1807 - 1940

Joseph F. Dracup
Coast and Geodetic Survey (Retired)
12934 Desert Glen Drive
Sun City West, AZ 85375-4825

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Geodetic Astronomy

Only 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 accuracy.

Astronomic 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.

From 1847-1922 longitudes were determined by the telegraphic method whenever possible. After that time radio signals were employed.

Azimuths 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.

Broken Telescopes Introduced

Latitude 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.

Early 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.

In 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).

The 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.

Astro-Geodetic Deflections

Astronomic 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.

In 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.

Until 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 1985.

Towers, ... First of Wood

Classical 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.

While 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 tripods.

The 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.

Then of Steel

The 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.

The 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.

Bilby 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.

One of a Kind

Jasper 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

Lasting 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.

Reference marks serve several purposes: To aid in locating the station, to verify its position, to reset the monument and for use as substitute stations.

Versatile Concrete

Base 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.

About 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.

About 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.

In 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.

Bench 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.

Prior 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 marks predominate.

Field Communications

Communications 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 rare.

John 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.

Beginning 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.

Radios 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.

By 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.

More Territory ... More Work

Progress 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.

A 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.

The 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.

Continent-Wide Arcs

By 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.

West 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.

Great Hexagon and Davidson's Quadrilaterals

West 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 involved.

In 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.

Further 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.

Longest Line Observed

In 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.

Assistant 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.

The 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 globe.

It 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 French observation.

U.S. Lake Survey

The 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.

One 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 arc.

Special Surveys

In 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:

California-Nevada 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 between 1893-99.

Assistant 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.

Beginning efforts in Alaska over several decades, including work on the U.S.-Canada boundary in the 1890's.

The 1893-97 remonumenting of the U.S.-Mexico border made under the direction of Assistant Alonzo T. Mosman C&GS.

The 1872-85(?) triangulation of the Adirondack Mountains, NY by Verplanck Colvin, superintendent of the Adirondack and State Lands Surveys.

As 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.

BUILDING THE NETWORKS 1900-1940

At 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.

Geodetic Chief 1909-1936

During 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.

In 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 system.

He 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.

Another 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.

Bowie 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.

Bowie's Lieutenants

Members 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.

Gravity Surveys

Gravity 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.

Improvements 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.

Meters Replace Pendulums

About 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.

For 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.

Woollard's Contributions

Beginning 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.

Woollard 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.

As 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.

As 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.

The 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.