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geodetic surveying 1940-1990

1807 - 1940

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

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A Series of Name Changes

Hassler began using the title Superintendent of the Coast Survey in his 1836 report to the Secretary of the Treasury, the year the bureau was transferred to that department. This usage continued until 1865 where that and subsequent reports to 1877 showed United States Coast Survey. In 1878 Congress changed the name to U.S. Coast and Geodetic Survey (C&GS); in 1899 the U.S. was dropped. In the 1970's, the C&GS became the National Ocean Survey (NOS), later renamed the National Ocean Service (NOS) with the geodetic functions assigned to the National Geodetic Survey (NGS), presently an office in NOS.

The End of the Beginning

Hassler continued observing the triangulation southward and after about 10 years only had reached southern New Jersey involving but 24 stations. Upon completing the observations at station BURDEN he traveled to a station site in Delaware where during a severe storm he fell trying to protect his instruments and was badly injured. He returned to his home in Philadelphia where he died on November 20, 1843.

Thus we reach the end of the beginning. The groundwork for laying the foundation was done. By the turn of the century triangulation would span the nation north to south from Maine to Louisiana, east to west from New Jersey to California, over the Great Lakes and with work underway through the middle of the country between the Rio Grande and Canada.


The 1807-43 era in American geodesy was totally dominated by one person, Ferdinand R. Hassler. He was head and shoulders above all others. This was not the case for the 1843-1900 years, albeit several strong willed, very competent personalities prevailed for long stretches, some for almost the entire period, but none over shadowed all others as completely for as long a time as did Hassler. Among the elite group were Alexander Dallas Bache, George Davidson and Charles A. Schott.

A.D. Bache, Scientist

Alexander Dallas Bache was appointed the second superintendent of the Coast Survey in December 1843, being eminently well qualified for the post having a learned interest in many scientific activities. Born in Philadelphia in 1806, the great-grandson of Benjamin Franklin, a graduate of West Point at age 19 and served 3 years in the Engineers. Prior to his appointment, he was professor of natural history and chemistry at the University of Pennsylvania and served as superintendent of schools in Philadelphia. Later he helped form the National Academy of Sciences and was its first president.

Congressional Decisions

The Act of 1843, passed by Congress in April of that year, defined the functions and personnel responsibilities of the Coast Survey that, except for a few modifications, remained in place for more than a 100 years. In fact, the enabling act of 1947 did little more than restate the bases for the bureau that were essentially unchanged from the 1843 definitions.

One proviso in the 1843 act, unfortunately, limited the bureau's activities to the coast, and the narrowest belt of land to support the primary triangulation. Bache accepted the restriction, and immediately went ahead with a broad plan to divide the Atlantic and Gulf coasts into nine sections, where the ongoing work (geodetic, topographic and hydrographic) would be carried on simultaneously and to uniform standards. Two sections were added when the Pacific coast territories were annexed.

Congress lifted the restriction in 1871, during the superintendency of Benjamin Peirce, when it gave permission for the measurement of the Transcontinental arc. In 1878, the name was changed to U.S. Coast and Geodetic Survey to reflect the bureau's responsibilities for geodetic surveys inland.

Bache's Field Activities

Bache continued Hassler's practice of personal involvement in the field work, but didn't limit his participation mostly to geodetic surveys as did his predecessor; albeit he spent considerable time in making astronomic observations and measuring base lines. In regard to the latter, he designed and William Wurdemann, then a master mechanician in the Coast Survey, constructed compensating apparatus that was used until 1873. Seven principal base lines were measured using the Bache-Wurdemann equipment, the last being the ATLANTA base in Georgia. Bache personally directed the measurements for the first three bases at DAUPHIN ISLAND, near Mobile, AL in 1847, BODIE ISLAND, NC in 1848, EDISTO ISLAND, SC in 1850 and the sixth at EPPING, ME in 1857. Wurdemann later resigned from the Coast Survey and formed a very successful business making astronomic and geodetic instruments for clients here and abroad.

Alexander Dallas Bache died in 1867, age 61 at Newport, RI. America had lost one of its greatest scientists, however he left a legacy of excellence for his successors to build on and most were able to do so.


Bache was the last of the superintendents to participate extensively in field operations, the position had simply become too large and the responsibilities too great for such personal involvements. Following Bache's death in 1867, nine civilian superintendents/directors served until 1929 when the position was reserved for C&GS commissioned officers. The title was changed to director in 1920. Several of the civilian superintendents were highly regarded scientists including Benjamin Peirce (1867-74), Julius E. Hilgard (1881-85), Thomas C. Mendenhall (1889-94) and Otto H. Tittmann (1900-15). The last civilian, E. Lester Jones (1915-29) was a veterinary surgeon by profession with an outstanding record in public administration.

One of Jones' first acts was to propose a commissioned officer corps because of the difficulties in employing and retaining qualified personnel for the field work due to the frequent moves and other hardships. The proposal was accepted by the Congress and the corps came into existence during World War I. There was one complication. The formation of the corps forced the civilian personnel to accept a lesser role. In addition to the director, many of the top positions in both the field and office are reserved for officers causing occasional differences and/or resentment to arise in the civilian group towards the corps. All in all, however the arrangement has worked fairly well, both sides usually accepting the situation as that's the way it is.

One of the Giants

Bache was fortunate in recruiting a number of highly qualified engineers, mathematicians and other scientific types for the Coast Survey with the title of Assistant. In due course, many became master jack-of-all-trades being equally at home in the several Coast Survey activities that included in addition to geodesy, topographic mapping, hydrography, tides and currents, magnetism and others. Among the group was George Davidson, a native of Nottingham, England whose exceptional talents were recognized by Bache when he was head of the Philadelphia school system and Davidson was a student.

Joining the Coast Survey in 1845 at age 20 he trained in geodetic surveys and astronomy under the foremost engineers in the bureau including Bache and Robert H. Fauntleroy until 1850 when he was sent to California with 3 other civilian assistants. Travel between the Atlantic and Pacific coasts was not an easy journey in 1850. Overland routes were very limited, especially in the mountainous regions and west of the 98th meridian those that did exist often ran through hostile Indian territory. Once beyond rail and river boat services, travel was by foot or by horse or mule conveyances.

Transcontinental rail service wasn't available until 1869 and it was many years before a rail network was developed. Stage coach service from rail heads was unreliable at best and accommodations when available, were very primitive. Davidson and his colleagues travelled by ship to the Isthmus of Darien (now Panama), then by available means to the Pacific side and another ship to San Francisco, a trip that usually took several months.

There was no direct mail service with Washington for many years and the field records and computations were sent by ship as well. Duplicate and sometimes triplicate copies were made of all records and kept on site, even occasionally retained after their safe arrival was acknowledged for use in the ever expanding network.

Davidson remained on the west coast for about 45 years, the Civil War period and a few foreign assignments excepted. During his service, the Transcontinental triangulation, a k a the 39th Parallel arc was completed, primary surveys near Los Angeles and Santa Barbara Channel were underway and coastal triangulation was extended from Mexico to Canada.

Davidson is not credited with the earliest triangulation on the west coast, that accomplishment belongs to Robert D. Cutts, one of the assistants who came west with him in 1850, although we can be sure that he was a major participant. This small network covering San Francisco Bay was observed to a secondary accuracy between 1851-54, with scale provided by preliminary base lines, one at the Presidio measured in 1851 and the second, the PULGAS base near Palo Alto in 1853 and oriented by astronomic azimuths.

In 1876, Davidson was placed in charge of the western section of the 39th Parallel surveys and extensions to it. In the course of that work he measured base lines at Yolo County north of San Francisco (YOLO base 1881), LOS ANGELES base 1889, the angulation for the base expansions and the astronomy to orient the principal triangulation.

Survey vs. GLO

He was a man of strong personal opinions and was an ardent advocate for the Coast Survey to assume the surveying functions of the General Land Office (GLO), the present day Bureau of Land Management. While there was support for his proposals, it is very fortunate for both agencies that they never materialized. The Survey had done some section work for the GLO in the Florida Keys in the 1850's and the experience was not a happy one for either side.

There is no doubt that had his plan been accepted the section corners would have been located, at the very least, to a minimum geodetic accuracy thus creating a cohesive network supplemental to the national framework. However, the cost would have been enormous and Survey field personnel would assuredly have been bogged down executing time consuming short line surveys, often over rough terrain.

Sooner or later Congress would have viewed this as an endless project, and it very well might have been just that. GLO practices were right for the time, with means now available for a total upgrading of the system once all the legal ramifications for such an undertaking are settled.

Although this cause was lost, most others were successful. His ability to convince James Lick to finance the great astronomical observatory atop Mt. Hamilton is a good example of his successes. George Davidson was unquestionably the greatest field geodesist the Survey produced in the 19th century, the giant among giants for only men of that breed could have executed the triangulation through the mountain west of the time. In 1895 during William W. Duffield's superintendency (1894-97), Davidson in his 50th year of service was summarily dismissed, as were many other experienced employees. In his usual pragmatic fashion, Davidson accepted a professorship at the University of California and went on with his life doing things his way until his death in 1911.

Multi-Talented Schott

In the field of computations, Charles A. Schott stood alone during this period, much as George Davidson did in his area of expertise. Schott was born in Mannheim, Baden, Germany in 1826, a graduate of the University of Karlsruhe and joined the Coast Survey in 1848. Appointed chief of the computing division in 1856, he held the post until his resignation on December 31, 1899, after 52 years of service.

A world recognized expert on the earth's magnetic field, personally carrying out numerous field surveys in the eastern U.S. and instrumental in establishing magnetic observatories at Madison, WI and Los Angeles, CA. He had a life long interest in field activities that was culminated with the development of a contact compensating base line measuring apparatus of unique design in 1881 that bears his name and was used in the same year to measure the YOLO base in California.

As chief of the computing division he was responsible for a myriad of calculations and as was his way took a distinct interest in all of them. Much of his efforts was directed to geodetic computations, especially the adjustment of the observations and investigations of the results. One such examination carried out in 1878-79 relative to geodetic and astronomic data on the Eastern Oblique arc provided information for establishing the first national datum and conclusively showed that the Clarke spheroid of 1866 fit the U.S. better than the Bessel spheroid of 1841 then in use.

Both results were adopted, the New England datum of 1879 as it was named is the basis for all subsequent datums until 1983. The Clarke spheroid of 1866 still provides the best fit for the continental U.S., albeit the Geodetic Reference System of 1980 (GRS 80) was adopted in 1983 for other reasons.

Schott was a prodigious writer authoring more than 160 scientific articles, papers and reports including two volumes detailing the results of the primary triangulation that were hailed by geodetic circles worldwide. The volumes are The Transcontinental Triangulation and the American Arc of Parallel 1900, 871pp. and The Eastern Oblique Arc of the United States and the Osculating Spheroid 1902, 394pp. Charles A. Schott died in 1902. We will never see his kind again.

Quadrilaterals and Central Points

Early in Bache's tenure a decision was reached for the triangulation that only quadrilaterals with both diagonals observed, commonly called braced quadrilaterals or central point configurations would be permitted. The reason for this specification is these figures provide at least one geometric condition involving the angles, known as side conditions or equations. These equations utilize the sines of selected angles taken in a particular sequence and give a better evaluation of the worth of the angles than an examination of the triangle closures alone.

This requirement was met for all the principal triangulation and most of the secondary work except for parts of Hassler's early net and pieces of the U.S. Lake Survey triangulation observed later in the century.

Longitude by Wire

The first of several significant advances in American geodetic surveying that were to be made in the next 100 years was the development by Sears C. Walker in 1847 of a method for determining time differences between places using the electric telegraph, invented only a year earlier. As a result more accurate astronomic longitudes were determined leading to subsequent improvements in Laplace corrections to astronomic azimuths.

Once the transatlantic cables were in place later in the century the same principle was employed to determine a more accurate cardinal longitude for North America, relative to Greenwich, than had been obtained from 1065 chronometer exchanges during a previous time. This method was used until the 1920's before being replaced by radio signals.

Advances in Theodolites

Hassler's original theodolite was built by Edward Troughton of London during the period 1811-14, had a 24 in. circle, weighted more than 150 lbs. and required a special oversize carriage to transport it. In 1836 his Great Theodolite with a 30 in. circle, also constructed by Troughton arrived in the U.S. and was put to immediate use in the primary triangulation then being extended east and south of New York City.

In the early period, segments of the primary triangulation were observed with 10 and 12 in. repeating theodolites and acceptable results were obtained. The results notwithstanding, repeaters were rarely employed on primary work after this time.

In due course, several lighter, more accurate instruments were designed and/or built in the bureau's instrument division. The most notable being two 20 in. circle models in 1873 made by William Wurdemann, formerly of the Coast Survey and three improved versions of these models constructed in 1876-77 at his shops in Dresden, Germany.

The instruments were followed in the continuing evolution of smaller and equally accurate circles at 25-30 year intervals by the 12 in. circle - 3 micrometer theodolite constructed by the then chief of the division, Ernst G. Fischer about 1894 and the 9 in. circle - 2 micrometer theodolite designed by Douglas H. Parkhurst, head of the division in 1927.

All direction theodolites prior to the Parkhursts were read via 3 micrometers. In the early 1950's the Wild T-3 theodolite, with a 5½ in. glass circle read by an auxiliary telescope, replaced the 2 micrometer Parkhurst and was employed until the mid 1980's, becoming obsolete with the introduction of the Global Positioning System (GPS).

During the more than of 150 years of U.S. triangulation observations, diameters of horizontal circles ranged from 30 inches in the 1830's to 5½ inches in the 1980's. Despite the huge disparities in weight, construction and diameter of theodolite circles, the common denominator is the accuracy of the observed angles remain the same.

Instruments of the time were built to last. Hassler's 30 in. theodolite, for example was in constant use for about 37 years before being destroyed by a tornado. Its destruction is described in, p.47 as follows: ..... was in continuous use till November, 1873, when it met with an accident at station SAWNEE, in Georgia. It was struck by a tornado and, not withstanding its weight of 300 pounds, was hurled from its stand and irreparably damaged.

Few theodolites before the Parkhursts had vertical circles attached. Vertical angles were observed using separate instruments with circles only slightly smaller than the horizontal ones.

Field Specifications

Prior to 1905 no published specifications existed for making horizontal observations, although the procedures adopted by the superintendent of the Survey in that year were those long in use. The lack of stated requirements was not of great concern since only 4 or 5 observing units were in the field on any given day and the observers, a very select group, were field trained, college- educated engineers.

This was not always the case in later periods. Between 1935 and 1970, for example, about 10 times that number of units often were working on a single night.

From the beginning, direction instruments were mostly employed in a pattern of taking direct and reverse measures on each point in turn seen by the station occupied, the sum total constituting a set. Several sets were observed with each set beginning on a different part of the circle. This method of observing is attributed to Bessel, albeit it was likely used in some form prior to his proposal.

Eventually sets became known as positions and the initial settings for each position were assigned specific locations on the theodolite circle. Sixteen positions evolved finally as a complete observation for the principal triangulation with a 4" rejection limit from the mean.

The Coast Survey rarely deviated from the general practice described above, but the Lake Survey did on occasion by employing the method of independent angles reasoning it would reduce the effects of tower twist, a rationale dismissed by some.

Coast Survey observers were trained to take their measures as rapidly as possible, without forcing any of the motions or miss 'em quick in the parlance.

Little information is readily available about the time required to complete observations in earlier periods and it really doesn't matter much because most occupations took a week or more and a few several months. Gardens were even planted at times. Estimates to complete the 32 pointings on each signal light when using the 12 in.- 3 micrometer theodolite was 15-20 minutes.

Once Parkhursts came into use the time was reduced by about one-third and some observers became very fast, with runs of 6-8 minutes per light and less recorded. Similar times were the rule with T-3's.

Most theodolites used by the C&GS prior to about 1950 required two observers for the most expeditious operation, one to point the instrument and read one of the micrometers and the second to read the other micrometer(s).

Personnel assigned to observing units as recorders had to have superior arithmetic skills so as to be able to instantly mean the 6 readings of seconds for 3 micrometer instruments and 4 readings for Parkhursts. Then continuing the process by reducing the readings to the initial station for each position and placing them on an abstract for inspection by the observer on completing his work. T-3's involved a slightly different process, the effort required was about the same as for the Parkhurst.

Signals: Cones, Helios, and Lights

In Hassler's time, observations were generally taken on earthenware cone targets mounted atop pole signals. Polished metal cones for reflecting sunlight, mounted in the same fashion, continued to be used for several decades at unmanned stations.

By the 1840's, heliotropes came into use continuing until 1902 when they were replaced by acetylene lamps, which in turn were replaced by automobile headlights in 1916. It was long known that daytime observations caused the triangulation to sway, probably due to unequal heating of the theodolite, despite precautions taken against it and several attempts were made in the 1880's to observe at night, even to using a selenotrope to reflect moonlight; none proved too successful.

Selenotropes required much larger mirrors than heliotropes' 2-4 inch diameter reflectors: i.e. 6 by 6 inch mirrors for 22 mile lines, 6 by 8 inch for 48 miles, 8 by 10 inch for 70 miles and lines longer than these were not uncommon in the triangulation of the time. The sway, of course could be controlled by inserting additional Laplace stations. After 1902 all primary observations were made at night.

On the Mark

Accurate plumbing of theodolites and targets over station marks is requisite in making geodetic surveys. Exact centering was always desired, however miscenterings of 0.1 to 0.2-in. and perhaps more when high towers were involved would not be unrealistic, nor cause large errors. i.e. A miscentering of 0.1-in. translate into a maximum error of 003 in an angle over 10 mile lines and for a 1-in. displacement, over the same length lines the largest error would be 03. Where tripod height stands and short towers were employed, the centering was usually done with plumb bobs.

Prior to about 1900, when optical plummets, known as vertical collimators, came into use, high signals were plumbed with a theodolite, from observations at two locations, 90 or 180 apart. The early collimators were designed for use from the top, down. Once Bilby towers replaced wooden signals, the ease in which the tripod head and light plate could be adjusted made it more practical to plumb from the station mark, upward, and the collimators were modified accordingly. Many theodolites introduced after 1950 had built in optical plummets, further simplifying the task.

Base Line Measuring Apparatus

A variety of base line measuring apparatus, usually rods or bars 2-6 meters in length, encased in tubes were developed, many ingeniously designed to resolve the problem of thermal expansion through compensating principles. The last of these apparatus and the most accurate, a duplex bar set was designed by Assistant William Eimbeck in 1897 and used by him to measure the SALT LAKE base in 1897. The equipment was employed as a field standard replacing the iced bar apparatus used previously for the standard kilometer section during the measurement of 9 bases on the 98th Meridian arc by Assistant Albert L. Baldwin with steel tapes in 1900. Eimbeck was a long time associate of George Davidson and one of those elite mountain men mentioned previously.

The first bases measured with steel tapes were the HOLTON base, IN and the ST ALBANS base, WV by Assistants Alonzo T. Mosman and R. Simpson Woodward in 1891 and 1892 respectively. Measurements were made mostly at night utilizing 4 tapes, two 50 meters in length and two 100 meters in length. A 100 meter field comparator and a one kilometer section of each base were measured using iced bar apparatus designed by Woodward when he was associated with the USLS. The apparatus consisted of a 5 meter steel bar immersed in melting ice in a Y shaped trough mounted on two 3 wheel vehicles that were moved along a portable track.

While highly accurate, it was a cumbersome device requiring about one hour to measure 100 meters. Similar equipment was long used by the Bureau of Standards to standardize tapes. No C&GS base line was completely measured with the apparatus. The 9 bases on the 98th Meridian arc were measured accordingly, except only the 100 meter comparators were established by iced bar measures, with Eimbeck's duplex bars serving as the field standard, as mentioned earlier.

The Impact of Invar

The thermal expansion problem was finally resolved with the discovery of invar, an alloy of nickel and steel having a very low coefficient of expansion by Charles E. Guillaume, a French scientist about the turn of the century. Tapes and wires became feasible for measuring distances.

Following the measurement of 6 primary base lines by Owen B. French in 1906, all bases were measured using tapes made of invar, 50 meters in length, a much more efficient and faster method than those previously employed and certainly as accurate. Base line tapes were kept in sets of 4, three for the measurements and one as a comparator, all standardized prior to and after use.

Prior to 1870, all primary base lines resulted from a single measurement with segments occasionally remeasured for verification purposes. Due to the singular nature of the observations, the validity of the measurements was primarily ascertained by knowledge of the equipment and procedures employed. Accuracy estimates were based on comparisons of the apparatus made prior to and at the completion of the work with the field standard and by duly considered error estimates for the various observations and actions involved.

In 1872-73, the ATLANTA base on Peach Tree Ridge was completely measured three times, forward in the fall, backward in the winter and forward again the following summer. Few base lines were ever completely measured twice prior to this time and to do so on 3 occasions was probably a geodetic first. Later, at least two complete measurements were made and when using invar tapes two of the measures were always made in opposite directions with the same person at the front contact or marking end of the tape to cancel the parallax effect.

Measuring base lines was a time consuming chore prior to tapes being employed. Sites often had to be graded to meet the 5% slope restriction and taking observations at 6-8 meter intervals made for slow progress, which averaged less than 0.5 mile per day for Hassler's equipment and slightly more than one mile with compensating apparatus. Once tapes were introduced, the grade allowance was increased to 10% and the much longer tapes made 5 miles and more progress per day routine.

Base line apparatus followed an evolutionary path similar to that taken by theodolites, with one distinct exception. Each new apparatus provided an improved accuracy, while there is no significant difference in the angles measured with the 3 footers in 1790 and those observed by the half footers after 1950. For example: The 3 base lines measured with Hassler's equipment had an average standard error (one sigma) of 1:200,000; compensating apparatus 1:310,000 and invar tapes 1:675,000 or better.

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