spires and other high structures for triangulation
station sites. Even when available very few of these buildings
were ever selected for primary station locations because of
stability problems and the need, in many instances for eccentric
setups. As a result the tall wooden towers or signals, as they
were also called, required to overcome various obstacles were
often engineering and architectural gems. In some cases, especially
in the high plains where earth curvature was the only obstacle,
shorter double towers were topped with slender, and sometime
equally as tall superstructures from which heliotropes, lights
or pole targets were displayed.
era of tall wooden towers ended in 1926 when Jasper S. Bilby,
then Chief Signalman C&GS, drawing on steel windmill technology
used throughout the west, erector set toys, gas pipe towers
built earlier by the U.S. Lake Survey and his own long experience
in constructing wooden signals, designed a double tower survey
signal built almost entirely of reusable steel bars and rods,
held together with bolts. These especially strong structures
could be erected in standard configurations to heights from
37 to 116 feet in 13 foot increments by a 5 man crew in a day
or less and dismantled by a 4 man team in about half that time.
occasional need to extend the height of in-place towers and
for additional height on the highest signals available was resolved
very early with one piece sections, each 10 ft. in height, that
were bolted to the tops of towers. As many as 3 sections, while
rare, have been added to a single tower. A few bases for 129
ft. towers were later available, but seldom employed because
of the much larger area required to anchor them.
tower components could be reused on numerous, even hundreds
of times and the towers were employed worldwide. Their first
use was in 1927 in southern Minnesota where during the working
season that included other projects in the state, 96 towers
were erected. As a point of interest, the tallest built was
156 feet (about the height of a 15 story building) on the Mississippi
River arc in 1929.
of a Kind
S. Bilby joined the C&GS in the 1880's as a young man, fresh
off an Indiana farm and immediately showed an uncanny ability
to locate trees obstructing lines of sight, an important attribute
in a time when it wasn't easy to move about. He became skilled
in signal building and reconnaissance (planning surveys), and
in fact, wrote the original manual on the subjects, among several
special publications he either authored or co-authored. He rose
through the ranks to Chief of Party and at the time of his retirement
in the 1930's, he was Chief Signalman, the highest civilian
position ever in the C&GS field service.
station monuments, for obvious reasons, were always of fundamental
importance in geodetic surveys. Where rock ledges or large boulders
were available, Hassler utilized drill holes filled with sulphur
or some other substance to reduce the effects of freezing. Elsewhere,
buried truncated earthenware cones were the rule. The center
of the smaller radius end marking the exact station. Sub-surface
(underground) marks also were usually set in the same fashion.
In most cases, at least one reference (witness) mark was established,
drill holes and cross cuts in rock structures and truncated
earthenware cones, smaller than the station marks were standard.
Hassler buried the reference cones in a specific pattern, providing
visible reference information to locate the general station
site, and in addition buried small pieces of rubble, sea shells
and the like found at the site, atop the station mark to aid
in the recovery.
marks serve several purposes: To aid in locating the station,
to verify its position, to reset the monument and for use as
line stations were usually marked by heavy stone posts until
about 1900 when poured concrete monuments replaced them. From
about 1850 to the turn of the century, stone posts (marble,
sandstone and limestone) 2-3 ft. in length, and for sub-surface
marks the same type of posts, bottles, earthenware jugs and
crocks and similar, generally replaced cones for marking stations.
However, in some instances, bolts and nails cemented in drill
holes, simple drill holes, cross cuts and in fact, almost any
conceivable mark, in any combination with these station markings
were utilized. When necessary to bury the marks, a ditch 4-8
ft. in diameter and 8-18 in. deep, surrounding the station location
was usually dug and filled with coal or charcoal. Once concrete
became readily available, 2-3 ft. long tile and tin pipes filled
with the substance, set over underground marks were often employed
with centers marked by bolts, nails, punch holes, etc.
1900, cast bronze disks were introduced and shortly thereafter
poured concrete monuments 3-5 ft. deep with sub-surface marks
became the standard, where rock ledges and boulders were not
available. Monuments of this type continued to be used until
the mid 1980's.
1965, steel rods driven to refusal with disks attached later
were set for many surveys and in fact, are the basis for what
are believed to be the most stable marks by today's standards.
the 1920's, two reference marks were specified for each station
and beginning in 1927, a third reference mark was set about
¼ mile distant for use in providing azimuth control for
local surveys and for determining magnetic declination. Standard
azimuth mark disks replaced azimuth reference marks about 1935.
mark monuments were of similar design until the late 1970's
when special steel rod type marks were introduced. In the 1930's,
precast concrete posts with bench mark disks attached were used
for several years.
to the late 1970's, all concrete monuments and disks were constructed
of non-magnetic materials. Once GPS became operational, sub-surface,
reference and azimuth marks were seldom set and rod type station
between observing units and on station personnel were kept simple
and brief. In the earliest days none were usually necessary
because the pole-target signals were seldom attended and when
they were, a few flashes with a mirror for identification purposes
and to indicate the observations were to begin and something
similar on their conclusion would generally suffice. That practice
continued when heliotropes came into use during the 1840's and
most stations were manned, until about 1900 when Morse code
was introduced. Only a few observing units were active in this
period and the need to signal more detailed information was
F. Hayford during his service with U.S.-Mexican Boundary Commission
in the 1890's resolved such a need for in field communications
by utilizing Morse code. Once lights replaced heliotropes at
the turn of the century, most observations were made at night
and there were more reasons for the observers to have direct
contact with the lightkeepers. For one, identification, also
lights often had to be dimmed or brightened, messages relayed
in emergencies and the like. In 1902 International Morse code
was adopted as the vehicle to obtain that end.
in the 1930's multiple observing parties became the rule and
angle information was often transmitted to the Chief Observer
(1st O) so that triangle closures could be computed and any
required reobservations made while still on station.
were tried early in the World War II period and caused enough
problems to delay their general use for about 15 years, the
major one being the conversations were picked up by nearby receivers.
In one case, locals hearing the jargon, compounded by flashing
lights thought foreign agents were in the area and reported
the incidents to police, who went looking for spies and found
instead, surveyors atop towers. And, as might be expected, there
were a few complaints about profanity.
about 1960 radio technology was improved and all units were
so equipped. Another era ended. No longer would lightkeepers
peer off into the darkness awaiting a light blinking Dash -
Dot - Dot pause Dash - Dash - Dot or DG, translated, Done here,
Go to next station.
Territory ... More Work
was slow on the principal triangulation during a few periods
in the 19th century when territorial acquisitions, especially
those with long coastlines such as Florida, Texas, the Pacific
Coast and Alaska created a need for immediate hydrographic surveys
and other required charting information. And, the Coast Survey
was a small bureau, personnel-wise.
continuing problem, political opposition to geodetic surveys
never really disappeared, although not as viciously as in the
Hassler years. One congressman loudly proclaimed when the C&GS
was authorized to carry the work to the interior that it was
proliferating worthless triangulation throughout the country,
and he probably had some supporters.
Civil War caused the longest delay as many employees went off
to join the military, north and south. In 1863 when it appeared
the thrust of Lee's Army of Northern Virginia was aimed at Philadelphia,
Bache and Davidson were sent there to aid in planning a defense
for the city. Fortunately, Gettysburg ended that threat. The
Spanish-American War brought more coastal territories, the Philippine
Islands and Puerto Rico among them and about the same time,
the Hawaiian Islands joined the U.S., all adding to the work
of the bureau.
the turn of the century the Eastern Oblique and the 39th Parallel
arcs and extensions north from central Kansas to Nebraska and
south from San Francisco to Santa Barbara were completed. The
39th Parallel triangulation is 2,750 miles in length, probably
the longest arc executed by a single government and connects
the lighthouses at Cape May, NJ and Point Arena, CA linking
the Atlantic and Pacific Oceans, symbolically as well as scientifically.
During the period primary triangulation was observed in much
of New England and in 1876 Assistant Charles O. Boutelle measured
an arc over the Mohawk Valley connecting this work with the
Lake Survey stations near Buffalo.
of central Colorado the 39th Parallel triangulation consists
of massive figures, many containing lines 100 miles and more
in length, the longest being 183 miles between UNCOMPAHGRE PEAK
near Ouray in Colorado and MOUNT ELLEN near Hanksville,in Utah.
In the 950 mile stretch from Colorado Springs, CO to San Francisco,
CA less than 40 stations were required with many of the observations
made by Assistant William Eimbeck between 1876-96.
Hexagon and Davidson's Quadrilaterals
of Salt Lake City is the Great Hexagon with WHEELER PEAK at
its center connecting the stations on the Wasatch Mountains
to the east with those about 200 miles to the west in Nevada.
Due to the remoteness of the area and short working season it
took 10 years to complete the observations at the 7 stations
the 1880's and 90's the only mode of travel to the station sites
in the mountain west was by horse, more likely mule, and wagon.
Actually, horse and mule drawn wagons were the only means of
transportation to most station locations everywhere until motor
trucks were introduced in 1913. The first was a White Motor
Co. 1½ ton truck, with a 30 HP engine and 25 MPH top speed
used by an astronomic party on the 104th Meridian arc. Instruments,
equipment and supplies were heavy and wherever it could be done,
roads were built up the mountain as far as possible. The one
at WHEELER PEAK remains today.
west the triangulation is carried over the Sierra Nevada near
Lake Tahoe by very large figures known as Davidson's Quadrilaterals
with sides ranging from 57 to 142 miles in length.
1878 Carlisle P. Patterson superintendent of the newly named
Coast and Geodetic Survey gave George Davidson authorization
to establish a station on Mount Shasta, a huge mountain in northern
California with an elevation of 14,162 ft. The real purpose
for the project being to measure the side MT SHASTA to MT HELENA
which at about 192 miles would make it the longest triangulation
line ever observed. The line MT LOLA to MT HELENA one of the
sides of Davidson's Quadrilaterals, 133 miles in length was
selected as the base for the triangle.
Benjamin A. Colonna was chosen to make the observations at MT
SHASTA and George Davidson at MT LOLA. Observations were not
secured at MT HELENA, only heliotropes were shown. Colonna's
description that follows of the day he was successful tells
the whole story.
complete article, Nine Days on the Summit of Mt.Shasta appears
in The Journal -Coast and Geodetic Survey, June 1953 Number
5, pp. 145-152. Friday August 1, (1878) proved to be the day
I had been waiting for. The wind had hauled to the northward
during the night, and the smoke had vanished as if by magic.
At sunrise, I turned my telescope in the direction of MT LOLA,
and there was the heliotrope, 169 miles off, shining like a
star of the first magnitude. I gave a few flashes from my own,
and they were at once answered by flashes from LOLA. Then turning
my telescope in the direction of MT HELENA, there, too was a
heliotrope, shining as prettily as the one at LOLA. My joy was
very great; for the successful accomplishment of my mission
was now secured. As soon as I had taken a few measures, I called
Doctor McLean (a visitor from Oakland,CA) and (Richard) Hubbard
(a guide) to let them see the heliotrope at MT HELENA, 192 miles
off, and the longest line ever observed over the world. In the
afternoon the smoke had arisen, and HELENA was shut out; but
on the following morning I got it again, and my mission on Mount
Shasta was finished. The French have been trying for some years
to measure, trigonometrically, some lines from Spain across
the Mediterranean to Algiers; they have only recently succeeded,
and it has been a source of great satisfaction to French geodesists.
Their longest line is 169 miles. The line from MT SHASTA to
MT HELENA is 192 miles long, or 23 miles longer than their longest.
And the glory is ours; for America, and not Europe, can boast
of the largest trigonometrical figures ever measured on the
is somewhat ironic that only a few years later a regular network
line mentioned previously, UNCOMPAHGRE PEAK to MOUNT ELLEN was
observed and at 183 miles is 14 miles longer than the longest
Corps of Engineers were responsible for mapping and charting
the Great Lakes, and recognizing that the Act of 1843 limited
Coast Survey responsibilities only to the Atlantic, Pacific
and Gulf coasts, setup the U.S. Lake Survey (USLS) within the
Corps to do the job. Between 1864-1900, this agency established
primary triangulation throughout the lakes' area including an
arc south from Chicago connecting to the 39th Parallel triangulation
at Parkersburg, IL.
event of unusual interest was the several very long lines across
Lake Superior they were able to observe despite the fact they
were theoretically not intervisible. While very rare, when found,
these observations, known as refracted lines because the signals
are seemingly lifted by atmospheric conditions so they can be
sighted on, generally involve sights across water, as was the
case here. One such line was reported in the 1930's Hudson River
the 1880's, the Coast and Geodetic Survey (C&GS) offered
a program to assist the states in establishing geodetic control.
As a rule, college professors directed the activities, with
students and local people carrying out the work. Several states
entered the program, but only the surveys in northeastern Pennsylvania
and in New York were of acceptable quality. Other surveys of
special note in this period were:
boundary from Oregon to Lake Tahoe and its continuation, the
oblique line to the Colorado River measured in 1873 by Alexis
Von Schmidt, U.S. Deputy Surveyor and the subsequent resurvey
of the oblique line by Assistant Cephas H. Sinclair C&GS
William C. Hodgkins' C&GS 1893 resurvey of the circular
boundary between Pennsylvania and Delaware originally set by
local surveyors in 1760 and verified by Mason and Dixon in 1763.
efforts in Alaska over several decades, including work on the
U.S.-Canada boundary in the 1890's.
1893-97 remonumenting of the U.S.-Mexico border made under the
direction of Assistant Alonzo T. Mosman C&GS.
1872-85(?) triangulation of the Adirondack Mountains, NY by
Verplanck Colvin, superintendent of the Adirondack and State
geodetic surveying in America entered the 20th century, it did
so on a solid foundation built on excellent surveying practices
where the quality of the observations was never compromised
and the quest for higher accuracies never ended.
THE NETWORKS 1900-1940
the dawn of the new century, as in any year, a generation of
geodesists continue to move along the path towards their rightful
place in the profession, wherever that maybe. In the U.S. one
man, William Bowie, by virtue of his fine analytical mind and
determined nature emerged early as the best of the best and
in the same fashion as Hassler, totally dominated American geodesy
for more than 35 years. Born in Anne Arundel County, MD in 1872,
a graduate of Trinity College, Hartford, CT with additional
work at Lehigh University, he joined the C&GS in 1895.
the next 14 years he demonstrated outstanding abilities in all
phases of the bureau's geodetic activities, both field and office,
leading to his appointment as Chief of the Computing Division
and Inspector of Geodetic Work in 1909 (a position that about
1915 became Chief, Geodesy Division), replacing John F. Hayford,
who had moved on to setup an engineering department at Northwestern
University. There were several huge accomplishments during his
tenure and their successful conclusions can be attributed primarily
to his personal involvement in each one.
1913, for example, he persuaded the governments of Canada and
Mexico to adopt the U.S. Standard datum for their mapping, resulting
in an entire continent being placed on one datum, renamed the
North American datum, a first anywhere. In another case, Bowie
pushed for the completion of sufficient primary triangulation
in the western half of the country so that a single adjustment
could be made and once Bilby towers were available, did the
same for the eastern half. At the same time he proposed a method
to adjust the two halves as separate pieces, yet as a single
supported leveling about equally as triangulation with the result
in 1929 a general adjustment for the entire country was made.
Also on his watch and with his complete support, the State plane
coordinate system came about in 1932 and for the first time
all surveyors could use the network data. Lastly, his grand
ambition was to complete the nation's primary horizontal and
vertical networks and for all intents and purposes he succeeded
by the time of his retirement in 1936.
of Bowie's interest was gravity surveys introduced in the U.S.
by the C&GS in 1875 which led him to become a strong and
vocal proponent for the theory of isostasy, joining Hayford
in this belief. The basic principle of isostasy is that the
gravitational effects of the continental masses above the geoid
are about equally compensated for by lesser density masses below,
the opposite being true in the case of the oceans.
was recognized nationally and internationally, a founder of
the American Geophysical Union and an early president. He was
also president of the Society of American Military Engineers
and the International Union of Geodesy and Geophysics. Bowie
was a captain in the C&GS commissioned corps, but preferred
the title major, the rank he earned in World War I. William
Bowie died in 1940 leaving behind a record of accomplishments
that is not likely to be matched soon, if ever.
of the Geodesy Division making significant contributions during
this period were Walter D. Lambert, Jacob A. Duerksen and Frederic
W. Darling in gravity and astronomy; Sarah Beall in astronomy;
Henry G. Avers, Howard S. Rappleye and Walter F. Reynolds in
computations; Clarence H. Swick in gravity, astronomy and computations;
Walter D. Sutcliffe in records and archives and Hugh C. Mitchell
in promoting surveys in metropolitan areas, plane coordinate
systems and authoring Sp.Pub.no. 242 Definition of Terms Used
in Geodetic and Other Surveys, published after his retirement,
the first and still the best of geodetic glossaries. Others
in the division are cited elsewhere for particular efforts.
surveys began in the U.S. in 1875 under the direction of Charles
S. Peirce following the acquisition of Bessel reversible pendulum
apparatus from Europe. The initial measurements with the equipment
were made at Hoboken, NJ after connecting to known gravity values
in France, Switzerland, Germany and England. In 1882 international
connections were made with New Zealand, Australia, India and
Japan, and in 1900 to Europe again.
were made to the apparatus by Peirce, Thomas C. Mendenhall,
superintendent of the C&GS (1889-94) and others, the most
significant being the replacement of the bronze pendulum with
one made of invar in 1920. Work began on the first national
gravity network in 1891 and completed in 1949, involving 1,185
base stations, all observed with pendulums.
the same time, the first geodetic quality gravimeter, the Worden
gravity meter came into use and was adopted by the C&GS
in 1952 for differential measurements. Early devices of this
type appeared about 1930 for use in oil exploration and were
not accurate enough for geodetic work. The long reign of pendulum
measured gravity was coming to a close after about 75 years,
albeit the apparatus would continue to be used in absolute determinations
for another 25 years.
most of the period the C&GS was the primary mover with significant
contributions made prior to 1900 by Assistants Edwin Smith,
Erasmus D. Preston and George R. Putnam, in addition to Peirce
and Mendenhall. After 1900, William Bowie and Walter D. Lambert
led the way, with Donald A. Rice coming along after 1950 to
continue their work. About 1955, plans were laid to complete
the long desired 100 mile spacing network and to expand the
existing 900,000 square miles of area coverage at 10 mile intervals
over the entire country.
in 1954, George P. Woollard began observations using quartz
pendulum apparatus and Worden gravimeters to create a nationwide
net and completed the work in 1958 with about 175 stations established,
most at regional airports. By 1963, he had extended the net
worldwide involving some 1,300 points.
began making gravity measurements in the late 1930's, while
at the University of Wisconsin, running traverses across the
country and between the Gulf of Mexico and Newfoundland. He
also played a part in getting S. P. Worden to build his geodetic
gravimeter in 1948.
the space age began, the need for higher accuracy gravity networks
greatly increased. To meet that requirement, the U.S. National
Gravity Base Net (NGBN) was established in 1966 in a cooperative
effort by the Army Map Service, USAF 1381st Geodetic Squadron
and the University of Hawaii placing stations at airports in
59 cities throughout the country. Four LaCoste & Romberg
geodetic gravimeters were used and travel was by commercial
airlines. In 1971, the NGBN was incorporated in the International
Gravity Standardization Net 1971 (IGSN1971) along with observations
from various sources connecting stations in 36 additional cities
and a number of calibration line pendulum measurements. There
are 1,854 ISGN stations, 379 are in the conterminous U.S.
part of the continuing effort to improve the IGSN system, the
National Geodetic Survey (NGS),between 1975 and 1979, reobserved
most of the NGBN using 4 LaCoste & Romberg G meters in a
simultaneous mode and ground transportation. This new network
is identified as the National Geodetic Survey Gravity Network
(NGSGN) and includes stations in 54 cities observed in cooperative
efforts between NGS and other federal agencies. Calibration
lines established by 1990 are East Coast, Blue Ridge, Mid-Continent
and Rocky Mountain.
general availability of geodetic gravimeters after 1960 and
ease of operation has induced other federal agencies including
the U.S. Geological Survey (USGS), State and educational institutions
and private companies to carry out observations for several
purposes, other than exploration. Marine gravity remains a giant
undertaking that continues to be pursued. A safe prediction.
The last two decades of the 20th century will be known as the
period when the determination of absolute gravity, to a high
accuracy became commonplace.