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  Surveying Instrument Collection 


AGA, Sweden


Geodimeter NASM-2A

Serial Number



Measuring unit (Length 880 mm, Width 430 mm, Height 350 mm)

Optical unit (Length 870 mm, Width 460 mm, Height 530 mm)


Geodimeter NASM-2A (front view)Geodimeter NASM-2A (side view)Spherical mirrors of optical unit


This Geodimeter NASM-2A is an electronic-optical instrument for first-order geodetic distance measuring purposes and is capable of measuring distances up to 50 km. The instrument consists physically of two units, the 'measuring unit', housing the electronic equipment, and the 'optical unit', containing the light conductor and the two large spherical mirrors. With the two units combined, the instrument weighs approximately 100 kg. 

The first image above shows the front control panel view of the assembled instrument consisting of the two units. The second image shows the side view of the instrument, and here the two units can be clearly differentiated with the measuring unit on the left side of the image and the optical unit on the right. The third image represents a rear view of the Geodimeter (opposite side of control panel) where the two large "transmitting" and "receiving" spherical mirrors are clearly visible. 

The mirror lenses with their focusing devices are mounted on a beam attached to the front of the optical unit by means of ball links. This makes the position of the complete optical system fixed with regard to the measuring unit and hence adjustments are reduced to a minimum. In order to protect the Aluminium coated glass surfaces of the mirrors they are covered with a very thin layer of silicium-monoxide.

The "electronic system" is mounted inside the measuring unit and contains the crystal oscillator with a power amplifier for the light modulator, the photo multiplier with associated circuits for detecting and indicating the zero points and the power supply that converts the mains voltage to the operating voltages for the electronic tubes. The purpose of the "optical system", which is housed inside the optical unit, is to project the modulated light on the reflector and to concentrate the reflected light on the phototube. The built-in light conductor offers a possibility of an internal calibration of the instrument which considerably increases its precision.

The primary power required to run the Geodimeter is about 140 W, which is supplied by a 220 V petrol generator. The instrumental accuracy is primarily limited by the accuracy of the modulation frequency, as this directly determines the unit length. Experience has shown that after a few months running-in-time, the frequency error will be smaller than one part per million (1 ppm).

History & comments

The NASM-2A was the first commercially available electro-optical distance meter. 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. Earlier versions of the NASM-2A include the NASM-1 and NASM-2 which were first put into practical use in 1953 and 1955 respectively.


Stored in 2 large wooden boxes




Plane Mirror Reflector

Spherical Mirror Reflector

  • There are two types of reflectors used with the Geodimeter, and these include the 'Plane Mirror Reflector' and the 'Spherical Mirror Reflector' (both are shown in the images on the left). 
  • When using the Geodimeter, the observations can only be made in low light conditions (eg. at night).
  • Weight of Measuring Unit is 46 kg.
  • Weight of Optical Unit is 48 kg.
  • According to A. J. Robinson, this instrument was received from the (Federal) Department of Interior (National Mapping Div).
  • Catalogued by T. Ko
  • Updated by F. Pall


Manufactured in 1959 (approx). Catalogued in 2000.

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