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  Surveying Instrument Collection 


AGA, Sweden


Geodimeter Model 6A

Serial Number



Length 360 mm, Width 220 mm, Height 530 mm,

Telescope length 355 mm


Geodimeter 6A (objective end)Geodimeter 6A (control panel view)


This Geodimeter Model 6A is a portable and precise medium range electro-optical distance meter capable of measuring distances up to 10 kilometres in daylight and up to 25 kilometres in darkness. The instrument transmits a light beam (wavelength 550 nm), to which is applied a modulation of approximately 30 MHz (10 m), i.e. the light is intensity-modulated. The beam is directed onto a reflector at the distant station, from which it is reflected back to the instrument. The incoming light is received by a photo-multiplier (hence converted to an electrical signal) and the difference in phase between the transmitted and received modulations is measured.

The Geodimeter basically consists of the following components: (1) Light source, (2) Electro-optical shutter, (3) Modulator, (4) Transmitting and receiving optical systems, and (5) Photo-multiplier and phase comparison unit. The light source is, for night and short-range daylight work, an ordinary tungsten-filament lamp; and for daylight operation over longer lines, a mercury-vapour lamp.

The instrument has two optical systems, one for emitting the modulated light and one for receiving this light after reflection from the reflector at the other end of the line. There is also a searchlight and a coarse sight, which are used for easier location of the reflector. The searchlight is placed under the main tube and consists of a 2 W bulb, placed at the focal point of the projection lens. The course sight has a field of view and a magnification selected to facilitate the reflector location, and is equipped with a reticle which is illuminated when the panel light is on.

The magnification of the "receiver" optics, which is placed in a tube coaxial with the central tube, is 28x, with a focal length of 0.6 m and an objective diameter of 50 mm. The "transmitter" optics, located on the outside of  the receiver optics, has a focal length of 0.6 metres, and inner and outer objective diameters of 55 mm and 105 mm respectively. 

The power source required for the instrument is a 12 V DC battery. The distance meters average power consumption, which depends upon the operating conditions of the thermostats and lamps, is approximately 2.5 A. The precision using the standard method of measuring, with a measuring time of about 5 minutes, is less than (5 mm + 1 mm/km). However, when using the precise method of measuring, the readings could take as long as 30 minutes with a standard deviation of less than (1 mm + 1 ppm). 

History & comments

The Geodimeter (acronym for geodetic distance meter) was the invention of the Swedish Physicist Dr. Erik Bergstrand who designed and applied it to the measurement of the speed of light over known distances in 1947. His original device, introduced in 1949, weighed more than 100 kg and was the very first EDM instrument ever made.


Aluminium transport case (280 x 480 x 630 mm)



  • This instrument is one of the two model 6A's in the collection (see also 0413)
  • Weight of instrument is 16.3 kg.
  • Weight of transport case is 8.5 kg.
  • Catalogued by T. Ko
  • Updated by F. Pall


Manufactured in 1966 (approx). Catalogued in 2000.

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