Smithsonian - National Museum of American History, Behring Center

Physical Sciences Collection - Surveying and Geodesy

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EDM (Geodimeter Model 4B)

Catalogue number:


13 inches high, 12.75 inches wide, 19 inches deep


The Geodimeter Model 4, introduced in the spring of 1959, measured 12 inches square, weighed about 35 lbs (plus the 12 volt battery and inverter), cost less than $5,000, and had a range of 50 feet to 3 miles. Because of its small size and low cost, it was attractive to independent surveyors, especially once advertisements promised that the initial outlay was "easily returned by the savings gained in just a few projects use." Model 4B was similar to the Model 4, but had a slightly different housing. Model 4D used a high-pressure mercury vapor lamp. Advertisements in 1963 boasted: "Now up to 3 miles in brightest sunshine" and "only one instrument, only one operator."

In 1965, having found that the Model 4 offered the same accuracy as the Model 2, the United States Coast and Geodetic Survey, began phasing out the heavier instrument. The Survey was also realized that by equipping a Model 4D with a laser they could increase its range, and thus its productivity. George "Bud" Lesley, who had used the Model 2 at Cape Canaveral and several Model 4Ds on the transcontinental traverse, and who had read up on lasers, was given the task of designing this modification. Within a year Lesley had replaced the mercury vapor lamp with a 2-mw Perkins-Elmer helium neon laser, replaced the Kerr cell with a KDP cell (an electronic shutter), and replaced the original 1P21 phototube with a 56TVP phototube. The Coast and Geodetic Survey then put out a call for someone to put lasers in a number of 4Ds. As the low bidder, AGA got Lesley and his prototype for 60 days, and was soon producing the Model 4L. This instrument required an ac generator, but had a range of 42 km at night, and about 21 km in daylight. On one test in Nebraska, the laser Geodimeter measured a 30 km line, "nearly twice the length of previous measurements, with accuracy exceeding one part in a million." A July 14, 1968 press release stated that "A major breakthrough in surveying has been achieved with lasers," and quoted Lesley to the effect that "lasers have enabled survey teams to expand by 20 to 50 percent the amount of terrain covered and to increase their accuracy." For this work Lesley received the Colbert Medal of the Society of American Military Engineers

This Geodimeter at the Smithsonian is a Model 4B that belonged to the United States Coast and Geodetic Survey. It was made in the period 1960-1964, and was still in use in 1976. The original lamp was replaced with a 2-milliwatt laser in 1969, and a 6-milliwatt laser in 1971.

Ref: AGA, Geodimeter System Bergstrand Model 4D (March 15, 1964).

George B. Lesley, "Laser Geodimeter," Surveying and Mapping 27 (1967): 625-631.

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