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EDM (Geodimeter Model 2A)

Catalogue number:
1998.3094.01

Inscriptions:
"AGA Stockholm Sweden"

Dimensions:
measuring unit 15 inches high, 35 inches wide, 17 inches deep; optic unit 21 inches high, 35 inches wide, 35 inches deep

Discussion:

In 1938 Erik Bergstrand, a physicist with the Swedish Geographical Survey Office, began to investigate the possibilities of using an optical shutter (Kerr cell) rather than a toothed wheel to measure of the speed of light. Using an instrument that emitted pulses of light at a frequency controlled by a crystal oscillator (typically 10 megahertz), Bergstrand compared the phase difference between the outgoing light and that reflected by a distant mirror. In 1947 Bergstrand took his instrument to a 6-km baseline in Orland and obtained a measurement of 299,793.1 ±0.2 km per second.

At the meeting of the International Association of Geodesy held in Oslo in August 1948, Bergstand read a paper explaining that one could reverse the process, and use the speed of light to measure distances. Before the meeting Bergstrand asked AGA to make a cover for his instrument. After the meeting Bergstrand asked AGA to developing a commercial product. This work was undertaken by Ragnar Schöldström, assisted by Gustav Westerlund.

American geodesists kept abreast of Geodimeter developments. In 1951, for instance, Carl Aslakson of the United States Coast and Geodetic Survey described the new instrument as promising. "The time required for an observation is reported to be two hours, with two additional hours required for computation of the results," he noted. "The instrument is reported to be portable, and adaptable to field use." In fact, however, the early Geodimeter weighed over 200 lbs--the measuring unit weighed some 108 pounds and the optical unit weighed some 113 pounds--and it required a generator to supply power. It had a useful range of 30-35 km.

Of the ten Model 1 Geodimeters produced in 1953, five were purchased by geodetic agencies of the United States government. Tests by the Army Map Service in cooperation with the British Ordnance Survey produced results that were accurate to 1:300,000, thus showing that the Geodimeter was comparable to the best invar tapes. Moreover, the Geodimeter was especially valuable in areas where the terrain made conventional tape-measuring procedures difficult and expensive. The Bureau of the Budget appreciated the money to be saved by using a Geodimeter rather than traditional measuring techniques.

The Geodimeter Model 2 resembled the Model 1, but used three modulating frequencies rather than two. A mercury vapor lamp increased the range to about 50 km. The Model 2 was introduced in 1955, and by 1957 some 50 units had been delivered. Each cost about $25,000. The Model 2 at the Smithsonian was acquired by the United States Coast and Geodetic Survey in 1957. One part is marked "Measuring Unit / 151 X590228." The other is "Optic Unit / 51 X590271."

The first rigorous test of this instrument came in the late spring of 1960 when the United States Air Force asked the Coast and Geodetic Survey to determine the positions of nine tracking cameras located from 40 to 50 miles from Cape Canaveral. The Air Force was planning a test of its new Azusa Mod II tracking system in the Atlantic Missile Range, and demanded an accuracy of better than 1:400,000. In the hands of George "Bud" Lesley, the Geodimeter produced measurements accurate to about 1:1,200,000. This was said to be the greatest accuracy ever attained in any extensive geodetic survey, and ten times greater than the rigid standards prescribed for first-order triangulation. Soon thereafter the Survey adopted the Geodimeter as their basic unit for measuring first order baselines. The Cape Canaveral survey was extended to camera stations at Homestead, Florida, (200 miles south) and at Savannah, Georgia, (250 miles north), and eventually morphed into the high-precision transcontinental traverse, conducted jointly by the Coast and Geodetic Survey and the Army Map Service.

Ref: AGA, Geodimeter System Bergstrand Type NASM-1.

[J. Clendinning], "The Geodimeter: An Instrument for the Accurate Measurement of Distances by High Frequency Light Variations," Empire Survey Review 11 (1952): 290-301 and 363-371.

Austin C. Poling, Geodimeter Manual (Washington, D.C., 1959; rev. ed. 1961).

Lansing G. Simmons, "A Singular Geodetic Survey," United States Coast and Geodetic Survey Technical Bulletin 13 (September 1960).

Further Information:

AGA
EDM

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